The influence of grinding media hardness on an abrasion index

Transcription

1 The influence of grinding media hardness on an abrasion index Item Type text; Thesis-Reproduction (electronic) Authors Decker, John David, Publisher The University of Arizona. Rights Copyright is held by the author. Digital access to this material is made possible by the University Libraries, University of Arizona. Further transmission, reproduction or presentation (such as public display or performance) of protected items is prohibited except with permission of the author. Download date 27/07/ :32:17 Link to Item

2 THE INFLUENCE OF GRINDING MEDIA HARDNESS ON AN ABRASION INDEX by John David Decker A Thesis Submitted to the Faculty of the DEPARTMENT OF METALLURGICAL ENGINEERING In Partial Fulfillment of the Requirements For the Degree of MASTER OF SCIENCE In the Graduate College THE UNIVERSITY OF ARIZONA

3 STATEMENT BY AUTHOR This thesis has been submitted in partial fulfillment of requirements for an advanced degree at The University of Arizona and is deposited in The University Library to be made available to borrowers under rules of the Library. Brief quotations from this thesis are allowable without special permission, provided that accurate acknowledgment of source is made. Requests for permission for extended quotation from or reproduction of this manuscript in whole or in part may be granted by the head of the major department or the Dean of the Graduate College when in his judgment the proposed use of the material is in the interests of scholarship. In all other instances, however, permission must be obtained from the author. SIGNED: APPROVAL BY THESIS DIRECTOR This thesis has been approved on the date shown below; W E. HORST Associate Professor of Metallurgical Engineering DATE

4 I wish that we Could share the difference Between the things we say And the things we understand Lois Wyse LOVE and other signs of life iii

5 PREFACE This investigation was conducted to gain a closer insight in predicting grinding media consumption based on laboratory testing. Perhaps in time, the mineral industries will be able to more completely rely on laboratory test results and reduce the time and expense spent on long term actual production plant testing. This investigation was performed in the Grinding Research Laboratory of CF&I Steel Corporation in Pueblo, Colorado with the consent of the Graduate College and Department of Metallurgical Engineering of The University of Arizona, Tucson, Arizona. The author wishes to acknowledge, with sincere thanks, the guidance and encouragement given by Dr. W. E. Horst, his thesis advisor. Special appreciation is extended to the Mining Sales Department of CF&I Steel Corporation for allowing their facilities to be used for this study and to Mr. G. Dopson and the Magma Mining Company, San Manuel, Arizona, which supplied the raw material used in this investigation and actual plant operating data. iv

6 TABLE OF CONTENTS Page LIST OF T A B L E S vii LIST OF ILLUSTRATIONS.... viii ABSTRACT... '... '.... ix INTRODUCTION... LITERATURE REVIEW Abrasion Testing Types of Grinding M e d i a... 9 Grindability Testing Using Gr indab ility Test Results EXPERIMENTAL WORK General Discussion Materials Sample Preparation Abrasion Test Procedure Rod Mill Grindability Test Procedure Ball Mill Grindability Test Procedure EXPERIMENTAL RESULTS Special Abrasion Tests Grindability Tests DISCUSSION OF RESULTS Abrasion Tests Grindability Tests SUGGESTIONS FOR FUTURE WORK APPENDIX As ANALYSIS OF VARIANCE COMPUTER RUN APPENDIX B: EXAMPLE OF ABRASION TEST DATA SHEETS APPENDIX C: EXAMPLE OF ROD MILL GRINDABILITY TEST DATA SHEETS v

7 vi TABLE OF CONTENTS--Continued Page APPENDIX D: EXAMPLE OF BALL MILL GRINDABILITY TEST DATA SHEETS R E F E R E N C E S '. 82

8 LIST OF TABLES Table Page 1. Test Coupon Hardness (Rockwell C) Special Abrasion Test Results (San Manuel Ore) Special Abrasion Test Results (Quarts) Chemical Analysis Results of Test Coupons Ball Mill Grindability Tests - Test Data Rod Mill Grlndability Tests - Test Data Predicted Power Consumption - Ball Mill Grindability Tests Predicted Power Consumption - Rod Mill Grlndability Tests vii

9 LIST OF ILLUSTRATIONS Figure Page 1. Photograph of abrasion test m a c h i n e Photograph of rod mill grindability test m a c h i n e Photograph of ball mill grindability test machine Predicted metal consumption from the Bond abrasion index Relationship between abrasion index and brinell hardness Relationship between mesh size and work index for rod and ball mill grindability tests Relationship between mesh size and grams produced per test revolution - rod mill Relationship between mesh size and grams produced per test revolution - ball mill Relationship between work index and power requirements at varying product sizes for a 10 foot diameter ball mill. 57 viii

10 ABSTRACT The influence of grinding media hardness on an abrasion index has been investigated using the Bond abrasion test method. A porphyry copper ore and massive quartz were used as the materials tested. The results presented here show the variation in projected metal consumption rates at different media hardness levels with different materials being ground. Also presented are the ball and rod grindabilities of a porphyry copper ore at varying product sizes. This is presented to show the change in power requirements5 and consequently grinding capacity, for a given operating mill as a function of different product-size requirements. ix

11 INTRODUCTION In milling, one of the greatest costs incurred is steel consumption. For some properties being explored and considered for develop- ment, the cost of steel consumption could well be the contributing factor as tp whether or not a property could be profitably developed. Over the years many people have tried to find ah easy, fast and relatively inexpensive way of predicting metal consumption in order to help in evaluating the costs incurred in openiftg up new Ore bodies or to evaluate grinding media of different composition or manufacture in an existing operation. Mr. Fred C. Bond (10), while working at Allis--Chalmers Manufacturing Company, used an abrasion testing machine, developed originally by the Pennsylvania Crusher Division of Bath Iron Works Corporation (13), with which he was able to test hundreds of different ores and derive empirical formulas to predict metal consumption--balls, rods, liners--for wet or dry grinding or for crushing. This testing did not include predictions for Ni-Hard balls or liners or for special steels but only for the common steels used for balls, rods and liners during the late 19508s and early I9608s. One of the empirical formulas derived, predicts grinding ball consumption for wet grinding and is (10): 1/3 Ib/kwhr (Ai ) 1

12 Where Ai is the abrasion index of the ore and is a measure of the weight loss in grams of a piece of AISI 4325 steel (27) hardened to 500 BHW (4340 is now commonly used) during an abrasion test. This empirical formula gives no consideration to the chemistry of the grinding ball used (carbon, alloy, etc.) or its hardness. For example, a one- inch grinding ball may have a volumetric hardness of 712 BHN (Brine11 Hardness Number), whereas a 3-1/2 inch carbon grinding ball may have a volumetric hardness of 379 BHN. (Volumetric hardness is explained in DISCUSSION OF RESULTS.) This same argument can be applied to grinding rods using a different empirical formula (10): Ib/kwhr = 0.35 (Ai ) '20 The top size of the grinding ball to be used in a wet ball mill grinding operation can be calculated by taking into account the feed size, product size, specific gravity of the ore, the work index of the ore, the mill diameter and the fraction of mill critical speed (8). Hardness of a commercial grinding ball produced is dependent upon its chemical composition and manufacturing techniques. Usually, larger balls are of a lower hardness to reduce internal stresses set up in the manufacturing process.. Alloy grinding balls may require special heat treating to reduce internal stresses, whereas carbon or cast balls may require no heat treatment. This study was undertaken to determine a correction factor which would be used with the abrasion index and which would take into account grinding ball hardness and which should more accurately predict grinding ball (or rod) consumption.

13 3 A series of abrasion test coupons wefe obtained at different hardness levels which encompass the range of hardnesses found in forged steel grinding balls, 712 to 235 BHN. These coupons were abrasion tested using a porphyry copper ore and quartz. The results were analyzed to determine if a more accurate prediction of grinding ball consumption can be made based on laboratory-scale test results. This study takes into account only the affect of hardness on the wear performance of the grinding media. Work index values change as the reduction ratip changes, increasing or decreasing as the product size becomes finer in heterogeneous ores. Rod mill work index values often differ from ball mill work index values in the same type ore (40). In homogeneous ores the work index values tend to remain the same for different product sizes. As a supplement to this study, rod and ball mill grindability tests were run on the porphyry copper ore at sizes of 4 through 65 mesh for the rod mill tests and 14 through 325 mesh for the ball mill tests. These tests were run to determine the changes in work index values, since grinding media consumption is a function of the work index.

14 LITERATURE REVIEW Abrasion Testing ' The economic significance of wear cian easily be recognized when one considers the high cost of replacing #orn parts in mactiipery. A few ounces of metal worn from critical surfaces in an engine may cause the engine to cease working entirely, while tons of metal may be worn from a modern crusher in the matter of a few weeks. wear respectively are adhesive and abrasive wear. These types of The other two of the four principle forms of wear are corrosive and surface fatigue. Abrasive wear can be further broken down into three areas. Low-stress abrasion would occur in situations like pumping slurries or screening. An example of high-stress abrasion would be the wear of balls or rods in grinding mills. Gouging abrasion occurs when coarse abrasive materials cut into a wearing surface with considerable force, an example being the wear of jaw crusher liners. One company (23) reported a distribution of wear costs as follows: Material Worn Percent of Cost Rod mill liners 6.4 Grinding rods 35,2 Ball mill liners 2.5 Grinding balls 35.8 Crusher liners and misc. 20.1

15 ' ' Many tests are presently being used to study wear. The Grinding Research Laboratory at CF&I Steel Corporation has undertaken a study to determine the effect of oxidation-corrosioti on steel grinding balls. In this study, jar mills are used with small diameter grinding balls, slurries of controlled ph, and varied atmospheres. The Department of Energy, Mines and Resources in Ottawa, Canada is using a similar technique for an oxidation-eorrosion study (20). The Research Laboratory of Climax Molybdenum Company uses a rubber wheel abrasion test apparatus to determine the resistance of materials to low-stress abrasion (12) and the Department of Energy, Mines and Resources in Ottawa, Canada has used a similar type of test to study the combined erosion-corrosion properties of materials (21). High-stress abrasion testing, to evaluate ball, rod and liner wear, is varied. For some time, the only way a comparative test could be run would be to completely change to new liners, or to charge new rods or balls (18) into a mill gradually until the complete charge was made up of the test material. Not only being very time consuming and expensive, milling conditions, such as ore changes, could vary during this period masking test results. Bard (3) reports that as a rule of thumb, using wear theory, three times the weight of the ball charge must be charged before a mill can be claimed to be purged of the old grinding balls. For rods, it would be twice the weight of the charge. Some milling operations will charge one type grinding ball into one mill and another type ball into a companion mill in order to test a new grinding ball. Nordquist and Moeller (31), in a study to

16 determine the relative wear rates of various diameter grinding balls, showed that this type of testing may result in misleading information, when, in their test work, the grinding balls in one mill began wearing appreciably faster than those in a second mill. Some short term tests have been used in order to more quickly and economically evaluate ball, rod and liner ttear. Hull et al. (23) have reported a rod test where a few rods of many different types were charged into an operating mill with the standard rods used. In this test, one end of each test rod was drilled and tapped for standard pipe plugs. Rod identification was stamped on each plug. Wear rates and costs were evaluated. Nordquist and Moeller (31) mention a test where the relative wear rates of different sized forged steel grinding balls were studied. In this test, marking consisted of drilling a small hole into the center of the ball, dropping in an identification disk and plugging the hole with a low melting point alloy which could be melted out in boiling water. These balls were then tested in production ball mills which were using cast balls as the standard media. Norman and Loeb (34) and others have done extensive testing where the balls tested were notched and introduced into a production mill for a short period of time. Charging of the test balls was done at such a time that they could.be recovered during a normal mill maintenance down time. Gilbert and Wingham (19) report a test used to evaluate grinding balls where the balls tested were larger than the standard

17 7 grinding balls used in the production mill, Identification of the test balls was made from size. Although the wear rates of the larger balls should be slightly greater than the standard balls, the wear rates of different types of larger balls should be relative. Climax Molybdenum Company uses a five inch diameter grinding ball, tested in their production mills with the standard three inch diameter balls, to evaluate potential liner alloy material. The five inch diameter ball was chosen so that cooling rates and heat-treatment of the test balls would develop about the same structure^ as in liner sections (33). Batch tests have been performed on smaller diameter grinding balls using jar mills (25). Often, in these tests, wear rate results may not have an accurate correlation with production mill steel con^ sumption. The Research Department of the Cleveland Cliffs Iron Company uses a batch ball mill test for grinding ball wear rate evaluation. They claim to have found very good correlation between batch testing and actual production consumption data. Others (3, 36) have reported results of grinding ball wear rate tests in which groups of different balls to be tested were exposed to radiation at various levels and then charged into a production mill for testing. This test procedure allows for more wear than notched balls and the recovery of the irradiated test balls was around 98 percent (versus approximately 50 percent for notched ball tests). Handling and detection and identification equipment are more costly than

18 a notched ball test and care has to be taken to Insure that there are no psychological problems with the plant personnel. For gouging abrasion, Borik (12) and others are using modified laboratory jaw crushers. In this test, the relative weight loss of the stationary and movable wear plates is determined using a standard feed material to evaluate various crusher liner materials. A variation of this test is used (41) where the wear plates are of a standard material and various ores are tested. Stern (44) mentions an abrasion test applicable to hammer mills in which a laboratory mikro pulverizer is used to evaluate materials to be crushed. Bond (10) has done extensive laboratory abrasion testing using an apparatus originally set up by the Pennsylvania Crusher Division of Bath Iron Works Corporation (13). This test device was used mainly to determine the relative abrasiveness of various ores but can be used to test the relative abrasive resistance of metals. From his extensive test results with various ores, Bond derived a series of empirical equations which could be used to predict rod, grinding ball, and mill and crusher liner wear. When derived, the formulas did not include Ni-Hard applications, since his testing data did not include enough tests to correlate this material. In general, abrasion testing of grinding media, either in the laboratory or in an actual plant, show test results which are quantitatively comparable. These tests do not give results from which an. accurate prediction of metal consumption can be made.

19 Types of Grinding Media Domestic grinding rods presently in use are either a high carbon steel, usually 1090, 1095 or a modification thereof, or an alloy steel. High carbon rods are hot rolled, with square cut ends and are machine straightened with surface hardnesses in the 240 to 285 BEN range. Alloy grinding rods (manufacturers will not publish the alloying elements) have some additional alloying elements and are also heat treated for higher hardnesses which may be in the 510 to 545 BEN range on the surface. Alloy rods may be chosen for use in situations where their lower wear rate overcomes their increased cost. Cast iron grinding balls are cast from scrap usually by foundries located in or near mining regions or by some mining companies for their own use. These balls are often inferior in wear performance to any other type grinding ball. However, the cost can be low enough to permit their use. Ni-Hard, a nickel-chromium white cast iron grinding ball, is made under license from the International Nickel Company. Although used in some wet grinding applications, Ni-Hard balls are mainly used in dry grinding (16). These grinding balls tend to be somewhat brittle and the smaller diameter grinding balls are mainly used. A deterent for their use in the cement industry would be the non-smooth surface of this ball, which could reduce mill capacity. Costs of these grinding balls are near the prices of forged steel grinding balls. Magotteaux Corporation produces high chromium cast grinding balls which are particularly favorable, cost-wise, in dry grinding

20 applications. While the cost of these balls may be up to three times as much as.forged grinding balls, they may last about seven times as long depending on the dry grinding application. In wet grinding applications the cost of these grinding balls per on of ore may be higher than other types of grinding media (26). Cast steel balls have low amounts of alloy elements added to them and are comparable in cost and wear in most applications to forged steel balls, and are an improvement over conventional cast iron balls. Forged steel grinding balls are distinguished from cast balls by several factors. under impact (33). They are less brittle and less subject to breakage They are more perfectly spherical and are homogeneous, with no casting holes or cavities. They are also usually more uniform in hardness and, therefore, have a tendency to maintain their shape as wear reduces their diameter. The alloy forged-steel grinding balls contain small amounts of alloying 6laments which aid in depth hardening and increase wear resistance (24, 47). In the past, instead of using the smaller sizes of balls cyl- pebs or grinding slugs were used. These were made by cutting a round bar into short pieces. Although this practice is almost obsolete, one company has reported (23),.that they have cut rod scrap into slugs and as high as 15 percent slugs were substituted for grinding balls without production loss. Alumina, 9eramic, glass or other types of balls are used for specialty grinding such as in the paint manufacturing industry.

21 11 Because of their low specific gravity the efficiency ig low but iron contamination is eliminated. Flint pebbles are also used for this purpose (37). The cost of steel used in ball or rod mills can often equal or exceed the power cost. To reduce or eliminate this cost, rock media of suitable size is prepared in the crushing circuit, then fed to the mill continuously with the material to be ground. This type of grinding is called pebble milling. Grindability Testing Grindability testing is often done on a material in order to try to assess the properties of a material so that judgment can be made as to the size of the size-reduction equipment and the power conj. sumption required for a given throughput and product size. The Hardgrove grindability test is most widely used in the coal industry for this purpose. The grindability indeg, as determined from the Hardgrove,test, is a measure of the ease with which the coal can be ground, i.e., the softer the coal, the higher the grindability index. The machine used for this test as well as the test procedure is described in ASTM Standard D This test takes about one hour to complete. The Hardgrove test does not show the effect of excess surface moisture on a circulating load as might be noticed in a full scale plant. Test results vary widely with inherent moisture in the coal. Results may also vary as the relative humidity of the laboratory atmosphere varies or as the ash content of tha coal varies (22).

22 12 The most widely used method of grindability testing in the minerals industry is the Bond grindability test. This method of measuring grindability is discussed under EXPERIMENTAL WORK. Each Bond grindability test can take a trained technician about eight hours or longer to complete (43). Bond has proposed an empirical formula (8) by which an equivalent wet-grinding work index can be determined from the Hardgfove grindability rating, Hughes (22) ran both types of tests on sixteen materials and noticed test result variations. He concluded that the variations occurring between the two test types were probably due to the difference in breakage behavior in the two mills, since the Hardgrove mill is a guided track grinding process and the ball mill is a ffee-path grinding process. A modified Hardgrove mill, based on a German design, was tested at the Warren Spring Laboratory in England. This was developed in order to try to reduce testing time. Test results from this machine correlated very closely with the Bond ball mill test results only on homogeneous materials. As was stated previously, the Bond grindability test can take eight or more hours to run. Some operating mills, however, wish to use a grindability test for control purposes and such a test should be performed in as short a time as possible. Smith and Lee (43) ran a series of tests on eight different materials where standard Bopd grindability tests were performed on each material grinding at various mill revolutions. In each test, the same mill, mill speed, ball charge

23 13 and feed volume were used. The results of these tests showed it would be possible to estimate Bond grindabilitias from batch grindabilities if Bond and batch tests are first run on a material to establish the relationship between the two type- of grindabilities. Then one could run a short term batch test for plant control purposes. It has been learned that some companies from the copperbelt of Zambia use a batch grinding test that is said to give good results on minerallogically similar ores (5). Here, a batch grinding test gives a certain size distribution in 20 minutes on an ore known to produce close to this same distribution for an energy input of ten kilowatt hours per ton. Then for a similar size distribution on an unknown ore that takes 15 minutes to batch grind, the kilowatt hours per ton could be assumed to be 7.5. Using Grindability Test Results What happens after a grindability test, or a series of tests, has been run on a material? How is this number, that has been obtained, used to project power consumption, mill size, media size, etc? Certainly, at this time, a decision will already have been made regarding daily tonnage expected, product size required for optimum economics and perhaps the type of grinding to be used - autogeneous, semiautogeneous, tube, ball or rod-ball milling. When power calculations are derived from a work index, these must be correbted, where applicable, for dry grinding, open circuit ball milling, mill diameter, oversized or scalped feed, transmission,

24 14 loss (4), extra fine grinding and, especially in rod mills too large or tod small a reduction ratio. 1 Depending upon the hardness of the material, rod mills are usually able to accept feed sizes up to two inches and produce a product with maximum sizes in the range of 4 mesh to 35 mesh. Due to the nature of the action of the grinding rods in the mill, the largest particle sizes tend to be preferentially ground (14, 29, 45). In milling applications, grinding in a rod mill is usually done wet in open circuit, and the rod mill is used to reduce a crusher product to a size suitable for ball-mill feed. A grinding mill is operated most efficiently when the power drawn is at a maximum for the given mill. This is at a point where the grinding charge, including void volume, occupies around 45 to 50 percent of the mill volume (30). %n rod mills, the grinding charge is swollen by particles of feed which septarate the rods (6), and due to this, the rod mill is usually operated with a charge of 35 to 40 percent by volume (rods plus void volume). Wet grinding rod mills generally operate more efficiently when fines are left in the mill feed than when scalped (39). A correction factor for power requirements is used when the reduction ratio in a rod mill is above 20 or less than 12. Occasionally rod mills will be used for dry grinding. In this case, the recommended rod charges are 30 to 35 percent of the volume (39). Tests have also shown that scalping, removing fines from the rod mill/feed, seems to imprpye dry rod milling capabilities. Often, closed circuit grinding is used for this application.

25 15 In general the rod-mill length to diameter ratios vary from 1.2:1 to 1.6:1. The proper diameter of make-up steel grinding rods can be calculated (6) when the feed size, work, index, specific gravity of the ore, mill diameter and percent mill critical speed (usually between 60 to 68 percent) are known. When the reduction ratio of the ore is less than eight, the rod make-up size should be increased by 1/2 inch (8). Manufacturers of grinding mills are able to supply customers with the rod size distribution requirements for a start-up charge. Maximum rod lengths presently being used in the United States are 19.5 feet. In plant design, there is a choice of using many small grinding mills or a lesser number of larger mills. It can be shown that the cost of building a grinding operation of a given installed horsepower decreases as the size of the mills used increases (30). The Bond work index is based on the grinding power requirements for an eight foot diameter mill (8) and a correction factor for power requirements is used when the mill diameter varies from eight foot. This correction factor shows that the power requirements per ton of ore ground decreases as the mill diameter increases. Studies have indicated that this gain in.grinding efficiency with increasing mill diameter does not continue for mills having a diameter greater than 12.5 feet inside liners (40). Bond's grindability test procedures (7) requires that a natural feed size distribution be used. Should a scalped feed, one where the fines have been removed, be used the theoretical relationship

26 16 between feed size and product size is altered, and a correction for power requirements must be made. When grinding to a product size finer than 200 mesh, it has been found that additional energy, above the predicted amount, is required. In dry grinding, the balls may become coated with the firie particles thereby cushioning the metal contacts and decreasing grinding efficiency (2), Grinding aids are often used to reduce ball coating (42). Wet grinding to a fine size can change the pulp viscosity causing ball cushioning and decreasing grinding efficiency. In many cases the ball sizes used are too large to efficiently grind to the finer sizes (9). One author (35) has stated that a rod-ball compartment mill may help in saving power and reducing metal consumption. A drawback to this type of mill is that a compromise has to be made concerning power and operating percent critical speed, In dry grinding applications these rod-ball mills are normally operated at 68 to 70 percent critical speed whereas dry rod mills normally run 62 to 65 percent critical speed and dry ball mills operate around 75 percent critical speed for best efficiency.

27 EXPERIMENTAL WORK General Discussion The objective of this- study, was to determine the effect of hardness bn an abrasion index as defined by Bond (10). For this study, special test coupons had to be obtained. In order to differentiate between the standard test coupons and the non-standard test coupons used, the tests using the non-standard test coupons will be called special abrasion tests. Materials The test coupons used for the special abrasion tests were obtained from the Colorado Springs Machine Corporation in Colorado Springs, Colorado. In order to obtain the hardness levels required for this study, the company was to use M-2 steel, a high speed tool steel, for the highest two hardnesses requested and 0-1 steel, an oil.hardening steel, for the remainder, They were, however, unable to 'heat treat the 0-1 steel to the low.hardness of Rockwell C 27 as requested and used instead a resulphurised free machining steel,. Table 1 shows the hardnesses of the test coupons as ordered and as received. For the ball and rod mill grindability testing and the stani. dard abrasion testing, a porphyry copper ore was used, which was 17

28 18 Table 1. Test Coupon Hardness (Rockwell C) As Ordered Manufacturer's Test Author's Test ± and 32* *Non uniform heat treatment showed one surface as Rc25 and the other as Rc32.

29 19 obtained from the Magma Copper Company, San Manuel, Arizona. The ore sample was rod mill feed which was obtained from the mill circuit during March, For the special abrasion testing, a porphyry copper ore and quartz were used. The porphyry copper ore was again obtained from the Magma Copper Company, San Manuel, Arizona. This material was secondary crusher feed and was taken from the crusher circuit during June, The massive quartz was obtained from Colonna and Company of Colorado, Incorporated in Canon City, Colorado and was received as Flamingo Quartz Size D, which is minus 1-3/8 inches plus 7/8 inches. Sample Preparation For special abrasion tests, the porphyry copper ore and the quartz samples were both prepared in the same manner. After drying, the raw material sample was stage crushed to minus 3/4 inches, using a 4 inch by 6 inch Massco laboratory jaw crusher and Sweco screens. The minus 3/4 inch plus 1/2 inch size fraction was separated, blended and split into portions of approximately 1600 grams each. Prior to being tested, each portion was placed on a 1/2 inch Tyler screen to remove any undersize and then split into four portions of 400 grams each. For the standard abrasion tests, and the ball and rod mill grindability'tests, the rod mill feed was first dried at 200 F for 24

30 20 bourg Using the Sweco screens, the plus 3/4 inch size fraction was separated and stage crushed to minus 3/4 inch. All material was then coned and quartered Using the Sweco screens and laboratory - crpsher, quarter A was stage crushed to minus 6 mesh. This was then blended and one half was split into portions of approximately 450 grams each for ball- mill grindabhity testing. The remaining material was retained in the event of future need. Quarter C was stage crushed to minus 1/2 inch using the laboratory ja*? crusher and Sweco screens. After blending, one half of this material was split into portions of approximately 1^00 grams each for rod-mill gr inability testing, the remaining material being retained in the event of future need. The minus 3/4 inch plus 1/2 inch size fraction was screened out of quarters B and D, This was split into 400 gram portions for standard abrasion testing. Abrasion Test Procedure Standard abrasion test coupons consist of a 3 x 1 x 1/4 inch piece of AISI 4340 steel hardened to 500 Brine11 (Rc51). Originally, SAE 4325 steel was used but it is no longer as readily available. The test coupon is inserted for one inch into a 4.5 inch diameter rotor, which rotates on a horizontal shaft at 632 rpm through falling ore particles. Two square inches of coupon surface are exposed to abrasion, and the uoupon tip, with a radius of 4.25 inches, has a linear speed of 1,410 fpm. _ '. J'":

31 21 The rotor is enclosed by a concentric drum 12 inches in diameter and 4.5 inches deep, which rotates at 70 rpm, 90 percent of critical speed, in the same direction as the coupon. The inner circumference of the drum is lined with expanded metal to furnish a rough surface for continuously elevating the ore particles and showering them through the path of the rotating paddle. Initially, the coupon is polished to remove any scale or decarburization, cleaned and weighed on a semi-micro analytical balance, before being inserted into the rotor. Screened ore particles minus 3/4 inch plus 1/2 inch in size are used as test feed. Four hundred grams of 3/4 by 1/2 inch feed are placed in the drum, the end cover is attached, and abrasion is continued for 15 minutes; then the drum is emptied, another 400 grams of feed are added, and the abrasion continued. In each complete test, four 400- gram samples are each abraded for 15 minutes. The coupon, after being abraded for one hour, is removed, cleaned to remove dust particles and weighed. The loss of weight in grams is the abrasion index, Ai, of the material. The exposed outer corners of the coupon are visibly worn after a single hour test but the coupon can be reversed for a second test and it is then discarded. The abraded material is combined and its size, distribution is measuredi, - The test unit, Figure 1,. can also be used to evaluate the abrasion resistance'of different metals. Coupons of the same size can be made of any desired metal and given the desired heat treatment.

32 Fig. 1. Photograph of abrasion test machine. N> N3

33 23 The abrasion loss can be compared with that of the standard coupon, using a standard rock material. In this experimental program, quartz was used as the standard raw material. The abrasion index obtained from the standard test can be used to estimate metal rod, ball or liner wear in crushing and grinding operations. Any correlation between the work index and abrasion index is very slight (10, 25). - For all tests run, the procedure was to use the copper ore on side 1 of the first coupon tested and quartz on side 2. Then the quartz was used on side 1 of the second coupon tested and the copper ore on side 2. The alternating of the copper ore and quartz was continued throughout the testing. Rod Mill Grindability Test Procedure For each test that was run, approximately 45 pounds of raw material was stage crushed to minus 1/2 inch, using a 4 inch by 6 inch Masseo laboratory jaw crusher and Sweco screens. This crushed material was then blended and split into sixteen portions of approximately 1,275 grams each for grindability testing. Usually, one portion is used to determine the particle size distribution of the crushed test feed. In this study, since ten tests were to be run and since the crushing and splitting was done for all tests at the same time, triplicate screen analyses were made and the results were averaged. " -

34 24 A 1,250 cubic centimeter portion of feed material was packed into a graduated 2,000 cubic centimeter cylinder, using a Syntron model DLlA vibrator, and was then weighed. This material was then dry ground, in closed circuit, in a 12-inch diameter by 24-inch long tilting rod mill, Figure 2, running at 46 rpm, with a wave-type lining and revolution counter. The grinding charge consists of six 1.25 inch diameter and two 1.75 inch diameter steel rods, 21 inches long, and weighing 33,380 grams. During grinding, in order to equalize segregation at the mill ends, the mill was rotated level for eight revolutions, then tilted up five degrees for one revolution, down five degrees for another revolution, and returned to level for eight revolutions, continuously throughout each grinding period. Tests are made at all mesh sizes from 4 to 65 mesh. (A variation, of the way the make up feed is added for each cycle, is used by the Colorado School of Mines Research Institute and by the CF&I Steel Company Grinding Research Laboratory, for test sizes of TO or 14 mesh and coarser. This variation allows for test completion with accurate results in a much faster time.) At the end of each grinding period, the mill was discharged by tilting downward at 45 degrees for 30 revolutions, and the product was screened on sieves of the mesh size tested. For the 65 mesh test, the product was split into four portions for screening, using a 65 mesh sieve and a breaker sieve two sizes coarser, in this case a 35 mesh sieve. In all other product sizes, the product was split into two portions, using the breaker and mesh-size-tested sieves. The sieve undersize

35 Fig. 2. Photograph of rod mill grindability test machine. N5 Ln

36 26 i. was weighed and fresh unsegregated feed was added to the oversize to make its total weight equal to that of the 1,250 cubic centimeters originally charged into the mill, This new feed was returned to the mill and ground for the number of revolutions calculated to give a 100 percent circulating load. During testing, the net grams of sieve undersize produced tend to increase or decrease with each consecutive grinding period cycle. The grinding period cycles were continued until the net grams of sieve undersize produced per revolution reached equilibrium and reversed its direction of increase or decrease. A minimum of seven cycles was run for each test and when the net grams of sieve undersize produced per revolution of the last three cycles were within three percent (some test facilities use five percent) of one another, the test was called completed. The particle size distribution of the circulating load was determined. If this contained more than 5 percent undersize (some test facilities use other figures)--due possibly to screen blinding or too much material on one screen--the test was repeated, using more sieve nests to more closely separate the circulating load from the undersize product. The undersize product from the last three cycles was blended and quartered. One quarter was screen analysed to determine P, which is the size in micrometers of which 80 percent of the undersize product passes. The average of the last three net grams per revolution (Grp) is the rod mill grindability.

37 The rod mill work index, Wi, is calculated from the following equation (8):. Wi = 62 Where F is the particle size in micrometers which 80 percent of the new rod mill feed passes and Pi is the opening in micrometers of the sieve, size at which the test was made. Ball Mill Grindability Test Procedure For each ball mill grindability test, approximately 16 pounds of sample was stage crushed to minus 6 mesh (Tyler),, using a 4-inch by 6-inch, Massco laboratory jaw crusher, a 10-inch by 6-inch Denver laboratory roll crusher and Sweco screens. (Test results are not influenced by the use of the jaw crusher alone, the roll crusher alone or a combination use of these to crush the test feed material to minus 6 mesh.) This crushed material is then blended and split into sixteen portions of approximately 450 grams each, for grindability testing. Grindability tests can be run with a finer feed size when the reduction ratio, Rr, is greater than 6:1. One segment is screen analysed In this study, since eleven tests were to be run and since the crushing and splitting was done for all tests at the same time, a particle size distribution was determined bn each of three portions and the results were averaged... Test feed material, in the amount of 700 cubic centimeters, was packed in a 1,000 cubic centimeter graduated cylinder, using a

38 28 Syntron vibrator, and then weighed. This material was placed in a ball mill. Figure 3, and ground dry at a 250 percent, circulating load. The ball mill used was 12-inches by 12-inches with rounded corners, and a smooth lining, except for d 4-inch by 8-inch hand-hole door for charging. It has a revolution counter and the ball mill ran at 70 rpm. The grinding charge consisted of 285 iron balls weighing 20,125 grams. The grinding charge should consist of about inch balls, inch balls, 10 1-inch balls, inch balls, and inch balls, with a calculated surface area of 842 square inches. Due to presently available manufactured grinding ball sizes, this grinding charge consists of balls of nominal 1-1/2, 1-1/4, 1, 3/4 and 1/2 inch diameters. Ball mill grindability tests are normally made at sieve sizes below 28 mesh. After the first grinding period of 100 revolutions, or more or less depending upon what may be known about the material, the mill is dumped, the ball charge is screened from the material being tested and the 700 cubic centimeters of material is screened on sieves of the mesh size tested, with coarser protecting (breaker) sieves. Two nests, each consisting of a screen of the mesh size tested and a breaker sieve two mesh sizes coarser, are used for sieve tests, unless blinding is noted, then four bests would be used. Wet screening should be used when the undersize is 100 mesh or finer, in order to obtain more accurate test results. The undersize is weighed, and. fresh unsegregated feed is added to the oversize to bring its weight back to that of the original 700 cubic centimeter charge. Then it is

39 Fig. 3. Photograph of ball mill grindability test machine. ro VO

40 returned to the mill, ground for the number of revolutions calculated 30 to produce a 250 percent circulating load, dumped and rescreened. The number of revolutions required is calculated from the results of the previous period to produce sieve undersize equal to 1/3.5 of the total charge in the mill. The grinding period cycles are continued until the net grams of sieve undersize produced per mill revolution reaches equilibrium and reverses its direction of increase or decrease. A minimum of seven cycles are run for each test. Also, the net grams of sieve undersize produced per revolution of the last three cycles must be within three percent of one another (some test facilities use five percent). A particle size analysis is made on the circulating load. If this should contain more than three percent undersize (some test facilities have different limits), the test should be repeated using more sieve nests to more closely separate the circulating load from the undersize product. The undersize product from the last three cycles is blended and quartered. One quarter of this is screen analysed to determine P, the size in micrometers of which 80 percent of the undersize product passes. The average of the last three net grams per revolution (Gbp) is the ball mill grindability. The ball mill work index, Wi, is calculated from the following equation (8): Wi = 44.5

41 Where F is the siae in micrometers which 80 percent of the new ball mill test feed passes and Pi is the opening in micrometers of the sieve size at which the test was made.

42 EXPERIMENTAL RESULTS Special Abrasion Tests Results from the special abrasion tests were analyzed statist! cally to determine if all results were valid, at least consistent in a statistical sense. Two tests (quartz 59^1 and quartz 48-1) were found not to fit the remaining data and these tests were repeated. An analysis of variance was then made on the test data using a time sharing computer. The analysis of variance test results showed that there was a very high significance in testing between types of ore and a high significance between abrasion index and test coupon hardness. This computer run, with the results, is shown in Appendix A Tables 2 and 3 show the test result of the special abrasion tests and Table 4 shows the chemical analyses of these test coupons. One coupon from each hardness was analyzed. Figure 4 shows the predicted metal consumption for rods and balls as computed from the theoretical Bond abrasion index. The abrasion index using standard test coupons is also listed iri Tables 2 and 3. This is coded as tests 5B, 7A, and 7B for the quartz and as tests 13B, 14A and 14B for the San ' Manuel ore. Grindabillty Tests Table 5 lists the data obtained from the Bond ball mill grindability tests which were run at product sizes of 14 through 325 mesh; and Table 6 lists the same data for the rod mill grindabillty tests y.-, ' ' - ' ' ^. 32; ; y

43 33 Table 2. Special Abrasion Test Results (San Manuel Ore) Test No. Hardness BHN Rc Actual Hardness (Rc) Abrasion Index ± ± ± ±

44 34 Table 2. Continued Test No. Hardness BHN Rc Actual Hardness (Rc) Abrasion Index 13B A B St ± (A) (A) Combined with Rc30 for ANOVA computations.

45 35 Table 3. Special Abrasion Test Results (Quartz) Test No. Hardness BHN Rc Actual Hardness (Rc) Abrasion Index ± (A) ±

46 36 Table 3. Continued Test No. Hardness BEN Rc Actual Hardness (Rc) Abrasion Index ± B A B ± (A) ±

47 Table 3. Continued Test Hardness Actual Abrasion No. BHN Rc Hardness (Rc) Index (B) (A) Discarded--out of range--f test (48) (B) Combined with Rc30 for ANOVA computations

48 Table 4. Chemical Analysis Results of Test Coupons Percent c Mn P S Si Cu Cr Ni Mo V A

49 METAL CONSUMPTION (POUNDS/KWHR) u> KJ U l o GRINDING RODS GRINDING BALLS BOND ABRASION INDEX (Ai) Fig. 4. Predicted metal consumption from the Bond abrasion index. w vo

50 40 Table 5. Ball Mill Grindability Tests - Test Data Grind Size Test Tyler FgO PgO No. Mesh Micrometers Micrometers Micrometers Gb Wi * * * * * * wet F80 is the 80 percent passing size of the test feed. PgO is the 80 percent passing size of the test product. Gbp is the grams of product produced per mill revolution - i.e., the grindability. Wi is the work index.

51 41 Table 6. Rod Mill Grindability Tests - Test Data Grind Size Test Tyler FRO P80 No. Mesh Micrometers Micrometers Micrometers Grp Wi Fgo is the 80 percent passing size of the test feed. PRO is the 80 percent passing size of the test product. Grp is the grams of product produced per mill revolution i.e., the grindability. Wi is the work index.

52 42 run at product sizes of 4 through 65 mesh. The rod mill grindability test at 65 mesh was repeated using four nests of screens during testing since the original test showed a final oversize product containing eight percent undersize. The repeated test is the only one reported at this size. Appendix B contains the standard abrasion test results for the San Manuel ore. This same test procedure was used for all other abra-, sion tests. Examples of the individual rod and ball mill grindability test data sheets are included in Appendices C and D respectively.

53 DISCUSSION OF RESULTS Abrasion Tests The actual hardness of a grinding ball or rod usually varies from center to surface. A quantitative index of heat treated hardness for any ball size is expressed as average volumetric hardness (AVH)3 based on internal hardness values along a transverse diameter according to the equation (47). AVH = Hc H1/ H3/ HS The coefficients weight the hardness contribution of each radial position as indicated by the subscripts so that proportional volumes are represented. One manufacturer presently produces 3-inch diameter forged carbon steel grinding balls which have a published average volumetric hardness of 397 BHN and 3-inch diameter forged alloy grinding balls which have a published average volumetric hardness of 660 BHN. The carbon steel grinding balls had been used in the past to grind a certain ore; presently the harder alloy balls are being used. Standard Bond abrasion tests were run on this ore and the predicted grinding ball consumption from these tests was very close to the actual plant consumption figures using the standard carbon steel grinding balls. The alloy balls presently being used have a significantly lower consumption rate than would be predicted from laboratory tests. 43

54 44 Based on the experimental data obtained during this invest!^ gation, Figure 5 shows that the abrasion index decreases as hatduess increases. In other words9 the amount of grinding media that would be consumed in an operating mill would decrease as the hardness of the grinding media increases. This fact, along with the aforementioned plant results, indicates that a correction factor should be incorporate ed with the empirical equations used to project metal wear. This correction factor would adjust for the influence of the hardness of the grinding media. From the data shown in Figure 5, it can be seen that the abrasion index varies with hardness. It can also be seen that the hardness versus abrasion index curve differs with different ores. At the lower hardnesses it can be seen that the different materials tested generated similar curves. Perhaps a small amount of gouging abrasion occurred. However, at the higher hardness values the curves were somewhat different due possibly to varying degrees of surface work hardening from the different ores. The results of the standard abrasion tests are included as single data points. These do and should differ from the special test results at this same hardness due to the difference in alloy composition of the test coupons. Also from Figure 5, compare the abrasion index for the San Manuel ore at two hardness levels. This company presently uses makeup grinding balls of 50 percent 1.5 inch diameter balls and 50 percent 2 inch diameter grinding balls. The approximate average volumetric hardness for carbon grinding balls of these sizes is 620 BEN and for

55 ABRASION INDEX o w /\ Quartz o tv) Q San Manuel Ore HI Standard Test Coupons BRINELL HARDNESS NUMBER Fig. 5. Relationship between abrasion index and brinell hardness. ^Ui

56 alloy grinding balls of these same sizes is around 680 to 695 BHEL The corresponding abrasion indices for these hardnesses with this ore from Figure 5 are approximately 0.19 and 0.15 respectively. The difference between these two indices is approximately 27 percent. Actual plant data (17) has shown that, when carbon and alloy balls are used in companion grinding mills of equal size and operating speed, the metal consumption between carbon and alloy grinding balls differs by approximately 29 percent. It would seem therefore, that for forged grinding balls, grinding media consumption as based on media hardness is the difference between the abrasion indices at the different hardnesses. The manufacturer of the cast alloy grinding ball used at this same plant states that the average volumetric hardness of the cast ball is as hard as or harder than the forged alloy grinding ball. Fl,ant data show that grinding media consumption is slightly higher for the forged ball. There are many factors which could influence this, however, laboratory abrasion testing should be done using test coupons made of the cast material before any evaluation could be made as to why there is a difference in media consumption between the two types of grinding balls. For the volumetricaily softer grinding rods, the plant data from San Manuel show that the grinding consumption for alloy rods is 20 to 24 percent lower than for carbon steel rods. Using the data in Figure 5, and the average volumetric hardness for carbon and alloy grinding rods the difference between the abrasion indices is

57 47 approximately 19 percent. This difference based on laboratory results also is comparable with plant data from the San Manuel operation. This same plant uses two different sized ball mills which differ in power consumption by over 13 percent. The difference in alloy grinding media consumption between these ball mills, is approximately 15 percent, thus showing that grinding media consumption is directly related to power as stated by Bond (10), One author (15) has stated that steel consumption should decrease with larger diameter mills. These larger mills may be able to reduce slippage and attrition grinding and more closely approach an impact, crushing and nipping action. This is not the case at San Manuel, since the plant operating data shows a higher media steel consumption in both the larger rod and ball mills than in the smaller diameter mills. From the plant data on power consumption that are available, it is not possible to relate the abrasion index to plant grinding media consumption data as was possible with the previously mentioned grinding operation which used the 3-inch diameter grinding balls. Since it has been, shown that grinding media consumption varies with media hardness as the abrasion index varies with hardness, this relationship can be used to predict grinding media performance. Let us assume that a company is planning to open, a new property that will use grinding balls or has an existing property using grinding balls of a certain hardness. A series of abrasion tests, with test coupons of different hardnesses, would be run on the given ore since each ore

58 48 will generate a different curve for abrasion index versus hardness. The data would be evaluated to determine if it would be more economical to use a carbon steel grinding ball or a harder alloy grinding ball. - From the results of this study, it appears that grinding media consumption varies proportionately with iqedia hardness. This relationship appears to be linear, however, more tests would have to be made before this linearity could be positively established,- Figure 5. Assuming the relationship is linear, the grinding media consumption for a given ore could be more accurately calculated. A standard Bon4 abrasion test would be run and the abrasion index, Ai, would be determined. If needed, a second abrasion test would be run using a test coupon of a hardness level equivalent to the average volumetric hardness of the grinding media to be used and the special abrasion index,?ai, would be determined. The grinding media consumption would then be calculated from the Bond empirical formulas (10), using Ai, and this would be then multiplied by the ratio between SAi and Ai. Grindability Tests Tables 7 and 8 show the predicted power consumption for ball and rod mills used at San Manuel for various product sizes based on the results from this investigation. The feed sizes to the ball and rod mills used in these calculations were obtained from actual plant data (35). The predicted power consumption is the power required at the pinionshaft of the mill which includes mill bearing and gear and

59 49 Table 7. Predicted Power Consumption - Ball Mill Grindability Tests Tyler Mesh Bond Work Index Work Input (Kwhr/ton) (A) Work Input (Kwhr/ton) (B) (C) (D) (D) (C) 2.382(D) 2.713(D) (E) (E) (E) (A) Corrected for 10 foot diameter mill. ID equals 9 1 < liners. FgO = 1,270 micrometers (17). (B) Corrected for 12-1/2 foot diameter mill. ID equals inside liners. f80 = 1,200 micrometers (17). (C) (D) (E) Correction for increased power usage when reduction ratio less than 3 - out of range of experience. Corrected for increased power usage when reduction ratio less than 3. Corrected for increased power usage due to fine grinding.

60 50 Table 8. Predicted Power Consumption - Rod Mill Grindability Tests Tyler Mesh Bond Work Index Work Input (Kwhr/ton)(A)(B) Work Input (Kwhr/ton)(A)(C) (D) (A) Fro = 12,600 micrometers (17). (B) Corrected for 10 foot diameter mill. ID equals 9' 3" inside liners. (C) Corrected for 12-1/2 foot diameter mill. ID equals 11' 9" inside liners. (D) Second test at this size.

61 51 pinion losses but does not include motor losses or losses in any other drive component such as reducers and clutches (38). Bond (6) and others have concluded that rod mills are sensitive to unfavorable reduction ratios and if this ratio is smaller than about 12 or larger than about 20, the power required for grinding increases. From the San Manuel operating data, the presently used reduction ratio for the rod mills is around 10. No correction, however, was made in the rod mill power requirement calculations for an unfavorable reduction ratio. For grindability testing it is recommended that the test feed size should be six times greater than the product size (11). Normally the feed size for rod mill grindability testing is minus 1/2 inch by down and the feed size for ball mill grindability testing is minus six mesh by down. Finer feed sizes can be used when the reduction ratio is greater than 6:1 (40). Grindability tests are usually not run at sizes coarser than 10 mesh for the rod mill test and 28 mesh for the ball mill test since the reduction ratio would be less than 6:1. Power calculations based on the results of grindability tests at these coarser sizes may lack gome degree of accuracy. 1 The work index for homogeneous ores should remain constant over all size ranges (11), In heterogeneous materials the work index increases or decreases as the product size becomes smaller (1). When materials have natural grain sizes or incipient cracks, the work index tends to increase with finer grinding as the natural grain size is approached, remains constant at meshes where this natural grain size

62 " " ' 52 persists, and usually decreases at meshes finer than the natural grain size. Cushioning by fines qau increase the work index, as can be seen in Figure 6 at the 270 and 325 mesh sizes. When the 80 percent passing size, P, is less than 70 micrometers, the work index is corrected by multiplying by an empirical factor (8). Bond (7) has listed ball m$ll gtipdahilicy test results which were performed on 15 different ores at.sizes from 28 through 100 mesh. Most of his results show fhe work index to be fairly constant oyer the range of sizes tested or to increase es the test size becomes finer, The test results from this investigation. Figure 6,- show that the work index decreases generally as the product test size becomes finer and then increases due to cushioning at the finest sizes. This same decrease of work index at finer product sizes was noticed also in test results from grindability tests run oh other ores (28), Figures 7 and 8 show the grindabiiity results at various sizes;1, grindability being the number of grams produced per revolution of the test mill. Published data (7, 28, 43) have shown this decrease in grindabiiity to be at different rates for different ores. Grindabiiity has been broadly defined (46) as the resistance of a material to fine comminution. The work index is the comminution parameter which expresses the resistance of a material to crushing and grinding and which is defined (7) as the kilowatt-hours required to reduce a ton of material from theoretically ihfinite size to 80 percent passing 100 micrometers. When, referring to work index, the grindabiiity is then directly defined as the grams of product produced

63 MICROMETERS I 1 ID u> -O In ID UJ 4> in <J\ 00 VOO in O' 00 \D O o o o o o o o o O o o o o o o o o o Q Rod Mill Tests A Ball Mill Test WORK INDEX TYLER MESH ASTM Fig. 6. Relationship between mesh size and work index for rod and ball mill grindability tests. In U>

64 54 MICROMETERS o O o o o O o o o o o o o o O o o o O o o o o o o o CM <t m < > r>. coos o o o o o t-l CM cn <r m PER REVOLUTION 10 o TYLER MESH ASTM Fig. 7. Relationship between mesh size and grams produced per test revolution - rod mill.

65 55 o m o o o o vo oo o\ o MICROMETERS o o o o oco o>* om o o o o o CN <o cxd on o PER REVOLUTION o ocn o m o o m so o CN CN TYLER MESH ASTM Fig. 8. Relationship between mesh size and grams produced per test revolution - ball mill.

66 56 per mill revolution. Therefore, when one talks about the grindability of an ore, it should be made exactly clear, what is meant by this term. Some people may become confused when they notice that the work index of a material decreases with decreasing product size. Although the work index value itself may increase or decrease with decreasing product size, the power required to grind to a specific size, as calculated from the work index, increases as the product size becomes finer as shown in Figure 9, It has been stated (40) that a rod mill requires less power per ton of ore ground for its range of work than a ball mill. This statement would hold true by comparing the predicted power requirements in Tables 7 and 8, using a ball mill product of between 35 and 48 mesh for the 12,5 foot diameter mills and slightly finer than 48 mesh for the 10 foot diameter mills. Actual plant operating data (17) only lists the power used on the grinding floor, in kilowatt hours per ton of ore milled. Using averages of mill operating hours, kilowatt hours used, horsepower and tons of ore, then the power used per ton of ore milled can be calculated for each size rod and ball mill. When these calculations are made however, the individual ball mills are seen to use less power per ton of ore than the rod mills. Since each rod mill feeds two ball mills, if the power from both mills is taken into account, then these two mills combined use more power per ton of ore ground than does the single rod mill.

67 57 /\ Work Index O Work Input WORK INPUT (KILOWATT HOURS PER TON) WORK INDEX (Wi) TYLER MESH ASTM Fig. 9. Relationship between work index and power requirements at varying product sizes for a 10 foot diameter ball mill.

68 SUGGESTIONS FOR FUTURE WORK Since only the hardness variable of the abrasion test coupon has been studied in this work, further studies would require the use of test coupons of different chemical compositions and heat treatments. A study has been started at the CF&I Steel Corporation Grinding Research Laboratory using abrasion test coupons cut from rounds,of the same chemical composition as used for the manufacture of forged 3 to 4 inch diameter grinding balls. varying degrees of hardness. These coupons have been heat treated to The test materials to be used will be quartz as a standard, a porphyry copper ore (the same as used for this study) and an ore from the operations of the Climax Mine of Climax Molybdenum Company. The Climax ore will be used to determine if there is a sufficient correlation between the Bond abrasion test results and the test results of Norman and Loeb (34). Should there be a good correlation or should a proper correction factor be derived, further testing will be done with the cooperation of the Climax Molybdenum Company, using test coupons of mill liner materials. The objective of this test work will be to try to obtain a faster method for evaluating potential mill liner material. Test results will be compared against past test work (32). Future grindability tests on the porphyry copper ore used in this study need only be made if there is a drastic change in the composition of the ore body which, at this period of time, is unlikely. ^... s s -

69 APPENDIX A ANALYSIS OF VARIANCE COMPUTER RUN ( 59

70 60 Tymshare System S Line 4 Node 1065 Ready, System J03 July 29, :14 Last Log In July 22 19:34 -Math Mathematical Programs +++A0V Computer Complex s Analysis of Variance Ver Input from terminal? Yes Save Input Data? Enter File Name: /ABRHN/ New File? Enter Number of Data Matrix Rows: 24 Enter Number of Data Matrix Columns: 3 Enter Parameters In Response To The Matrix Row Number , , , , , , , , , , , , , , , ,

71 , , , , , , , , , , , ID.35878, , , , , , , , , , , , , , , , , , , , , :he type of design: Two of levels for factor A : 2 of levels for factor B : 12 Two factor factorial design Source of variation Sums of Squares Degrees Freedom Mean Squares Variance Ratio Total about origin Total about mean Due to Replicates

72 62 Due to Treatments Factor A Factor B A&B Analytical Error Correction Factor = Fixed Effect Model Replication Factor A Factor B Interactioi Standard Error Mean Mean Mean Mean Mean Mean Mean Mean Mean Mean Mean Mean Means for Interaction (A&B) B 1 B 2 A

73 A H- EXIT - LOG TIME USED CPU: 8 SEC CON: 0.10 HOURS

74 APPENDIX B EXAMPLE OF ABRASION TEST DATA SHEETS 64

75 65 Average of Three Abrasion Tests Standard Coupon--San Manuel Ore (Charge each test - 1,600 grams, -3/4" +1/2") Abrasion Index, g Ai = Formulas: ( A ^ 0.020) Wet Rod Mills: Wet Ball Mills (A): Rods = 0.35 (Ai-0.020)0, Liners = (Ai-0.015) Balls = 0.35 (Ai-0.015)0 '33 Liners = (Ai-0.015)0,30 Ib/kwhr Dry Ball Mills (B): Balls = 0.05 (Ai) Liners = (Ai)0* Crushers -- gyratory, jaw and cone types: Liners = (Ai+0.22)/II Roll Crushers: Roll = (Ai/10)0-67 Shells (A) Overflow and grate discharge. (B) Grate discharge (AjX 0.22).

76 6 6 Screen Analysis of Abrasion Test Product (Average of Three) Screen Product ASTM Ell-70 (Tyler) Mesh Weight Percent Cumulative Direct Passing -3/4 " + 1/2 " /2» +3/8 " /8 "

77 67 28, " 20,000 3/4' 15,000 10, ,000 1/2 3/8' 4,000 3,000 M ic ro m e te rs 2,000 1,500 1, Tyler Mesh ASTM o o o o o o O' oo r- vo m s o co o CM W eight Percent OCT' co r~» Passing in ro

78 APPENDIX C EXAMPLE OF ROD MILL GRINDABILITY TEST DATA SHEETS 6 8

79 69 Screen Analysis of Rod Test Feed (Dry) (Average of Three) Screen Product Weight Percent ASTM Ell-70 Cumulat: (Tyler) Mesh Direct Passii -1/2 " +3/8 " /8 " * * * * * * * 1.6 *wet

80 70 28, ' 20,000 3/4' 15,000 1/2 10, t OOO 6,000 5, /8' Micrometers Tyler Mesh ASTM o o o o o ov oo r- vo o m o m o <N W eight Percent o <r> oo vo in Passing cn m

81 71 Rod Mill Grindability Tests Test Number 2 Purpose: Sample: Procedure: To determine the rod mill grindability of the test sample in terms of a Bond work index number. San Manuel rod mill feed. The equipment and procedure duplicate the Bond method for determining rod mill work indices. Test Conditions: Mesh of grind: 14 Weight of undersize product for 100 percent circulating load: g Weight percent of undersize material in ball mill feed: 36.5 Results New Stage! Feed No. g In Feed 8 Undersize To Be Ground. 8 Revolutions Undersize In Product 8 Undersize Produced Per Mill Total Revolutio , , Average last three =

82 72 Rod Mill Grindability Test Number 2 (Continued) Work Index Computations Wi = (v23)bo-sb)(r$ 62. Where: = 100 Percent Passing Size of Product = 1,180 micrometers Grp = Grams per Revolution = P = 8 0 Percent Passing Size of Product = F = 8 0 Percent Passing Size of Feed = 900 micrometers 7,050 micrometers Wi = 13.83

83 73 Rod Mill Grindability Test Number 2 (Continued) Screen Analysis of Test Product Oversize (Dry) Screen Product ASTM Ell-70 (Tyler) Mesh Direct Weight Percent Cumulative Passing -1/2 " +3/8 " /8 " Undersize (Wet) Screen Product ASTM Ell-70 (Tyler) Mesh Direct Weight Percent Cumulative Passing * * * * * * * 12.5 * wet

84 28, ' 74 20,000 3/4' 15,000 1/2 10,000 3/8' 8,000 7,000, 6,000 5,000 4,000 3,000 Micrometers 2,000 1,500 1, <0( Tyler Mesh ASTM o o o o o o o o o o oo vo in m CM CM W eight Percent Passing

85 APPENDIX D EXAMPLE OP BALL MILL GRINDABXLITY TEST DATA SHEETS 75

86 76 Screen Analysis of Ball Test Feed (Dry) Average of Three Screen Product Weight Percent ASTM Ell-70 Cumulat: (Tyler) Mesh Direct Passii * * * * * * * 4.8 * wet

87 Micrometers 77 28,000 20,000 15,000 10, ,000 4,000 3,000 2,000 37:i 1,500 1, " 3/4" 1/2" 3/8" o o ro eg W eight Percent oos oo vo Passing Tyler Mesh ASTM 11-70

88 78 Ball Mill Grindability Tests Test Number 3 Purpose: Sample: Procedure: To determine the ball mill grindability of the test sample in terms of a Bond work index number. San Manuel rod mill feed. The equipment and procedure duplicate the Bond method for determining ball mill work indices. Test Conditions: Mesh of grind: 150 Weight of undersize product for 250 percent circulating load: g Weight percent of undersize material in ball mill feed: 10.9 Results Undersize Undersize Produced New In To Be Undersize Per Mill age Feed Feed Ground In Product Total Revolution >. 2 g g Revolutions g g g Average last three 1.506

89 79 Ball Mill Grindability Test Number 3 (Continued) Work Index Computations Wi = 44.5 / 0.23W 0.82\/10-10\ v 1 )(Gbp j k t ) Where: = 100 Percent Passing Size of Product = 106 micrometers Gbp = Grams per Revolution = P = 80 Percent Passing Size of Product = 83 micrometers F = 80 Percent Passing Size of Feed = 1,875 micrometers Wi = 12.55

90 Ball Mill Grindability Test Number 3 (Continued) Screen Analysis of Test Product Oversize (Dry) Screen Product Weight Percent ASTM Ell-70 Cumulative (Tyler) Mesh Direct Passing Undersize (Wet) Screen Product Weight Percent ASTM Ell-70 Cumulative (Tyler) Mesh Direct Passing * * not standard

91 28, " 81 20,000 3/4' 15,000 10, ,000 1/2 3/8' 4,000 3,000 Micrometers 2,000 1,500 1, Tyler Mesh ASTM o at qo vo CM Percent Pining