ABSTRACT INTRODUCTION

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1 Title: Authors: Increasing Capacity of Existing Limestone Grinding Systems While Reducing Grind Size Mr. Gary Harper, Tri-State G&T Association Ms. Melissa A. Hagan, Burns & McDonnell Mr. Paul Dyer, Burns & McDonnell Mr. William J. Breuer, Burns & McDonnell ABSTRACT Increased limestone grinding capacity may be required at existing FGD systems in response to changes in coal sulfur content, increasingly stringent limitations on SO2 emissions, and escalating SO2 allowance prices. In a conventional wet grinding circuit the capacity can be easily increased, but at the expense of particle size. The resultant coarser grind can decrease limestone utilization, leading to operating problems. The challenge faced by the FGD system operator is how to increase limestone slurry production capacity while maintaining or improving the fineness of the limestone grind achieved. This paper presents the results of a successful full-scale application of a novel approach to achieving this goal by placing a new vertical ball mill in series with an existing conventional horizontal mill. Advantages of this approach are documented, including minimal disruption to operations and installation with minimum outage time. Grinding system test results before and after the modifications are presented. INTRODUCTION This paper describes an alternative method to increase limestone reagent production for existing FGD systems while preserving existing in-place grinding assets. In this paper we will discuss the changes made to the limestone preparation system at Craig Generating Station. The requirement to increase limestone production resulted from the need to both increase the SO2 removal percentage and to eliminate the capability to bypass the FGD system. Tri-State Generation and Transmission Association, Inc. (Tri-State) implemented a project entitled the Yampa Environmental Project at Craig Station Units 1 and 2 in Craig, Colorado. This project included FGD system performance improvements based on conversion to 100% scrubbing of the flue gas and FGD absorber upgrades to enhance removal efficiency. Craig Station Units 1 and 2 consist of two 455 MW coal-fired boilers equipped with four spray tower absorbers per unit for sulfur dioxide emission control. As a result of the elimination of scrubber bypass and the increase in removal efficiency, the capacity of the limestone slurry preparation system was increased from under 5 tons of reagent per hour to 11 tons of reagent per hour with product slurry having a grind of 90% by weight passing 325 mesh. THEORY Communition is a mechanical process where large particles are acted upon to produce smaller particles. This is the fundamental process used by ball mills in FGD systems to produce limestone slurry. For the purpose of this paper we will concentrate on two types of ball mills. The most common type of ball mill used in the FGD industry is the horizontal ball mill. In this configuration, a horizontal tube or shell containing pre-crushed feed (limestone) and media (steel balls) is rotated, resulting in collisions between the feed material, grinding media, and the shell. These collisions continually reduce the size of the feed. Water is added to the mill and flushes particles out of the mill into Page 1 of 14

2 a mill product tank. The mill product pump transfers the slurry from the mill product tank to the classifiers. The classifiers separate the limestone slurry based on particle size. When properly set up and operating, the fine material flows from the classifier overflow to a limestone storage tank where it is stored until needed by the FGD system. The classifier underflow (heavier material) returns to the ball mill to be re-ground to a smaller size. The operational parameters which have a major impact on horizontal mill operation and throughput are feed size, feed hardness, and output size. The strengths of a horizontal mill lie in its ability to handle a wide range of operating parameters to produce a consistent slurry particle size. The weakness of this design is the potential reduction in capacity resulting from increases in feed size or feed hardness or decreases in target output particle size. The second type of ball mill considered in this paper is the vertical ball mill. Vertical ball mills, as the name suggests, have the mill shell oriented in the vertical position. Unlike their horizontal counterparts, the mill shell does not turn. However, feed size reduction is accomplished through a similar mechanism as the horizontal mill. Attrition grinding occurs with collisions between the grinding media and feed. The movement of the feed and media is the result of an internal screw which stirs the material on the inside of the mill and continuously circulates the material from the bottom of the mill to the top. This grinding action results in an extremely fine grind of the limestone. The operational parameters which have a major impact on vertical mill operation and throughput are the same as those for the horizontal mill. The strength of this mill design is its ability to produce a consistent finely ground limestone when operated in-series with a horizontal ball mill. The weakness of the design is the inability of the system to utilize feed greater than ¼ inch when operating as a stand-alone ball mill system. As stated above, both mill types are sensitive to feed size and feed hardness. Increasing the feed size in a horizontal mill rapidly reduces the mill s output capacity at a given product size. Increasing the feed size can upset the balance in a mill to the point where recycle flows, ball size distribution, and classifier tuning may require adjustment. A solution to achieve the same mass flow through the mill with increased feed size would be to let the product size float relative to the original design. This would mean that a horizontal mill originally designed for -325 mesh product size may be able to produce the same throughput in tons/hr with a larger feed size and a larger product size, for instance -200 mesh. The same general variations and relationships (although the relationship of throughput and particle size would likely be different) could be seen by varying the hardness of the feed (Bond Work Index). The Bond Work Index (BWI) is a measure of the amount of work necessary to reduce a representative sample of a material to a specified size. BWI is measured in KWH/ton. As the Bond Work Index increases, the capacity of the mill to produce a constant product size decreases. A reduction in BWI should also lead to an increase in production, however the amount of the increase may be reduced based on other factors. Vertical ball mills appear to be somewhat less sensitive to changes in feed size and BWI when operated in series with a horizontal ball mill. This is because the horizontal ball mill has done most of the work in normalizing the feed size to the vertical mill. By configuring the grinding circuit with an existing horizontal ball mill followed by a vertical ball mill, the utility can preserve an asset, gain flexibility, and increase production using the strengths of the two ball mill technologies while minimizing their weaknesses. ORIGINAL SYSTEM DESIGN The original limestone grinding system supplied with the FGD systems for Craig Station Units 1 and 2 included two 100% closed circuit ball mill trains. Each train included a KVS 7 x14 horizontal ball mill rated at a capacity of 10 tons per hour while producing an 80% passing 200 mesh grind. This provided enough capacity to allow one mill to Page 2 of 14

3 act as a standby spare even at the original design worst case coal sulfur and SO2 removal levels. Prior to the Yampa Environmental Project, the grinding circuits were modified to produce a finer grind, and thus typically operated at about 4.7 tons per hour each and at times as low as 3 to 4 tons per hour, but that was still adequate to meet the needs of the FGD systems for both units at the coal sulfur and SO2 removal levels while effectively providing a spare mill. However, the increase in limestone feed rate required to increase removal efficiency in the scrubbers meant that the limestone grinding rate was not adequate and needed to be increased to maintain the original sparing. There are two main reasons that the original mill capacity effectively dropped by 50 percent or more. First, the original grinding circuit for each mill was designed to produce limestone slurry with a particle size such that 80 percent by weight would pass a 200-mesh screen. This relatively coarse grind was selected by Peabody Process Systems as part of its standard process design at the time. The FGD system process design incorporated the use of six Krebs D15B-852 absorber recycle slurry classifiers (four operating and two spare) intended to separate the recycle slurry into a low-solids overflow containing comparatively little limestone and a high-solids underflow containing a higher limestone content. The classifier overflow was sent to the wash slurry tank for washing the interface tray between the slurry spray zone and the mist eliminators, while the underflow was returned to the absorber reaction tank. According to Peabody s process description, this feature should have allowed the absorbers to operate at an internal stoichiometry (in the absorber spray zone) higher than the external stoichiometry (in the slurry reaching the mist eliminators). When the recycle slurry classifier system proved to be unable to function as intended, Tri-State was forced to change the operation of the limestone grinding system to produce a finer limestone grind, typically 90 percent or more passing a 325-mesh screen. This necessitated a significant reduction in the mill throughput, because more passes through the mill were required to achieve the finer grind. It is important to understand that conversion to the finer limestone grind was necessary to allow operation of the absorber recycle slurry at reagent utilization levels that would be conducive to minimization of scale in the mist eliminators. Minimization of the unreacted limestone content of the slurry reaching the mist eliminators is very important to scale-free operation, and became even more so as the absorber velocity increased when modifications were implemented to scrub 100 percent of the flue gas. The secondary reason for reduction in the mill production capacity was the elimination of the original Pennsylvania Crushers, model JR7-1, from the flow stream. The original design was for limestone feed up to 1-3/4 size, with hammermill crushers provided to reduce the top size to ¼ entering the ball mill. The hammermill crushers at Craig were problematic from a maintenance standpoint, and had not been used for many years. The effective increase in limestone feed size to the ball mills above that for which they were designed contributed to the decreased production rate. One way for Tri-State to reduce this effect would have been to procure stone with a smaller top size. A flow diagram of the limestone grinding system as it existed prior to the Yampa Environmental Project is shown in Figure 1. Page 3 of 14

4 Figure 1: Original Limestone Grinding System Flow Diagram PROCESS AUDIT The first step in determining the potential of the existing system to support future operating conditions was to have the system evaluated by the OEM. This evaluation, called a Process Audit, was used to determine the existing operational condition of the mills. Metso Minerals was consulted to assist in the investigation of the ability of the mills to increase capacity while operating at the fine grind needed for proper FGD system performance. Two separate investigations were conducted. Metso began by examining the physical conditions of the ball mills. Metso evaluated the current grinding system configuration, the specifics of the feed material and media, mill liners and circuit control. In addition to the mechanical audit, Metso determined the ball mill operating performance by taking samples from the circuit along with various measurements of inlet and outlet flows to the mills. After collecting the baseline mechanical and process data, Metso ran a series of tests to determine the accuracy of water flow and conveyor speeds. June 2000 Assessment In June 2000, Svedala Industries (now called Metso Minerals) conducted a site visit for the purpose of investigating the possibility of increasing the capacity of the existing ball mills for the Craig Station Unit 1 and 2 FGD systems. The process audit data showed that the Unit 1 mill circuit was achieving a limestone grind of 90.2 percent passing 325 mesh, whereas the Unit 2 circuit was tested at 84.6 percent passing 325 mesh when operating at a higher than normal feed rate. A parametric test was conducted on the Unit 2 mill to determine the effect of continuing to increase the feed rate to the mill on fineness of grind. The percent passing 325 mesh decreased as the feed rate increased, degrading to 80.2 percent at a feed rate control setting of 55 percent. Page 4 of 14

5 The report failed to reach a definitive conclusion regarding the capability for increasing the capacity while maintaining the current fine grind. Instead it made recommendations for the installation of additional instrumentation that would be needed before optimization could be done. However, regarding the effect of the oversize feed material (due to retirement of the hammermill pre-crushers) on mill capacity, the report asserted that the benefit of returning the hammermills to service would be very small. The possible capacity increase cited due to reinstatement of the hammermills was less than two percent when operating at the desired fineness of grind. The report also included the results of a Bond Work Index test on the limestone sample taken at that time. The reported work index was kw-hr/short ton. This parameter would be important in the design basis for any new mill to be purchased as part of the Yampa Environmental Project. September 2001 Assessment Metso Minerals was contacted again in July 2001 about performing an assessment of the capability of the Craig Unit 1 and 2 limestone grinding circuits to operate at increased capacity when producing the desired fine grind. It was shortly thereafter determined that the mill circuit control and instrumentation upgrades recommended in their June 2000 report had not been made, and would likely not be made. Therefore a detailed on-site test program would not be possible. Nevertheless, personnel from Metso Minerals visited the site in September to assess the capability of the grinding circuit for increased capacity. Metso Minerals concluded that the best that could be obtained from the existing mills was to increase the current production of each mill from about 4.5 tons per hour to the range of tons per hour. A flowsheet illustrating this result was included with the Metso documentation, and is shown in Figure 2. This 5.2 tons per hour per mill falls just short of the nominal 11 tons per hour production rate target, even if both mills were operated simultaneously at full capacity. Metso again confirmed that a reduction in the top size of the feed would not have a significant impact on production rate. Page 5 of 14

6 Figure 2: Process Audit Flowsheet Original System Product Solids (stph) = 5.2 % solids = 28 Slurry GPM = 60.8 Slurry S.G. = 1.22 CYC Feed Solids (stph) = 5.2 Cyclone Feed % moisture = 1 Grind Water Solids (stph) = 31.2 gpm= 38.4 % solids = Slurry GPM = Slurry S.G. = 1.56 WEIGHFEEDER Recycle Solids (stph) = 26 % solids = 70 Slurry GPM = 81.5 Slurry S.G. = 1.82 BALL MILL Mill Discharge Solids (stph) = 31.2 % solids = 60 Slurry GPM = Slurry S.G. = 1.63 Dilution Water gpm= 14.8 SUMP PUMP Metso did propose a novel arrangement involving the use of each existing mill in series with their Vertimill vertical grinding mill to greatly increase the overall production rate. A mill circuit flowsheet for Craig Station showing the modified system flows for each grinding circuit was included in their documentation and is shown in Figure 3. Page 6 of 14

7 Figure 3: Process Audit Flowsheet - Vertimill System EVALUATION Burns & McDonnell evaluated the results of the grinding circuit process audits conducted by Metso Minerals in June 2000 and September 2001, and compared options for meeting the increased limestone grinding requirements associated with the higher SO2 removal rates. Evaluation of the options for increasing the grinding system capacity was based on maintaining the original design and operating philosophy of providing a standby mill at all operating conditions. Based on the grinding system evaluations conducted by Metso, it was confirmed that an additional ball mill would be required. Burns & McDonnell considered two options for increasing the grinding capacity to provide a spare mill at all operating conditions. Each option is described and discussed in the following subsections. The evaluation of each option includes consideration of feasibility and reliability. Cost was also evaluated at the time of the evaluation, but is not included in this paper because of changes in the market. Option 1 The scope of Option 1 included the addition of a new weigh feeder, ball mill (7 diameter 14 long), mill product tank and classification circuit, and reagent slurry tank identical in capacity to the original equipment. Feasibility Burns & McDonnell s review identified the possibility for locating the third ball mill in an extension of the existing limestone preparation building enclosure. There is extensive underground piping and electrical duct banks that pass through the area immediately north of the existing Unit 1 silo. Due to these underground piping considerations, the only alternative was to consider extension of the building to the south. Comparison of this possibly available space versus the size of the existing mills and their associated mill product tanks and maintenance access platforms indicated that the fit would be extremely tight. Even if the new horizontal ball mill could be made to fit in this space, the problem of how to get the limestone to the mill needed to be solved. A conveyor could be extended southwestward from the bin discharger outlet for the Unit 2 Page 7 of 14

8 limestone silo to a new weigh feeder that would serve the new third mill to be located in the building extension as described above. However, this conveyor would need to pass through the concrete silo skirt/support wall. Also, a way would need to be found to feed both the new conveyor and the existing feeder for the Unit 2 ball mill, or to feed both at the same time. The chutework below the bin discharger would need to be fitted with a diverter gate that would accommodate these operating modes. There is no room within the available space for the building extension as described above to locate a third limestone slurry storage tank. It was presumed that the hydrocyclone classifier assembly for the new third mill would be located above the existing slurry storage tanks, so that its overflow could be routed to either tank. Based on the space constraints and the feed scenario difficulty, it was clear that there would be several technical challenges associated with this option. Reliability If a new third ball mill were sized to match the capacity of that for Units 1 and 2, it would presumably be constructed to match the production rate of the original ball mills, which was nominally about five tons per hour each. So the sparing situation with a third mill would be that two mills would be required to operate full time at the worst case future condition, while one mill was available as the standby spare. If a feasible chutework and conveyor scheme could be developed to allow location of a third ball mill in the southward building extension as described above, the mill would act as a third 50 percent capacity mill when compared to the total maximum limestone demand. One mill would be available as a standby in the event of problems with one of the other two. However, the reliability would be affected by the fact that, of necessity, two of the three mills would share the same silo and the same bin discharger. If any problem developed in this common equipment, both downstream mills would be put out of commission, and the total limestone grinding capability would drop to about five tons per hour. Option 2 Metso Minerals, in its evaluation of ways to increase the capacity of the limestone grinding system at Craig Station Units 1 and 2, identified the possibility of adding a vertical ball mill in series with each of the existing horizontal ball mills. According to their proposal, the capacity of the grinding train for each unit would be upgraded to 11 tons per hour. This would actually exceed the original design capacity of 10 tons per hour per mill, and would provide 100 percent spare grinding capacity at the target maximum limestone usage rate. The Metso proposal was based on the use of two ball mills in series. The existing horizontal ball mill would be operated in open circuit (once through) mode, to produce a grind of 100 micron particles, which would then be fed to the closed circuit vertical stirred ball mill. The grinding circuit for the vertical mill would include a new hydrocyclone classifier. The underflow from the hydrocyclone would be returned to the vertical mill for further grinding. The hydrocyclone overflow would be sent to the limestone slurry storage tanks. Metso proposed to produce a grind of 90 percent passing 325 mesh. The flow scheme is shown in Figure 3 above. According to Metso, the ball charge in the existing mills would be changed to optimize the coarse grind. The existing mill product pumps could be used to transfer the coarse-ground slurry from the horizontal mill to the inlet of the vertical mill. Feasibility Vertical stirred mills have been used in limestone grinding service for FGD applications for over 20 years. One of the largest FGD installations of Metso s Vertimill is at Mill Creek Station of Louisville Gas & Electric. Burns & McDonnell confirmed that the mills there work extremely well. Although there was no other application of series grinding of limestone exactly like Metso proposed for use at Craig, similar applications had proven successful in other industries. Page 8 of 14

9 The most important factor lending support to the use of vertical stirred mills for fine grinding of limestone at Craig Station is the consideration of plot space. As described in the previous section, an extension of the existing limestone grinding building could be accomplished to the south. The available space described previously, which would barely be able to accommodate a 7 x14 horizontal ball mill, appeared to be able to house the two proposed Vertimills. One notable feature of the vertical mills is that the enclosure would need to be high, with the topmounted motor drive extending above the roof line of the original building. Another consideration favoring the use of the vertical mills was the fact that no limestone rock feed was required to feed the Vertimills. The slurry product from the horizontal mills would be pumped to the vertical mill inlets. It was assumed that it would be possible to route the new slurry lines from the mill product discharge lines to the proposed location of the vertical mills without much difficulty. As with the single horizontal mill, the hydrocyclone classifiers for use with the vertical mills would be situated to allow overflow to be diverted by gravity into either of the existing limestone slurry storage tanks. The high sections of the building extension that would house the Vertimills would provide areas for location of these hydrocyclones. Vertical stirred mills provide fine grinding at significantly lower power consumption than would be needed using horizontal mills. The VTM-200 mills utilize 200-horsepower drive motors. It was expected that the combined power consumption of the horizontal and vertical mill in each circuit would be less than that which would have been needed to operate two horizontal mills producing 11 tons per hour limestone. Reliability Compared to the other option evaluated, the combined use of the horizontal mill and the new vertical mill for limestone grinding should provide greater reliability. It would be possible to pipe the mill product discharge to provide a backup flow path so that each Vertimill can receive feed from either existing mill. Recommendations Based upon Burns & McDonnell s evaluation, the addition of a separate closed circuit vertical stirred ball mill in series with each original horizontal mill was selected as the preferred method for increasing the limestone grinding system capacity. The new vertical mills would be located in a southward extension of the existing limestone preparation building. A cost comparison of the two options found that the price for two vertical ball mill circuits was comparable to that of one horizontal ball mill. The power consumption of the horizontal and vertical mills in series would be less than the power consumption of two horizontal mills operating in parallel. The series grinding configuration would provide the greatest degree of backup at the lowest operating cost. CONCEPT Two main options are available when seeking to expand the capacity of an existing plant. These options are installation of additional grinding circuits, and modification to the existing grinding circuits. The installation of a Vertimill to operate in series with an existing grind circuit is a combination of these two alternatives, and provides distinct advantages over either individual alternative. The Vertimill has been proven to operate more efficiently than the traditional horizontal ball mill, with 65-70% of the power requirement of a horizontal ball mill (see Figure 4). This power savings translates into increased capacity for a given installed power. Installing the Vertimill in series with an existing ball mill allows the plant to take advantage of this power savings without experiencing some of the disadvantages of an entirely new grinding circuit (additional peripheral equipment, increased floor space requirements, etc.). Page 9 of 14

10 Figure 4: Grinding Power of Vertimill vs. Ball Mill Another distinct advantage of this configuration is the ability for the circuit to handle a coarse feed. Typically, the Vertimill can only be installed with a feed size of 1/4. However, since the horizontal ball mill is still handling the fresh feed, and thus breaking it down before it reaches the Vertimill, this is no longer a restriction. Not requiring a 1/4 feed is an advantage over a stand-alone Vertimill circuit in that less expensive limestone can be purchased without requiring a crushing stage prior to grinding. Some considerations that must be made when installing a Vertimill in series with an existing grind circuit include modifications to the existing ball mill and modifications to the grinding circuit logic. These modifications could include feed chute design, proper media size, pump speed control, or any number of other design points. These issues can vary greatly based on the specifics of the installation, and due diligence should be taken to consider any required modifications. The concept to increase limestone grinding capacity through a two-stage series grinding circuit using horizontal and vertical ball mills was proposed to Tri-State Generation and Transmission as a low cost option to increase limestone throughput while reducing particle size. The concept of series grinding allowed Tri-State to leverage their existing grinding equipment into a higher capacity system with minimal investment. The concept called for de-tuning the existing horizontal mills by converting them to once-through open circuit operation. This conversion required the elimination of the classifier system. With these modifications in place, the horizontal mill would be able to produce more gross tons of limestone, but at a larger particle size. The mill discharge would then be pumped into a storage tank. From the storage tank, the limestone slurry would be processed through a vertical ball mill operating in closed Page 10 of 14

11 circuit. New Krebs hydrocyclone classifiers were also provided. The vertical ball mill would act as a fine grind mill producing 11 tons per hour of 90% passing 325 mesh limestone. A summary of the modifications made to the limestone grinding system is as follows: Convert horizontal ball mill to once-through operation Demolish cyclone classifiers for horizontal ball mill Install VTM-200 WB vertical ball mill system New cyclone classifiers Separating tank Recycle tank Install mill product tank and mill product pumps The existing feed and storage infrastructure and horizontal ball mill circuit were utilized, offering a lower capital cost compared to adding a third ball mill operating parallel to the existing circuits. One grinding circuit is able to provide limestone slurry to both units, keeping the second limestone grinding train as a backup. Refer to Figure 5 for an overview of the limestone preparation system modifications. Figure 5: Series Limestone Grinding Flow Diagram IMPLEMENTATION Construction of the limestone preparation modifications began in May Initial start-up occurred in November Tie in of the vertical ball mill system with the existing limestone preparation equipment was accomplished without a unit shutdown. One of the challenges faced during initial design of the grinding modifications was the space constraint in the existing limestone preparation area. The limestone preparation building layout is shown in Figure 6. The horizontal ball mill circuits were located at grade, underneath the limestone storage silos. Column Row 1 located to the West of the original grinding equipment is the exterior wall of the FGD Building. A road runs adjacent to the FGD Page 11 of 14

12 building and limestone storage silo to the South. The vertical ball mill circuits fit into a 31-9 by 57-7 space between the FGD building, original limestone preparation building, and the road. Figure 6: Limestone Preparation Building Layout An elevation view of the vertical ball mill grinding equipment is shown in Figure 7. While the footprint is significantly smaller than a horizontal grinding system, the height of the building is much taller. The overall height of the new vertical ball mill building is 78 feet. A picture of one of the Vertimills is shown in Figure 8. Page 12 of 14

13 Figure 7: Vertical Grinding Circuit - Elevation View Page 13 of 14

14 Figure 8: Vertical Ball Mill Start-up of the new grinding system went smoothly, however some problems were encountered with the original horizontal ball mills. Metso Minerals evaluated the grinding circuit in the fall of 2003 in order to prepare for their performance testing. Their objective was to evaluate the reported lack of capacity of the horizontal ball mills and the performance of the vertical ball mills. Metso found that the feed chute of the horizontal ball mills plugged with a feed rate greater than 6 tons per hour and concluded that the feed chute needed to be modified. After operating the horizontal mill at a feed of approximately 6 tons per hour (which corresponded to 55-58% feeder speed), the ball mill output contained coarse particles, which prevented a consistent ball mill operation. Metso determined that the feed chutes needed to be modified to avoid plugging and ball size should be increased to improve the grinding of coarse particles before the output capacity of the grinding circuit could be verified. The recommended modifications were completed and the grinding circuit proved effective at producing 11 tons per hour of 90% passing 325 mesh limestone slurry. SUMMARY The installation of Metso Mineral s Vertimills was a novel solution to the need for increased limestone slurry production at Tri-State s Craig Station. The existing horizontal ball mill circuit was converted from a closed circuit system to a once-through horizontal grinding train feeding a new vertical ball mill. The vertical mills offered a comparable capital cost, lower operating cost, and smaller foot print option compared to the addition of a third horizontal grinding train. The limestone grinding system at Craig Station Units 1 and 2 consistently produces 11 tons per hour reagent with product slurry having a grind of 90% by weight passing 325 mesh. Page 14 of 14