The principle objective for controlling grinding mill operation is to produce a product having an acceptable and constant size distribution at optimum cost. To achieve this objective an attempt is made to stabilize the operation by principally controlling the process variables. The main disturbances in a grinding circuit are:
The mill control strategy has to compensate for these variations and minimize any disturbances to the hydrocyclone that is usually in closed circuit. The simplest arrangement is to setup several control loops starting from the control of water/solid ratio in the feed slurry, sump level control, density control of pulp streams at various stages and control of circulating load. Presently most mills use centrifugal pumps for discharging from the sump. This helps to counter surges and other problems related to pumping. For feed control the most likely option is to use a feed forward control while for controlling the hopper level and mill speed and other loops the PI or PID controller is used. The control action should be fast enough to prevent the sump from overflowing or drying out. This can be attained by a cascade control system. The set point of the controller is determined from the level control loop. This type of control promotes stability.
Each of these is controlled by specific controlled inputs, i.e., feed rate, feed water and discharge water flows. The overflow solids fraction is controlled by monitoring the ratio of total water addition (WTOT) to the solid feed rate. The ratio being fixed by the target set point of the overflow solid fraction.
Usually the charge volume of SAG mills occupy between 30-40% of its internal volume at which the grinding rate is maximized. When the charge volume is more, then the throughput suffers. The fill level is monitored by mill weight measurement as most modem mills are invariably mounted on load-cells.
During the operation of SAG mills, it is sometimes observed that the sump levels fall sharply and so does the power draft. This phenomenon is attributed to flow restrictions against the grate. When this occurs it is necessary to control, (or in extreme circumstances), stop the incoming feed.
The power draft is the result of the torque produced by the mill charge density, lift angle of the charge within the mill and fill level. The relationship between these parameters is complex and difficult. Therefore to control mill operation by power draft alone is difficult.
For the purpose of stabilization of the circuit, the basis is to counteract the disturbances. Also the set points must be held. The set points are attributed by dynamic mass balances at each stage of the circuit.
In modern practice the structure and instrumentation of the control systems of tubular grinding mills are designed to operate in three levels or in some cases four levels. The control loops and sensors for a SAG-mill and the levels of control are illustrated in Fig. 18.22. According to Elber [10,17], the levels are:
The function of Level 2 is to stabilize the circuit and to provide the basis of optimizing function in Level 3. Three cascade loops operating in level 2 controls that function in conjunction with level 1 controllers. The cascade loops are:
The set points are supplied by level 3 controllers for all the cascade loops. The mill load and percent solids in the two streams are calculated from signals received by sensors in the water flow stream, the sump discharge flow rate and the density readings from density meters in the pulp streams. The mill load cells supply the charge mass. The load cell signals are compensated for pinion up thrusts .
To determine the set point for the optimum mill load, a relation between load, consisting of different feed blends and performance (the maximum achievable throughput) is established. Similar observations are made for mill discharge density and mill discharge flow.
The primary function at Level 3 is optimisation of the SAG mill operation. That is, control of the product at optimum level. In an integrated situation where ball mill and cyclone is in the circuit, the optimisation must take place keeping in mind the restraints imposed by down stream requirements. This optimisation can best be achieved by developing a software for computer use. Usually a large database is required to cover infrequent control actions.
Ore samples were taken from a grinding mill operating as a batch process. The feed size distribution, breakage functions and size analysis of samples taken at intervals of 10minutes up to 30minutes are given. Determine:
An alkaline slurry from a bauxite grinding mill was scheduled to be classified using a spiral classifier at the underflow rate of 1100t/day. The width of the classifier flight was 1.3m and the outside diameter of the spiral flights was 1.2m. Estimate the pitch of the spirals if the spiral speed is 5rev/min and the bulk density of the underflow solids is 2000kg/m3.
The diameter of a typical hydrocyclone was 30.5cm. The apex was fitted with a rubber sleeve 12cm in length and 8.0cm in internal diameter. A quartz suspension at a density of 1.33 was fed to the cyclone at the rate of 1000L/min. The underflow measured 75% solids. The apex diameter was reduced by 10% twice. Estimate:
The volume flow rate of pulp fed to a hydrocyclone was 129L/min. Its solid content was held at 32% by volume. Samples of the feed, under flow and over flow streams were taken simultaneously, dried and a size analysis carried out. The results obtained were:
An hydrocyclone is to be installed in a closed circuit grinding circuit with a mill discharge containing 30% solids by volume. The solid density is 2800kg/m3 and the density of water is 1000kg/m3. Given that the maximum pressure deferential between the inlet and overflow was 50kPa and the throughput from the mill was 800t/h, estimate:
A hydrocyclone classifier is fed with quartz slurry at the rate of 20.8t/h from a grinding mill. The underflow is recirculated. The screen analysis of each stream were determined with the following results:
The input and output streams of an operating cyclone were sampled simultaneously for the same period of time. The dried samples were analysed for size distribution and the mass per cent retained on each size fraction was determined with the following results:
After a steady state operation the solid content of feed slurry was increased by 20% while all other conditions remained the same. Determine the size distribution of each stream under the altered condition.
If a second cyclone is added in series to the cyclone in problem 12.8, what is the effect of the overall efficiency of the classification. What will be the size distribution of the cyclone U/F of the second stage? The partition coefficient of the second stage cyclone is given as:
A crushing plant delivered ore to a wet grinding mill for further size reduction. The size of crushed ore (F80) was. 4.0mm and the S.G. 2.8t/m3. The work index of the ore was determined as 12.2kWh/t. A wet ball mill 1m1m was chosen to grind the ore down to 200microns. A 30% pulp was made and charged to the mill, which was then rotated at 60% of the critical speed. Estimate:
Everell  believed that the mechanism of breakage of particles in a grinding mill was analogous to the slow compression loading of irregular particles and that the specific rate of breakage for a particular size of fragment is an inverse function of the average failure load of the particles. Everell et al  developed a model to describe the relationship between the grinding selection function (breakage rate) and the physio-mechanical properties of the rocks. The advantage in such a relationship lies in the wealth of rock strength data determined on drill core during mine development being available to predict energy demands in the comminution circuits.
Briggs  measured the tensile strength, using the Brazil tensile test, and the point load compressive strength of four rock types of different grindabilities. These results were compared to the Bond Work Index of the ores as measured by the Magdalinovic method . The results in Fig. 3.8 show that there is a good correlation between the Bond Work Index and the tensile strength and the Equivalent Uniaxial Compressive Strength (EUCS). Some of the scatter in the graphs are due to the structure of the rock. For example one rock type was a banded iron, heavily mineralised with sulphides with numerous planes of weakness on a macro scale. This affected the mechanical properties when tested on large specimens. However when the grinding tests were carried out at relatively small particle sizes, the planes of weakness were no longer present and the ore became more competent.
The correlation between Bond Work Index and tensile strength is an indication that the grinding mechanism in the work index test favours abrasion type breakage given that tensile strength is a fair indication of the abrasiveness of a rock.
Briggs also measured the breakage rate and breakage distribution function of the ores and compared the breakage rate and Bond Work Index. There was a good correlation between the rock strength data and the breakage rate with higher strength rocks having a lower breakage rate. However the data set for these tests was small and further work needs to be done to confirm the relationship.
Bearman et al  measured a wide range of rock strength properties and correlated these to the JKMRC drop weight test data. Conclusions were that this technique will enable the data required for comminution plant design to be obtained from mechanical tests on drill core samples.
During normal operation the mill speed tends to vary with mill charge. According to available literature, the operating speeds of AG mills are much higher than conventional tumbling mills and are in the range of 8085% of the critical speed. SAG mills of comparable size but containing say 10% ball charge (in addition to the rocks), normally operate between 70 to 75% of the critical speed. Dry Aerofall mills are run at about 85% of the critical speed.
The breakage of particles depends on the speed of rotation. Working with a 7.32m diameter and 3.66m long mill Napier-Munn et al  observed that the breakage rate for the finer size fractions of ore (say 0.1mm) at lower speeds (eg. 55% of the critical speed), were higher than that observed at higher speeds (eg. 70% of the critical speed). For larger sizes of ore, (in excess of 10mm), the breakage rate was lower for mills rotating at 55% of the critical speed than for mills running at 70% of the critical speed. For a particular intermediate particle size range, indications are that the breakage rate was independent of speed. The breakage ratesize relation at two different speeds is reproduced in Fig. 9.7.
Ultrafine grinding (UFG) has continued to evolve in terms of equipment development. A number of specialist machines are commercially available including Xstrata's IsaMill, Metso's Vertimill, Outotec's High Intensity Grinding (HIG) mill, and the Metprotech mill. UFG equipment has been developed with installed powers of up to 5MW.
Compared with conventional ball or pebble milling, the specialist machines are significantly more energy efficient and can economically grind to 10m or lower, whereas the economical limit on conventional regrind mills was generally considered to be around 30m. Coupled with improvements in downstream flotation and oxidation processes, the rise of UFG has enabled treatment of more finely grained refractory ores due to a higher degree of liberation in the case of flotation or enhanced oxidation due to the generation of higher surface areas.
In 1993, the Salsigne Gold Mine was reopened. Salsigne treated a gold-bearing pyrite/arsenopyrite ore by flotation, with the flotation tails treated in a CIL circuit and the concentrate reground in a conventional mill to approximately 2530m. The oxygen demand for reground concentrate was high and the rate of oxidation was slow. The concentrate was initially oxidized for approximately 6h using oxygen injection via a Filblast aerator before cyanidation. Additional oxygen was added in in the second CIL stage and hydrogen peroxide was added into the fourth unit to maintain dissolved oxygen concentrations of >10ppm.
Goldcorp have commenced operations at the Elenore Gold Project in Quebec, Canada. The mineralogy of the ore and hence the circuit selection show similarities to those at Salsigne. The main sulfides are arsenopyrite, pyrite, and pyrhottite. The ore is floated, with the flotation tails passing to a tails CIL circuit and the flotation concentrate reground before passing to the concentrate CIL circuit via preaeration tanks designed to achieve 18-h contact with oxygen. The main difference between the Salsigne and Elenore projects is that the Elenore concentrate is ground to 10m and oxidation of the sulfides is substantially complete before cyanidation.
Large projects are typically associated with more complex contracting strategies but not necessarily greater flow sheet complexity. The complexity of the contracting strategy and the increased focus on key items of equipment, such as large grinding mills, elevate the manning requirements. Higher throughput, and associated larger equipment, does lead to increased complexity in service equipment such as lubrication, cooling, and control systems. Gearless motor drives on large semi-autogenous grinding (SAG) and ball mills require significant installation testing and commissioning effort. The 20 MW drive for a large Australian gold/copper project with a capital cost of approximately A$295M in 1998, took three technicians over 6weeks to test and commission with the total vendor cost (installation and commissioning) for the 20MW ring motor alone exceeding A$1M.
The first step in slag processing is size reduction to liberate metallic iron and iron-bearing minerals. This is done by crushers or by autogenous grinding, that is, the slag is ground on its own in the grinding mill without any balls. The latter process yields higher quality product as the iron product discharged from grinding mill contains as high as 80% Fe (Shen and Forssberg, 2003). Metallic iron and iron minerals are separated by magnetic separation. The phosphorus-bearing minerals occurring in steel slag are removed in the tailings of high gradient magnetic separation. The flow diagram is shown in Figure 8.2.
Reduction of iron oxide at high temperature has been shown to be an attractive low energy cost process (Olginskij and Prokhorenko, 1994). The iron-free mineral residue is suitable for applications in construction industry. An alternative route applies microwave heating with carbon and the recovery of iron by magnetic separation (Hatton and Pickles, 1994).
Low-competency ores such as oxides are unlikely to have a problem with generation of pebbles. They are more likely to have a problem with slurry viscosity. For smaller plant in the 1980s, these ores were treated through a single-stage SAG mill grinding to 75 or 106m. This type of circuit is still straightforward in its design concept. High-competency ores or ores requiring a finer product size frequently require two stages of grinding and a number of design issues become important.
Mill orientation. For smaller plant, the mills can be arranged in parallel if the products feed the same discharge hopper. This is more difficult for larger mills as the diameter of the mills drives the height required to maintain the discharge launder slope. In this situation the mills are often located at right angles. Alternatively, a transfer hopper can be used. This decision is usually based on the difference in cost and operability of each option.
Transfer size from the SAG to the ball mill. Transfer size prediction is somewhat uncertain but has an important impact on the balance of power between the two stages of the milling circuit. For feasibility studies, a combination of modeling and benchmarking is generally used.
Mill discharge arrangement. If the SAG and ball mills feed separate pump systems this is not an issue. If the two stages feed the same pump system, the SAG mill usually drives the discharge arrangement. This is because the SAG mill is usually of larger diameter and may have a requirement for screening of pebbles to protect the mill discharge pumps and cyclones, and to facilitate pebble crushing.
Mt. Baker Mining and Metals designs and builds hammer mills with longevity in mind. These units feature wire feed welds, and replaceable-wear parts. Each machine comes complete and ready to run, including hammer mill, inlet chute, belts, motor, full-enclosure guards, steel skid, and optional stand. Our standard-duty hammer mills are suited for a variety of projects, including:
Hammer mills are versatile machines for material size reduction. Applied to ore milling applications, our hammer mills can produce flour-fine rock powder. Glass pulverizing, rock recycling, porcelain recycling, and similar material reduction projects achieve slightly variable output sizes, from powder to rice-grain sized discharge, per operator specifications using slotted screens or punch plate holes as small as 1/4 in our printed circuit board (PCB) recycling hammer mills.
Mt. Baker Mining and Metals hammer mills are industrial grade, continuous duty machines. Hammers, protective liners, and screens are easily replaced at reasonable cost. Abrasive material grinding like granite or quartz ore, requires regular hammer mill maintenance. In less abrasive uses like breaking up and cleaning copper wire from motors or pulverizing printed circuit boards, the hammers and internal components last longer, reducing maintenance and wear.
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Ball mill is a type of grinder machine which uses steel ball as grinding medium, can crush and grind the materials to 35 mesh or finer, adopted in open or close circuit. The feed materials can be dry or wet, they are broken by the force of impact and attrition that created by the different sized balls.Types of ball milldry grinding ball mill and wet grinding ball mill; grate discharge ball mill and overflow ball mill.Applicationsmining, chemical, glass, ceramics, etc.Suitable MaterialsCement, silicate products, new building materials, refractory materials, fertilizers, mineral processing and glass ceramics.
Ball mill is a horizontal machine, contains a hollow cylindrical shell that rotates around its axis, Inside the cylinder, there are many different sized stainless steel balls. As the the cylinder rotates, the mill balls lifts and then drops, strikes the materials, that is the impact and attrition take place.The cylinder chamber which turning around the horizontal axis is partially filled with grinding mediums: mostly are steel balls, cast iron or porcelain balls. Filling rate best at 40%, steel balls diameter with 30 to 80mm.These grinding balls are initially 3-10 cm in diameter, but gradually became smaller as grinding progressed. So we usually just refill the big balls.The chamber is lined with a wear resistant material, such as manganese-steel or high quality rubber, to extend the service time.Thanks to the closed grinding chamber, the dust and pollution generated in the grinding process are avoided to emit to air.
Eastman provides you with complete rock crushers and full list of replacement parts, original ball mill parts, form and function are a perfect fit.If your equipment breaks down, the productivity of the whole factory will be threatened. Critical wear parts are shipped with the goods to ensure they are available when you need them and to reduce maintenance time.
Eastman is a crushing manufacturer with more than 30 years of experience, produces hammer crusher used for a variety of applications.We not only can provide you with various types of rock crusher, but also can design reasonable crushing process for you free.
Factors of ball mill product sizeWithin the rotating chamber the grinding balls rub and strike against each other.The final discharge size can be changed by changing the number and size of the steel balls, the material of the ball, rotate speed, and the what material to be ground. Besides, the ball mill production rate is directly proportional to the drum rotation speed. Check the ball mill critical rotation speed which indicated in the manufacturers technical specifications.