The purpose of these operations is the complete or partial opening of grains of gold-containing minerals, mainly particles of native gold, and bringing the ore into a state that ensures the successful completion of subsequent enrichment and hydrometallurgical processes. Crushing and especially fine grinding operations are energy-intensive, and their costs constitute a significant proportion of the total cost of ore processing (from 40 to 60%). Therefore, it must be borne in mind that grinding should always be completed at the stage when they are sufficiently opened for their final extraction or for their intermediate concentration.
Since the main method of extracting gold and silver for most ores is hydrometallurgical operations, the required degree of grinding should ensure the possibility of contact of solutions with open grains of gold and silver minerals. The sufficiency of exposure of these minerals for a given ore is usually determined by preliminary laboratory process tests for the extraction of precious metals. To do this, ore samples are subjected to technological processing after varying degrees of grinding with the simultaneous determination of the extraction of gold and accompanying silver. It is clear that the finer the inclusion of gold, the deeper the grinding should be. For coarse gold ores, coarse grinding (90% grade -0.4 mm) is usually sufficient. But since in most ores, along with large gold, there is also fine gold, most often the ores are crushed more finely (up to -0.074 mm). In some cases, the ore has to be subjected to even finer grinding (up to 0.044 mm).
An economically feasible degree of grinding is established taking into account a number of factors;
1) the degree of metal extraction from ore;
2) an increase in the consumption of reagents with more intensive grinding;
3) the cost of additional grinding when bringing the ore to a given size;
4) deterioration of the thickening and filterability of finely ground ores and the associated additional costs for thickening and filtering operations.
Crushing and grinding schemes vary depending on the material composition of the ores and their physical properties. Typically, the ore is first subjected to coarse and medium crushing in jaw and cone crushers with test screening. Sometimes a third stage of fine crushing is used, carried out in short-cone crushers. After two-stage crushing, material with a particle size of 20 mm is usually obtained; after three-stage crushing, the material size is sometimes reduced to 6 mm.
The crushed material is fed to wet grinding, which is most often carried out in ball and rod mills. Ores are usually crushed in several stages. Two-stage grinding has become most widespread, and for the first stage it is preferred to use rod mills, which produce a product that is more uniform in size with less overgrinding.
Currently, at gold mining enterprises, ore and ore-pebble autogenous grinding has become widespread in the ore preparation cycle. In ore autogenous grinding, the grinding medium is pieces of the crushed ore itself, unclassified by size; only some control is provided over the upper size of the pieces. In the case of ore-pebble auto-grinding, the grinding medium is a fraction of pieces of crushed ore (pebbles) specially selected for size and strength.
Ore autogenous grinding is carried out in an air or water environment in special mills, in which the ratio of the diameter to the length of the mill is increased compared to conventional ball mills. Since the grinding effect of ore pieces is worse than that of steel balls, the diameter of autogenous grinding mills reaches 5.5-11.0 m.
For dry autogenous grinding, an Aerofol mill is used. It is a short drum mounted on a massive foundation. On the inner surface of the drum along its generatrix, shelves made of I-beams or rails are installed at some distance from one another, which lift pieces of ore when the drum rotates. As they fall, the pieces crush the ore below, and in addition, when they hit the shelves as they fall, large pieces split. On the end covers of the drum there are guide rings of triangular cross-section, the purpose of which is to direct the pieces into the middle of the drum. The mill rotation speed is 80-85% of the critical one.
Grinding ores in Aerofol mills ensures a product that is more uniform in size compared to grinding in conventional ball mills. In Aerofol mills, overgrinding of ore is reduced, which improves the filterability and thickening of the resulting pulps. After grinding in these mills, the performance of hydrometallurgical processing also improves: the consumption of reagents (cyanide) is reduced by 35%, and gold recovery is increased (up to 4%). Dry ballless grinding of gold ores is in some cases more economical. However, it imposes strict requirements on the moisture content in the ore (no more than 1.5-2%). Increased humidity dramatically reduces the efficiency of the grinding and classification processes. In addition, dry grinding is accompanied by large dust formation, which requires a developed dust collection system and worsens working conditions. Therefore, self-grinding in an aqueous environment is more common.
Wet ore autogenous grinding is carried out in Cascade mills. This mill has a short drum with conicalend caps. Hollow axles and the drum rest on bearings. The ore from the mill is discharged through a grate. Cascade mills operate in a closed cycle with a mechanical classifier or hydrocyclones.
Ore-pebble autogenous grinding is carried out, as a rule, in an aqueous environment. The designs of ore-pebble and ball mills with unloading through a grate are similar.
The size of the ore galls used as a grinding medium is determined by the stage of grinding. At the first stage of grinding, galls with a size of -300+100 mm are usually used, at the second - 100+25 mm. Screening of galls is carried out on screens. The shape of the galley for grinding does not matter.
In gold ore processing schemes, a significant place is occupied by the operations of classifying crushed material by size. Recently, at most gold mining factories, hydrocyclones of various designs have become widespread as a classifying apparatus at all stages of processing, including in a closed cycle of primary grinding, instead of spiral, rack and bowl classifiers. Rough classification of mill products is in some cases carried out by screening in drum screens mounted at the discharge ends of the mills.
Before hydrometallurgical processing or enrichment by flotation, gold ores are deslimed if the sludge is depleted in gold and negatively affects technological operations. For desludge baths, hydrocyclones or thickeners are used. Using such techniques, up to 30-40% of sharply depleted material is sometimes removed to the dump, which not only improves technological performance, but also reduces the volume of equipment for subsequent operations.
Sorting and primary enrichment of lump ore
Usually, in the mined rock mass, along with pieces of gold-bearing ore, there are also pieces of waste rock, the exclusion of which from subsequent processing can significantly improve the technical and economic indicators.
Manual sorting is sometimes used to remove waste rock. In this case, waste rock is either removed from the rock mass or an ore fraction enriched in gold is isolated. The general rule for sorting is that the extracted rock should not be richer in gold content than the tailings of the gold recovery plant.
Typically, ore sorting is used for material larger than 40-5C mm. To improve the inspection of pieces, sorting conveyor belts are given a vibrating motion. However, manual sorting of ores is a labor-intensive and low-productivity process. Therefore, it is not currently used (with the exception of a few enterprises in South Africa).
In recent years, advances in science and technology have made it possible, instead of manual sorting, to use more rational and economically feasible methods of preliminary enrichment of relatively large lump ore, in particular, the enrichment process in heavy environments, which is completely mechanized and quite simple in design. The most promising application of enrichment in heavy environments is to sulfide ores, in which it is associated only with sulfides, is evenly distributed, and its content in the enriched raw material is almost proportional to the content of sulfides. Therefore, when enriched in heavy environments, it is concentrated together with sulfides in heavy fractions; The light fractions contain host rocks that are almost not mineralized for this group of gold-bearing ores.
Copper ore has a different composition, which affects its quality characteristics and determines the choice of method for enriching the feedstock. The composition of the rock may be dominated by sulfides, oxidized copper, or a mixed amount of components may be present. At the same time, for ore mined in the Russian Federation, the flotation enrichment method is used.
Processing of disseminated and continuous copper sulfide ore, which contains no more than a quarter of oxidized copper, is carried out in Russia at processing plants:
- Balkhash;
- Dzhezkazgan;
- Sredneuralskaya;
- Krasnouralskaya.
The technology for processing raw materials is selected in accordance with the type of source material.
Working with disseminated ores involves extracting sulfides from the rock and moving them into depleted concentrates using chemical compounds: blowing agents, hydrocarbons and xanthate. The primary method used is fairly coarse grinding of the rock. After processing, the lean concentrate and middlings undergo an additional process of grinding and cleaning. During processing, copper is freed from intergrowths with pyrite, quartz and other minerals.
The homogeneity of the porphyritic ore supplied for processing makes it possible to flotate it at large processing plants. A high level of productivity makes it possible to reduce the cost of the enrichment procedure, as well as to accept ore with a low copper content (up to 0.5%) for processing.
Flotation process diagrams
The flotation process itself is built according to several basic schemes, each of which differs in both the level of complexity and cost. The simplest (cheapest) scheme involves switching to an open ore processing cycle (at the 3rd stage of crushing), grinding the ore within one stage, as well as carrying out a subsequent additional grinding procedure to obtain a result of 0.074 mm.
During the flotation process, the pyrite contained in the ore is subjected to depression, leaving in the concentrates a sufficient level of sulfur necessary for the subsequent production of slag (matte). To carry out depression, a solution of lime or cyanide is used.
Solid sulfide ores (cuprous pyrites) are distinguished by the presence of a significant amount of copper-containing minerals (sulfates) and pyrite. Copper sulfides form thin films (covellite) on pyrite, and, due to the complexity of the chemical composition, the floatability of such ore is somewhat reduced. An effective beneficiation process requires careful grinding of the rock to facilitate the release of copper sulfides. It is noteworthy that in a number of cases, thorough grinding is not economically feasible. We are talking about situations where pyrite concentrate, subjected to a roasting process, is used in blast furnace to extract precious metals.
Flotation is carried out by creating an alkaline environment of high concentration. The following are used in the process in specified proportions:
- lime;
- xanthate;
- fleetoil.
The procedure is quite energy-intensive (up to 35 kW h/t), which increases production costs.
The ore grinding process is also complex. As part of its implementation, multi-stage and multi-stage processing of the source material is provided.
Intermediate ore beneficiation
Processing of ore with a sulfide content of up to 50% is similar in technology to the processing of solid sulfide ore. The only difference is the degree of its grinding. Material of a coarser fraction is accepted for processing. In addition, the separation of pyrite does not require the preparation of an environment with such a high alkali content.
At the Pyshminsky concentrating plant, collective flotation followed by selective processing is practiced. The technology makes it possible to use 0.6% ore to obtain 27% copper concentrate with subsequent extraction of over 91% copper. The work is carried out in an alkaline environment with varying levels of intensity at each stage. The processing scheme allows reducing the consumption of reagents.
Technology of combined enrichment methods
It is worth noting that ore with a low content of clay and iron hydroxide impurities lends itself better to the beneficiation process. The flotation method allows you to extract up to 85% of copper from it. If we talk about refractory ores, then the use of more expensive combined enrichment methods, for example, the technology of V. Mostovich, becomes more effective. Its use is relevant for the Russian industry, since the amount of refractory ore makes up a significant part of the total production of copper-bearing ore.
The technological process involves crushing raw materials (fraction size up to 6 mm) followed by immersing the material in a sulfuric acid solution. This allows sand and sludge to be separated and free copper to go into solution. The sand is washed, leached, passed through a classifier, crushed and flotated. The copper solution is combined with the slurry and then subjected to leaching, cementation and flotation.
In work using the Mostovich method, sulfuric acid is used, as well as precipitating components. The use of technology turns out to be more expensive compared to standard flotation.
The use of Mostovich's alternative scheme, which involves the recovery of copper from the oxide with flotation after crushing the ore subjected to heat treatment, allows to reduce costs somewhat. The technology can be made cheaper by using inexpensive fuel.
Flotation of copper-zinc ore
The process of flotation of copper-zinc ore is labor-intensive. The difficulties are explained by the chemical reactions that occur with multicomponent raw materials. If the situation with primary sulfide copper-zinc ore is somewhat simpler, then the situation when exchange reactions began with the ore already in the deposit itself can complicate the enrichment process. Selective flotation may not be possible when dissolved copper and cavellin films are present in the ore. Most often, this picture occurs with ore mined from the upper horizons.
In the beneficiation of Ural ore, which is quite poor in copper and zinc content, both selective and collective flotation technologies are effectively used. At the same time, the method of combined ore processing and the scheme of collective selective enrichment are increasingly used at leading enterprises in the industry.
Copper can be produced as a main product or as a co-product with gold, lead, zinc and silver. It is mined in the Northern and Southern Hemispheres and primarily consumed in the Northern Hemisphere with the United States as the main producer and consumer.
A copper processing plant processes copper from metal ore and copper scrap. The leading consumers of copper are wire mills and copper mills, which use copper to produce copper wire, etc. End uses of copper include building materials, electronic products, transportation and equipment.
Copper is mined in quarries and underground. The ores typically contain less than 1% copper and are often associated with sulfide minerals. The ore is crushed, concentrated, and suspended with water and chemicals. Blowing air through the mixture attaches the copper, causing it to float at the top of the slurry.
Crushing complex for copper ore
Large raw copper ore is fed into the copper ore jaw crusher, evenly and gradually, by vibrating feeder through the copper ore primary crushing hopper. Once separated, the crushed pieces of copper ore can meet the standard and will be taken as the final product.
After the first crushing, the material will be transferred to the copper ore impact crusher, copper ore cone crusher, secondary crushing conveyor. Then the crushed materials are transferred to the vibrating sieve for separation. The final production of copper ore will be taken away, and other copper ore parts will be returned to the copper ore impact crusher, forming a closed circuit.
The dimensions of the final copper ore product can be combined and rated according to customers' requirement. We can also equip ash removal systems to protect the environment.
Mill complex for copper ore
After primary and secondary processing in the copper ore production line, it can enter the next stage to grind the copper ore. The final copper ore powder produced by Zenith copper ore milling equipment typically contains less than 1% copper, while sulfide ores have moved to the beneficiation stage, while oxidized ores are used for leach tanks.
The most popular copper ore milling equipment is ball mills. Ball mill plays an important role in the copper ore grinding process. Zenith ball mill is an effective tool for grinding copper ore into powder. There are two grinding methods: dry process and wet process. It can be divided into table type and flow type according to different forms of material discharge. Ball Mill is a crucial equipment for grinding after crushed materials. It is an effective tool for grinding various materials into powder.
It can also use mills such as MTW European type trapezoidal mills, XZM ultrafine grinding mills, MCF coarse powder grinding mills, vertical mills, etc.
The mined mineral in most cases is a mixture of pieces of various sizes, in which the minerals have grown together closely, forming a monolithic mass. The size of the ore depends on the type of mining and, in particular, on the blasting method. In open-pit mining, the largest pieces are 1-1.5 m in diameter, in underground mining - somewhat smaller.To separate minerals from each other, the ore must be crushed and ground.
To free minerals from intergrowth, in most cases fine grinding is required, for example to -0.2 mm and finer.
The ratio of the diameter of the largest pieces of ore (D) to the diameter of the crushed product (d) is called the degree of crushing or degree of grinding (K):
For example, with D = 1500 mm and d = 0.2 mm.
K = 1500 ÷ 0.2 = 7500.
Crushing and grinding usually take place in several stages. At each stage, crushers and mills of various types are used, as shown in table. 68 and in Fig. 1.
Crushing and grinding can be dry or wet.
Depending on the final practically possible degree of grinding in each stage, the number of stages is selected. If the required degree of grinding is K, and at individual stages - k1, k2, k3..., then
The overall degree of grinding is determined by the size of the original ore and the size of the final product.
Crushing is cheaper the smaller the mined ore. The larger the volume of the excavator bucket for mining, the larger the mined ore, which means larger crushing units must be used, which is not economically profitable.
The degree of crushing is chosen so that the cost of equipment and operating costs are minimal. The size of the loading slot should be 10-20% larger than the transverse size of the largest pieces of ore for jaw crushers; for conical and conical crushers it should be equal to a piece of ore or slightly larger. The productivity of the selected crusher is calculated based on the width of the discharge slot, taking into account the fact that the crushed product always contains pieces of ore two to three times larger than the selected slot. To obtain a product with a particle size of 20 mm, you need to choose a cone crusher with a discharge slot of 8-10 mm. With a small assumption, we can assume that the productivity of crushers is directly proportional to the width of the discharge gap.
Crushers for small factories are selected to operate in one shift, for factories of medium productivity - in two, for large factories, when several crushers are installed at the stages of medium and fine crushing - in three shifts (six hours each).
If, with a minimum jaw width corresponding to the size of the ore pieces, a jaw crusher can provide the required productivity in one shift, and a conical crusher will be underloaded, then a jaw crusher is chosen. If a cone crusher with a loading slot size equal to the size of the largest pieces of ore is provided with one-shift operation, then preference should be given to the cone crusher.
In the ore industry, rolls are rarely installed; they are replaced by short-cone crushers. To crush soft ores, such as manganese ores, as well as coals, toothed rollers are used.
In recent years, impact crushers have become relatively widespread, the main advantage of which is a high degree of grinding (up to 30) and selectivity of crushing due to the splitting of ore pieces along the planes of mineral accretion and at the weakest points. In table 69 shows comparative data on impact and jaw crushers.
Impact crushers are installed to prepare material in metallurgical shops (crushing limestone, mercury ores for the roasting process, etc.). Mechanobrom tested a prototype of the inertial crusher design developed by HM with 1000 rpm, providing a crushing degree of about 40 and making it possible to produce fine crushing with a large yield of fine fractions. A crusher with a cone diameter of 600 mm will be put into mass production. Together with Uralmashzavod, a sample crusher with a cone diameter of 1650 mm is being designed.
Grinding, both dry and wet, is carried out mainly in drum mills. A general view of mills with end unloading is shown in Fig. 2. The dimensions of drum mills are determined as the product of DxL, where D is the diameter of the drum, L is the length of the drum.
Mill volume
A brief description of the mills is given in Table. 70.
The productivity of a mill in weight units of a product of a certain size or class per unit volume per unit time is called specific productivity. It is usually given in tons per 1 m3 per hour (or day). But the efficiency of mills can be expressed in other units, for example in tons of finished product per kWh or in kWh (energy consumption) per ton of finished product. The latter is used most often.
The power consumed by the mill is composed of two quantities: W1 - power consumed by the mill at idle speed, without loading crushing medium and ore; W2 - power for lifting and rotating the load. W2 - productive power - is spent on grinding and associated energy losses.
Total power consumption
The lower the ratio W1/W, i.e., the greater the relative value of W2/W, the more efficient the operation of the mill and the lower the energy consumption per ton of ore; W/T, where T is the mill productivity. The highest mill productivity under these conditions corresponds to the maximum power consumed by the mill. Since the theory of the operation of mills is not sufficiently developed, the optimal operating conditions of the mill are found experimentally or determined on the basis of practical data, which are sometimes contradictory.
The specific productivity of mills depends on the following factors.
Mill drum rotation speed. When the mill rotates, the balls or rods are influenced by centrifugal force
mv2/R = mπ2Rn2/30,
where m is the mass of the ball;
R - radius of rotation of the ball;
n - number of revolutions per minute,
are pressed against the wall of the drum and, in the absence of sliding, rise with the wall to a certain height until they come off the wall under the influence of gravity mg and fly down a parabola, and then fall on the wall of the drum with ore and, upon impact, perform crushing work. Ho can be given such a number of revolutions that the He balls will come off the wall (mv2/R>mg) and begin to rotate with it.
The minimum rotation speed at which the balls (in the absence of sliding) do not come off the wall is called the critical speed, the corresponding number of revolutions is the critical number of revolutions ncr. In textbooks you can find that
where D is the inner diameter of the drum;
d is the diameter of the ball;
h - lining thickness.
The operating rotation speed of the mill is usually determined as a percentage of the critical speed. As can be seen from Fig. 3, the power consumed by the mill increases with increasing rotation speed beyond the critical limit. Accordingly, the productivity of the mill should increase. When operating at a speed above the critical speed in a smooth-lined mill, the speed of movement of the mill drum is higher than the speed of movement of the balls adjacent to the surface of the drum: the balls slide along the wall, rotating around their axis, abrading and crushing the ore. When lining with lifters and without sliding, the maximum power consumption (and performance) shifts towards lower rotation speeds.
In modern practice, the most common are mills with a rotation speed of 75-80% of the critical one. According to the latest practice data, due to rising steel prices, mills with lower speeds (low-speed) are being installed. Thus, at the largest molybdenum factory, Climax (USA), mills are 3.9x3.6 M with a 1000 hp motor. With. operate at 65% of critical speed; at the new Pima factory (USA), the rotation speed of the rod mill (3.2x3.96/1) and ball mills (3.05x3.6 m) is 63% of the critical one; At the Tennessee factory (USA), the new ball mill has a speed of 59% of the critical speed, and the rod mill operates at an unusually high speed for rod mills - 76% of the critical speed. As can be seen in Fig. 3, increasing the speed to 200-300% can increase the productivity of mills several times with their volume remaining unchanged, but this will require structural improvement of the mills, in particular bearings, removal of scroll feeders, etc.
Crushing environment. For grinding in mills, rods made of manganese steel, forged or cast steel or alloyed cast iron balls, ore or quartz pebbles are used. As can be seen in Fig. 3, the higher the specific gravity of the crushing medium, the higher the mill productivity and the lower the energy consumption per ton of ore. The lower the specific gravity of the balls, the higher the mill rotation speed must be to achieve the same productivity.
The size of the crushing bodies (dsh) depends on the size of the mill feed (dр) and its diameter D. Approximately it should be:
The finer the food, the smaller the balls can be used. In practice, the following ball sizes are known: for ore 25-40 mm = 100, less often, for hard ores - 125 mm, and for soft ores - 75 mm; for ore - 10-15 mm = 50-65 mm; in the second stage of grinding when feeding with a particle size of 3 mm dsh = 40 mm and in the second cycle when feeding with a particle size of 1 mm dsh = 25-30 mm; When regrinding concentrates or industrial products, balls no larger than 20 mm or pebbles (ore or quartz) - 100+50 mm are used.
In rod mills the diameter of the rods is usually 75-100 mm. The required volume of crushing media depends on the rotation speed of the mill, the method of unloading it and the nature of the products. Typically, at a mill rotation speed of 75-80% of the critical load, 40-50% of the mill volume is filled. However, in some cases, reducing the ball load is more effective not only from an economic, but also from a technological point of view - it provides more selective grinding without sludge formation. Thus, in 1953, at the Copper Hill factory (USA), the ball loading volume was reduced from 45 to 29%, as a result of which the mill productivity increased from 2130 to 2250 tons, steel consumption decreased from 0.51 to 0.42 kg/t ; Copper content in tailings decreased from 0.08 to 0.062% due to better selective grinding of sulfides and reduced overgrinding of gangue.
The fact is that at a mill rotation speed of 60-65% of the critical one, in a mill with central unloading, with a small volume of ball loading, a relatively calm mirror of the pulp flow moving towards unloading is created, which is not agitated by the balls. From this flow, large and heavy particles of ore quickly settle into a zone filled with balls and are crushed, while thin and large light particles remain in the flow and are unloaded without having time to be re-crushed. When loading up to 50% of the mill volume, the entire pulp is mixed with the balls and the fine particles are re-ground.
Mill unloading method. Typically, mills are unloaded from the end opposite the loading one (with rare exceptions). The unloading can be high - in the center of the end (central unloading) through a hollow axle, or low - through a grate inserted into the mill from the unloading end, and the pulp that has passed through the grate is lifted by lifters and also unloaded through a hollow axle. In this case, part of the mill volume occupied by the grate and lifters (up to 10% of the volume) is not used for grinding.
The mill with central unloading is filled with pulp up to the drain level. weight Δ. Balls with ud. weight b in such a pulp become lighter per beat. weight. pulp: δ-Δ. i.e., their crushing effect decreases and the smaller the δ, the greater it is. In mills with low discharge, the falling vapors are not immersed in the pulp, so their crushing effect is greater.
Consequently, the productivity of mills with a grate is greater by δ/δ-Δ times, i.e., with steel balls - by about 15-20%, when grinding with ore or quartz pebbles - by 30-40%. Thus, when switching from central unloading to unloading through grates, mill productivity increased at the Castle Dome factory (USA) by 12%, at Kirovskaya - by 20%, at Mirgalimsayskaya - by 18%.
This is true only for coarse grinding or single-stage grinding. With fine grinding on fine feed, for example, at the second stage of grinding, the weight loss of the crushing body is less important and the main advantage of grate mills disappears, while their disadvantages - incomplete use of volume, high steel consumption, high repair costs - remain, which forces preference mills with central unloading. Thus, tests at the Balkhash factory gave results not in favor of grate mills; at the Tennessee factory (USA), increasing the diameter of the unloading journal did not give better results; at the Tulsikwa factory (Canada), when the grate was removed and the volume of the mill was increased due to this, the productivity remained the same, and the cost of repairs and steel consumption decreased. In most cases, it is not advisable to install mills with grates at the second stage of grinding, when work by abrasion and crushing is more effective (rotation speed 60-65% of critical) than work by impact (speed 75-80% of critical).
Mill lining. Various types of linings are shown in Fig. 4.
When grinding by abrasion and at speeds above critical, smooth linings are advisable; when crushing by impact - linings with lifters. The lining shown in Fig. is simple and economical in terms of steel consumption. 4, g: the spaces between the steel bars above the wooden slats are filled with small balls, which, protruding, protect the steel bars from wear. The thinner and more wear-resistant the lining, the higher the productivity of the mills.
During operation, the balls wear out and decrease in size, so the mills are reloaded with balls of one larger size. In a cylindrical mill, large balls roll towards the discharge end, so their efficiency is reduced. Tests have shown that by eliminating the rolling of large balls toward unloading, mill productivity increases by 6%. To eliminate the movement of balls, various linings have been proposed - stepped (Fig. 4, h), spiral (Fig. 4, i), etc.
At the discharge end of rod mills, large pieces of ore, falling between the rods, disrupt their parallel arrangement as they roll over the loading surface. To eliminate this, the lining is given a cone shape, thickening it towards the discharge end.
Mill size. As the amount of ore processed increases, the size of the mills increases. If in the thirties the largest mills had dimensions of 2.7x3.6 m, installed at the Balkhash and Sredneuralsk factories, then at this time they produce rod mills 3.5x3.65, 3.5x4.8 m, ball mills 4x3.6 m, 3 ,6x4.2 m, 3.6x4.9, 4x4.8 m, etc. Modern rod mills process up to 9000 tons of ore per day in an open cycle.
Power consumption and specific productivity Td are an exponential function of n - rotation speed, expressed as a percentage of the critical nk:
where n is the number of revolutions of the mill;
D - mill diameter, k2 = T/42.4;
K1 is a coefficient that depends on the size of the mill and is determined experimentally;
from here
T - the actual productivity of the mill is proportional to its volume and is equal to the specific productivity multiplied by the volume of the mill:
According to experiments in Outokumpu (Finland), m = 1.4, at the Sullivan factory (Canada) when working on a rod mill m = 1.5. If we take m=1.4, then
T = k4 n1.4 * D2.7 L.
At the same number of revolutions, the productivity of the mills is directly proportional to L, and at the same speed as a percentage of the critical speed, it is proportional to D2L.
Therefore, it is more profitable to increase the diameter of the mills rather than the length. Therefore, ball mills usually have a diameter greater than their length. When crushing by impact in mills of larger diameter, which are lined with lifters, when lifting the balls to a greater height, the kinetic energy of the balls is greater, so the efficiency of their use is higher. You can also load smaller balls, which will increase their number and mill productivity. This means that the productivity of mills with small balls at the same rotation speed increases faster than D2.
In calculations it is often assumed that productivity increases in proportion to D2.5, which is exaggerated.
The specific energy consumption (kW*h/t) is lower due to the fact that the ratio W1/W, i.e., the relative energy consumption for idling, decreases.
Mills are selected according to specific productivity per unit volume of the mill, according to a certain size class per unit of time, or according to specific energy consumption per ton of ore.
Specific productivity is determined experimentally in a pilot mill or by analogy based on data from the practice of factories operating with ores of the same hardness.
With a feed size of 25 mm and grinding to approximately 60-70% - 0.074 mm, the required mill volume is about 0.02 m3 per ton of daily ore productivity or about 35 mill volumes per 24 hours for class - 0.074 mm for Zolotushinsky, Zyryanovsky ores . Dzhezkazgan, Almalyk, Kojaran, Altyn-Topkan and other fields. For magnetite quartzites - 28 i/day per 1 m3 of mill volume according to class - 0.074 mm. Rod mills, when grinding up to - 2 mm or up to 20% - 0.074 mm, pass 85-100 t/m3, and for softer ores (Olenegorsk factory) - up to 200 m3/day.
Energy consumption when grinding per ton - 0.074 mm is 12-16 kW*h/t, lining consumption is 0.01 kg/t for nickel steel and mills with a diameter of over 0.3 mm and up to 0.25 /sg/g for manganese steel in smaller mills. Consumption of balls and rods is about 1 kg/t for soft ores or coarse grinding (about 50% -0.74 mm); for medium-hard ores 1.6-1.7 kg/t, for hard ores and fine grinding up to 2-2.5 kg/t; the consumption of cast iron balls is 1.5-2 times higher.
Dry grinding is used in the preparation of pulverized coal fuel in the cement industry and, less commonly, in the grinding of ores, in particular gold-bearing, uranium, etc. In this case, grinding is carried out in a closed cycle with pneumatic classification (Fig. 5).
In recent years, in the ore industry, short mills of large (up to 8.5 m) diameter with air classification have begun to be used for dry grinding, and ore is used as the crushing and grinding medium in the form in which it is obtained from the mine - with a particle size of up to 900 mm . Ore with a particle size of 300-900 mm is immediately crushed in one stage to 70-80% - 0.074 mm.
This method is used to grind gold ores at the Rand factory (South Africa); At the Messina (Africa) and Goldstream (Canada) factories, sulfide ores are crushed to a flotation size of 85% - 0.074 mm. The cost of grinding in such mills is lower than in ball mills, while the cost of classification is half of all costs.
At gold and uranium factories, when using such mills, it is possible to avoid contamination with metallic iron (abrasion of balls and lining); iron, by absorbing oxygen or acid, impairs gold extraction and increases acid consumption during leaching of uranium ores.
Selective grinding of heavier minerals (sulfides, etc.) and the absence of sludge formation leads to improved metal recovery rates, increased sedimentation rate during thickening and filtration rate (by 25% compared to grinding in ball mills with classification).
Further development of grinding equipment, apparently, will follow the path of creating centrifugal ball mills, which simultaneously perform the role of a classifier or work in a closed cycle with classifiers (centrifugal), like existing mills.
Grinding in vibration mills belongs to the field of ultrafine grinding (paint, etc.). Their use for grinding He ores has left the experimental stage; The largest volume of tested Bibromills is about 1 m3.
Machines used for crushing - crushers - can reduce the size of pieces to 5-6 mm. Finer crushing is called grinding and is carried out in mills.
In most cases, crushing together with grinding are preparatory operations before ore beneficiation. Although it is possible to crush in one unit from 1500 mm, for example, to 1-2 mm or less, practice shows that this is economically unprofitable, therefore, at crushing and processing plants, crushing is carried out in several stages, using the most suitable type of crusher for each stage: 1) coarse crushing from 1500 to 250 mm; 2) average crushing from 250 to 50 mm; 3) fine crushing from 50 to 5-6 mm; 4) grinding to 0.04 mm.
Most crushers used in industry operate on the principle of crushing pieces of ore between two steel surfaces approaching each other. For crushing ores, jaw crushers (coarse and medium crushing), cone crushers (coarse, medium and fine crushing), roller and hammer crushers (medium and fine crushing) are used.
Jaw crusher(Fig. 1, a) consists of three main parts: - a fixed steel vertical plate, called a fixed cheek, - a movable cheek suspended in the upper part, - a crank mechanism that imparts oscillatory movements to the movable cheek. The material is loaded into the crusher from above. When the cheeks come together, the pieces break apart. When the moving jaw moves away from the fixed one, the crushed pieces fall under the influence of their own weight and exit the crusher through the discharge hole.
Rice. 1 Crushers: a – jaw; b – conical; c – hammer; g – roller
Cone crushers They work on the same principle as cheek ones, although they differ significantly from the latter in design. A cone crusher (Fig. 1, b) consists of a fixed cone and a movable cone suspended in the upper part. The axis of the movable cone with its lower part enters eccentrically into the rotating vertical glass, due to which the movable cone makes circular movements inside the large one. When the movable cone approaches some part of the fixed one, pieces are crushed, filling the space between the cones in this part of the crusher, while in the diametrically opposite part of the crusher, where the surfaces of the cones are removed to the maximum distance, crushed ore is unloaded. Unlike jaw crushers, cone crushers have no idling, due to which the productivity of the latter is several times higher. For medium and fine crushing, short cone crushers are used, operating on the same principle as cone crushers, but slightly different in design.
IN roll crusher crushing of ore occurs between two horizontal steel parallel rolls rotating towards each other (Fig. 1, c).
For crushing brittle rocks of low and medium strength (limestone, bauxite, coal, etc.) hammer crushers, the main part of which (Fig. 1, d) is a rotor-shaft rotating at high speed (500-1000 rpm) with steel hammer plates attached to it. Crushing of material in crushers of this type occurs under the influence of numerous hammer blows on falling pieces of material.
Commonly used for grinding ores ball or rod mills, which are cylindrical drums with a diameter of 3-4 m rotating around a horizontal axis, in which steel balls or long rods are located along with pieces of ore. As a result of rotation at a relatively high frequency (~20 min -1), the balls or rods, having reached a certain height, roll or fall down, grinding pieces of ore between the balls or between the balls and the surface of the drum. The mills operate in a continuous mode - loading with ore occurs through one hollow axle, and unloading through another. As a rule, grinding is carried out in an aqueous environment, due to which not only dust emissions are eliminated, but also the productivity of the mills is increased. During the grinding process, particles are automatically sorted by size - small ones become suspended and are taken out of the mill in the form of a pulp (a mixture of ore particles with water), while larger ones, which cannot be suspended, remain in the mill and are crushed further.
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