Fundamentals of Roll Surface Functionality & Wear in Operation James Pownell Global Product Manager HPGR Minerals Technologies Abstract Unlike smooth rolls used in cements, the evolution of today s HPGRs has progressed through several design improvements in order to more effectively process minerals. Specifically, both roll surface design and material composition have seen considerable technological advancements to increase wear life and reduce slippage. To optimize the handling of rough, abrasive material and manage increased friction, the design of the HPGR s wear surface has progressed from a full hard-face welded smooth surface, to studded with hard faced edges, to studded with replaceable edges, to studded with cemented tungsten carbide edges. Along with these advancements, stud composition and positioning on the roll surface has also been improved. This paper provides direction in consideration when designing an ore-type roll surface and operator guidelines to optimize HPGR roller life and effectiveness in the grinding circuit. Introduction Mineral High Pressure Grinding Rolls (HPGRs) have developed significantly, evolving from the Hydraulic Roller Press (HRP) and the Hydraulic Roll Crusher (HRC) used in the cement industry. Traditionally, roll surfaces used in the cement industry featured a smooth, hard-faced surface, which had satisfactory wear characteristics and yielded a suitable final product. However, due to the abrasiveness of mineral ores, these rolls did not meet either required throughputs due to ore slippage on the smooth roll surface or the wear life requirements. To suit the requirements of minerals processing, one major development emphasis has been the optimization of the roll surface design and wear. With the introduction of cemented tungsten carbide studs, HPGRs have substantially increased surface wear life and throughputs. The subsequent escalation in end-use of this new technology has continued to fuel the continuous development and optimization of studded wear surfaces for this new breed of HPGRs. In addition to stud patterning, stud size, heat, material shape and composition, major factors that contribute to roll surface wear are the pressure curve profile and the nature and composition of the ore body. The pressure curve is derived from the forces exerted on the roll's surface by the installed hydrau-
lic cylinders. The press force is greatest at the center of the roll, diminishing towards the outer edges of the roll. The variation of this curve from center to the edges is dependent on: Press forces. Feed presentation. Particle density. Optimizing press forces and roll speeds is necessary to obtain efficient grinding and acceptable wear rates. There is general misconception that the higher the grinding pressure, the higher the efficiency. However, this is not always the outcome, as in most cases the increased grinding pressure simply increases the cake competency. In turn, this results in higher recirculation loads due to some cakes not fully de-agglomerating during the screening stage. Pressure curve relation to PSD of feed material Figure 1 shows an example of a HPGR pressure curve. The pressure drop zone is determined by the particle feed size and fine evacuation. For correct stud patterning and composition, an estimate of the pressure curve for the ore to be ground should be obtained. With this curve, percentage edge recycling can be more effectively realized. Normal / Natural Wear Figure 1: Redefined HPGR Pressure Curve from J. Pownell's findings. 2 Normal / Natural wear is the effect the pressure curve and the feed material mineralogy have on the wear surface. The longevity of the roll surface is determined by the rate of wear. This is typically measured as fractions of millimeters of tungsten carbide wear per day or per ton. When a roll experiences Normal / Natural wear, the maximum percentage of stud height is consumed or worn to where there is so little left in the base material that the stud falls out. For this Normal / Natural wear to occur, the following guidelines need to be met:
Feed top-size control is maintained. The HPGR process island is run with zero to minimal tramp events. Good maintenance practices are followed and regularly scheduled stud inspections and repairs are performed. Stringent and correct control philosophy is employed. Side to side pressure and skew control is continuously active. Feed presentation is consistent and free flowing. Choke feed conditions are always maintained. Cheek-plate gaps are maintained to their minimum allowable tolerance. A competent compacted mineral Layer forms on the roll surface. Excessive wear of a roller leads to roll change-outs at a more frequent, and financially, sub-optimal rate. This type of wear will often lead to a very irregular roller profile, difficulty in maintaining an efficient gap, and much of the tungsten carbide going underutilized. An incorrect gap has a negative effect on subsequent process performance, reducing overall plant efficiency. Irregular and excessive wear leads to higher maintenance costs and lower machine availability. Excessive wear is generally caused by: Incorrect feed presentation. Over pressing, running at a press force higher than the optimum press force. Inappropriate Tungsten Carbide grade selection. Inappropriate stud patterning. Poor or no tramp protection. Poor maintenance practices with too few or no roll inspections. No feed particle top-size control. Multiple starting and stopping of feed to the HPGR. No ability to run in a choke feed mode. Incorrect control philosophy employed. Inability to maintain a Compacted Mineral Layer. Figure 2: (Left): Roll surface at end of life with slight concave natural wear. (Right): An example of unnatural wear causing premature end of roll life. 3
Loss of the Compacted Mineral Layer (CML) In HPGR operation, autogenous grinding occurs in the compression chamber. The resulting grinding energy and hydraulic press force transfer to the roll as abrasion. This abrasion can be easily accommodated by the tungsten carbide, but not in the exposed roll body. This roll surface base material is significantly softer and thus more susceptible to rapid abrasion. Fortunately, the ongoing autogenous grinding particles are forced between the studs and become wedged and adhered. These wedged particles form a Compacted Mineral Layer (CML), which drastically reduces the wear rate on the roll base material. A competent CML is crucial for good wear performance, balancing the wear rate of the roller base and the tungsten carbide studs. It has been identified that there is a wear cycle which the CML experiences. Initial wear is experienced by the roll surface material; upon reaching the correct depth to maintain a competent CML, wear is transferred to the cemented tungsten carbide stud surface. The rate of wear is then far slower than initial roll surface material wear. The stud reduces in height until the CML starts to give way, and the whole cycle repeats. Figure 3: The CML cycle. The press force, depth of the spacing between the studs and the nature of the ground material contribute to the integrity of this formed layer. Factors such as moisture content, press force and ore body composition also have an effect on the CML. This layer reduces the effect of any side forces on the studs, protecting them from pressure-bending moments. It has been noted in preliminary test work that the competency of this CML needs to be identified. The roll surface material wear characteristics provide insufficient protection without a competent CML thus drastically reducing the life expectancy of the roller. This competency can be quickly determined in the HPGR Design and Suitability test work. In some cases, roll surface CML can easily be removed via a simple sweep of a paint brush. More competent CMLs can require removal with scrapers, brushes or even rotary wire brush grinders. 4
Figure 4: (Left): An incompetent CML. (Middle): Competent CML yet easily removed with a scraper. (Right): Very competent CML requiring electrical wire brushes to remove CML. Incorrect stud configuration Protection of the roll surface base material is of the utmost importance. Failure to protect this surface will result in a significantly reduced life expectancy for the roller. The roller currently accounts for 10% to 15% of the value of an HPGR and increased life expectancy translates to increased cost effectiveness. Generally, the higher the amount of installed tungsten carbide the higher the cost of the machine and the higher the wear resistance of the HPGR roll; however, the lower the installed carbides, the lower the cost of the machine. With optimally-designed stud patterning, the amount of installed tungsten carbide can be reduced and still achieve an optimal life with lower cost. As previously mentioned, the CML is not static; it is constantly renewing itself. Evidence of this movement can be seen on worn rolls. The material flow paths migrate through open stud pattern configurations. This pressurized material moment can have a negative effect on the CML. The distancing between the studs as related to the material top size, particle size distribution and shape from the upstream comminution device of the ground ore also contribute to the CML loss. Gouging out of the CML by rock fragments is common. The harder quartz type material has the greatest tendency to gouge out the CML and expose the roll surface material, and slabby or tabular material has a similar effect. Gouging can also occur when the spaces between the studs are too great. Gouging is thus a greater issue in a very open stud pattern. Figure 5: Stud pattern ore flow direction (Left to Right): Diagonal, Vertical, Horizontal. 5
Stud Damage Tungsten carbide studs are the main form of protection against poor wear performance. Generally, the harder the stud the more brittle it is. With high specific grinding forces this can result in stud chipping or breakage, which drastically reduces the life expectancy of a roll wear surface. Incorporating a higher wear resistant stud may not necessarily achieve higher roll life expectancy. Careful consideration needs be taken in roll surface design. The nature of the ore body should always be taken into consideration. The difference between a broken and chipped stud, is that the broken stud fractures across the entire horizontal plane of the stud, while a chipped stud can fracture from one side of the stud but the fracture or cleavage terminates by heading vertically to the top face of the stud. Stud chipping generally occurs on flat top studs during the run-in stage of the new rolls. As the top of the stud is flat, when material is pressed against it the point load force has no way to deflect, and given the brittle nature of the stud this lack of point load force deflection causes the corner or edge of the stud to chip off. Once in operation, flat top studs become more rounded, reducing the rate of chipping. Poor stud maintenance is one of the quickest ways to bring about a catastrophic roll surface failure. Cemented tungsten carbide has a high wear resistance but is very brittle and can fracture fairly easily. Because of this, care needs to be taken to ensure no tramp iron enters the grinding chamber. Tramp events can cause massive acute failures in isolated regions on the roll which left unchecked, or without replacement of broken studs, can quickly turn to a serious "pot-hole" occurring in the roll surface. Failure to replace broken studs results in roll surface wash out / pot-holing. The roll surface's resistance to wear, when not protected by the CML, is low and large wash outs can occur quickly. These wash outs, once started, are very difficult to stop. If they are left to run they will not only result in poor comminution but also could cause severe damage to the bearing due to the "camming" effect caused by the wash out. The camming effect occurs when the roll surface develops a wash out. The wash outs create an eccentric profile in the roll and results in a short release in pressure as well as a gap change. Automatic correction of these effects by the HPGR control system results in erratic gaps and losses in efficiency. Broken and chipped studs happen due to: Poor pressing procedure and manufacturing quality control. Incorrect grade selection for application. Poor feed presentation to the machine. Poor shape or stud extrusion length. Incorrect run-in procedures. Over-size ore. Tramp iron. Lack of fines. Multiple start / stop events. 6
Figure 6: (Left): Multiple stud breakage, (Middle): Chipped studs polished via continued production, (Right): Single stud breakage inside parent material. Shape of Tungsten Carbide Insert Squared top studs chip and break more often because they see a higher point loading from the higher torque due to the higher initial throughput with the fresh square stud tops. Grip of the rock increases, which increases throughput, power draw and torque loading. Squared top studs do not deflect the individual rocks. Instead, they increase the chances of point loading on the stud. Rounded top studs can deflect the point loading, just as a rounded shield deflects an arrow. Size: Diameter and Length of Tungsten Carbide Insert Diameter: Too small a stud diameter could result in substantial stud breakage causing exponentially fast wear rates. Thicker stud diameters will result in studs more resistant to breakage. Length: Simply increasing the stud length does not necessarily give longer wear life due to the fact that the "bath-tubbing" effect will be allowed to get so severe that not enough grinding will occur, thus causing an increase in the particle size distribution, and much higher recycle rates. Stud Composition Figure 7: Example of "bath-tubbing" causing premature end of roll life. Ore type, feed and control philosophy are not the only considerations to be taken into account for the efficient operation of an HPGR. Stud compositions play a vital role in the long productive life of a wear surface. The stud hardness and composition are important considerations; however, with increasing 7
content of more specialized (virgin) Tungsten Carbide material comes a rapidly rising price. It is important to ensure that the stud selection for the application is correct. Perceived cost savings that come with lesser quality studs can greatly impact machine availability, and therefore add cost once in operation. Cemented Carbide Basics The purpose of this section is to outline the basic steps in the production of cemented carbide. While cemented carbide may seem to be simply another wear material, it is actually comprised of a number of constituents and processing steps that need to be carefully considered to produce the chemical, physical and dimensional properties required to suit the application. Cemented Carbide production begins with the reduction of the raw material Ammonium Para Tungstate (APT) into Tungsten (W) via furnacing under hydrogen. Tungsten is then combined with carbon (lamp black) and carburized to produce Tungsten Carbide (WC). The term Cemented Carbide refers to the fact that the WC particles are bound or cemented together by a binder material producing a metal matrix that is both hard and tough. The most widely used binder material is Cobalt, but other binder agents are often more suitable for corrosive environments. Lubricant is required in order to press the powder into the desired shape with wax (paraffin) being the most common lubricant, especially where shaping or machining of the green carbide is required prior to sintering. Care must be taken when using wax as a lubricant as insufficient removal of this lubricant (de-waxing) can lead to carbon balance and porosity problems during final sintering. The WC, binder, lubricant and dopants for grain growth control, if required, are selected and weighed to produce the desired grade. This mixture is then blended and milled into a homogenous material. This can be accomplished by attritor or ball milling technology. Both methods have unique advantages and disadvantages in terms of time, quality and cost. This blended material is then dried using vacuum or spray drying methods which results in a Ready to Press (RTP) powder. The RTP can now be pressed to the desired shape and size at which point it is considered to be in the green state. Pressing the RTP can be accomplished by a direct hydraulic method using a fixed cavity mold or by cold isostatic pressing (CIP) methods using rubber bags or chambers filled with powder. Direct pressing is generally used for products that will not require further processing in the green state and for high volume products where robotic handling can be employed. CIP pressing is generally used to produce larger sections (billets) that can then be cut to the desired size and machined (shaped) in the green state to add shapes and features that cannot be achieved in the direct pressing method. The final step in the production is liquid phase sintering. The two most widely used methods for sintering cemented carbide are vacuum and sinter hip. In the green state, cemented carbide is not fully dense and during the sintering process the green carbide will shrink between 20% and 25% as it achieves full density. This requires careful consideration of the starting size in order to ensure the sintered part meets the required dimensions. 8
Figure 8: Schematic of cemented carbide production. Rigorous quality testing is undertaken at all of the major steps in the process (see above figure) as well as post sintering. These quality steps can include, but are not limited to, density, hardness, grain size, chemical content, porosity, fracture toughness and magnetic property testing. Below are six of the most common problematic occurrences in the production of studs and their causes and effects: Grain Contamination Grain contamination is caused by cross grade contamination due to improper cleaning or segregation of powder production equipment, and uncontrolled grain growth. Grain contamination reduces mechanical properties in that large areas or large amounts of smaller areas of grain contamination can cause catastrophic failure especially in high load or high shock applications. Figure 9: Grain contamination in dashed circle. 9
Cobalt Pools Cobalt pooling is caused by poor blending technique, insufficient press pressure, and using the incorrect sintering process. Cobalt pools reduce mechanical properties. Free Carbon Figure 10: Cobalt pools shown in dashed circles. Free carbon is caused by an incorrect stoichiometric carbon balance during sintering thus creating excess carbon. Free carbon reduces mechanical properties. Free carbon is "C" type porosity (porosity due to carbon inclusions), which can also develop into voids and pits at the surface of the carbide and negatively impact performance. Eta Phase Eta phases are caused by incorrect powder formulation, improper sintering, oxidation of green (pre-sintered) carbide, and incorrect stoichiometric carbon balance resulting in insufficient carbon levels thus creating a harder carbondeficient phase of carbide. Eta Phases reduce the Transverse Rupture Strength and Fracture Toughness because they embrittle the material. This increases the possibility of catastrophic failure of components if subjected to high levels of stress and pressure. Figure 11: Free carbon. Figure 12: Eta phases shown in dashed circles. 10
Cobalt Leaching Cobalt leaching is caused by the binder being attacked by a corrosive agent or using the incorrect binder for the application. Cobalt leaching causes the elimination of the binder leading to rapid failure in the affected area as the carbide particles have no method of attachment. This can progress to the point of catastrophic failure of the entire carbide component. Figure 13: Cobalt leaching shown in dashed circle. Micro Porosity Micro porosity has similar causes to cobalt leaching and include the binder being attacked by a corrosive agent and using the incorrect binder for the application Micro porosity reduces the mechanical properties and causes surface finish pitting. Lack of Fines / Truncated Feed Figure 14: Micro porosity. The presence of fines is ore-dependent but on average, fines content in the feed of less than 20% volume help with cushioning through filling of feed interstices, transferring grinding energy, inhibiting the feed from just simply moving in the compression zone without grinding, and helping absorb pressure shocks due to ongoing feed particle variation and movement. When there are too many fines, this increases the packed density in the crushing zone. That, in turn, prevents fines evacuation, which decreases the percentage comminution efficiency. The fines pack around the ore which can increase machine vibration. A truncated feed (a feed without fines) will increase the wear rate, as the CML is not as efficiently maintained. In summary, some fines are needed, but too many fines promote problems as well. 11
Inconsistent Feed The running of HPGRs under inconsistent feed conditions should be avoided. The highest power spike experienced in normal operation will occur during the start-up/feed-on phase. Inconsistent feed and variation in composition will cause an increased number of starts and stops to the running HPGR. Erratic gaps and poor comminution will be experienced. Motors are designed in accordance to specific power draw calculated from the designed laboratory test work. If feed to the HPGR is consistent then the additional safety factor percentage increase in power draw is minimized, helping to minimize the motor sizes, and thus reducing capital cost. Control philosophy may be used to help overcome power spikes upon startup events. Inconsistent feed increases the probability of broken studs and, in addition, causes a greater occurrence of power spikes. It affects the grinding efficiency by causing the operating gap to oscillate between zero and ideal. Hydraulic components will be activated more frequently, which could lead to a shorter life expectancy and/or higher maintenance. Choke feed HPGRs are not mechanical crushing equipment; instead they make use of 3 dimensional hydraulic compression. If the 3-dimensional hydraulic compression is not maintained, the optimal press force cannot be achieved. The vertical feed height is one of these critical dimensions and the impact is greater on the larger machines. This vertical mass greatly impacts the nature of the HPGR grinding zone and thus the machine operating gap. A choke feed is considered crucial to effectively control and stabilize the HPGR for optimal results. Starve feed In this case, there is simply insufficient feed material to fill the feed chute and fill the gap. HPGRs are designed to a certain tonnage for the application and it is important to maintain this tonnage for optimum grinding. If the grinding chamber is not fully occupied with material it is difficult to obtain a floating gap. The majority of the material passes through the center of the rolls and is not evenly distributed. The HPGR then resorts back to being a simple roll crusher and there is a reduction on the quality of the product. At the same time as increasing the treatment cost, a risk is created that the downstream processes may be compromised and overall plant efficiencies reduced. Starve/trickle feed will cause severe "bath tubbing" of the roll, which will drastically reduce the life expectancy of the roll. Excessive stud damage would also be experienced. Segregated Feed Skewing of the roll due to material feed generally occurs as a result of poor materials handling, angle of transfer, speed of belt, design of chute etc. For efficient grinding to occur, a constant and uniform feed needs to be achieved. A variation in feed size to different sections of the roll surface will result in skew- 12
ing of the roll, which results in constant correction and this also leads to inefficient grinding. Segregated feed will also cause uneven parallel roll surface wear. Specific Grinding Force The specific grinding force needs to be identified in the HPGR design and suitability laboratory or via onsite test work. The selected grind force is arguably the most crucial factor in achieving effective grinding and optimal roll surface wear life. Effects of over pressing Increased friction wear. Higher chance of stud damage. Higher power draw. Over competent cakes. Higher operating costs. Effects of under pressing A competent CML is never fully established resulting in excessive base material wear. Stud protrusion due to poor CML, providing a higher chance of studs either snapping or breaking. CML does not build up across the full pressure curve (across the roll width), causing an accelerated base material convex wear profile. Inefficient comminution. Higher operating costs. Conclusion The general perception regarding roll surface selection for optimal performance and longevity is thought to be determined by the stud grade alone; however, as seen above in this paper, there are additional variables that require careful consideration. It also can be seen that one mineral type surface may not be optimal for other mineral type usage. A thorough understanding of the wear characteristics resulting from each ore type, including the pressure curve, stud patterning, stud manufacturing specifications and company-specific QA procedures, is only the foundation upon which a substantial structure of knowledge must be built. At the very least, steps can be taken to ensure the greatest performance and longest roller life via inquiries to the manufacturer about stud grades, origin of studs and optimizing stud grades to the customer's ore type and region. Such inquiries and procedures may seem initially costly and time consuming but this investment of time and resources will result in a solution to the specific needs of the plant and will therefore be valuable to the customer in the greater time up and running attributed to greater roll life longevity. In summary, the least expensive option is not always the most cost effective option to ensure smooth operation and optimal longevity of the roll surface. 13