Grinding

Wet Autogenous Grinding in Tumbling Mills

The autogenous grinding technique has been progressively employed in mineral dressing crops over the past 15 years. The term has been used with considerably totally different meanings. In this paper the writer will use it in probably the most restricted means, for processes where run-of-mine ore (rock) in one step is decreased to a grind product by the action of the ore itself with out another grinding media present. The pebble grinding course of the place the main part of the ore enters the mill as finely crushed product which is floor further with selected coarse ore items appearing as grinding media, is just not thought-about autogenous grinding. Processes the place a minor quantity extraneous media as steel balls is added to supplement an autogenous mill cost can be referred to as semiautogenous.

Autogenous Grinding Mechanisms

Autogenous grinding is at present carried out in tumbling mills of the same varieties as used in ball mill grinding. The similarities in gear and procedure have led to the generally held assumption that the grinding mechanisms are the same in the two processes, with the coarse a part of the ore cost taking up the perform of the ball cost. This view is essentially right when considering pebble grinding, however the state of affairs is one other in the true autogenous grinding, where run-of-mine ore is floor positive without any intermediate crushing.

In ball mills relatively small ore particles are floor high-quality by the tumbling motion of a charge of bigger metal balls. Comminution in ball mills is often described as happening by “impact” and “attrition” grinding. When the ore particles are crushed by direct hits from falling balls we get influence grinding, whereas attrition grinding is extra loosely outlined because the comminution brought on by the sliding and rolling motion of the balls in the shifting charge. In both instances the comminution is brought on by stressing the ore above the breaking power restrict, which results in cracking of the ore particles. In addition to the cracking some abrasion takes place where particular person grains or grain fragments are torn unfastened from the ore particles by frictional forces. In ball mill grinding abrasive circumstances are often prevented owing to the associated excessive wear fee of the grinding balls.

In present autogenous grinding abrasion is often the primary discount mechanism, and the impression and attrition processes might even be detrimental to the grinding operation. The ore feed to an autogenous grinding operation sometimes consists of ore pieces from 10″ measurement and downward with 50% coarser than three – four″ and solely a small amount, say 10 – 20%, finer than 1″. The coarser ore items may be lowered either by impression cracking or by abrasion, and solely the best half is floor by attrition between the larger items by the identical actions as in ball and pebble grinding. Because the finer materials is just a minor part of the whole this “conventional” grinding is less necessary.

During autogenous grinding in a tumbling mill the ore items are rubbed towards each other and progressively worn down by abrasion until they are sufficiently small to be caught between the larger pieces. Observations on equilibrium ore costs from working mills present a typical progress in this process relying upon measurement of the ore pieces and the time they have spent in the mill. The ore feed from the coarse crushing plant consists of jagged irregular pieces, sometimes with 20 – 50% coarser than 4″ and 10 – 20% finer than 1″. The unique pieces of +1″ materials are rapidly rounded off, first by chipping off the outstanding factors and edges, and then by further smoothing of the surface. Because the rounding- off proceeds, the dimensions of the chips and the wear and tear fee decreases quickly giving a progressively transfer to the second stage, true abrasion of the rounded items (pebbles). The quantity of fabric removed in the first stage will often be 10 – 20% of the original piece weight, and will probably be in the type of chips and particular person mineral grains.

In the subsequent stage the rounded pieces are steadily lowered in measurement by a gentle removing of mineral grains from the surfaces until the items are sufficiently small to be “nipped” between the bigger pebbles present in the charge. This motion takes place when the items are decreased to about one tenth of the dominant feed measurement, often in the order of 10 mm (½ inch). It also relies upon to a degree upon ore power and surface roughness. The entire amount lowered by abrasion through the second stage will run from 60 – 90 % of the feed, and the product will include mainly particular person mineral grains and grain fragments.

When the pebbles are sufficiently small to be “nipped” the method enters the third stage, here referred to as “Conventional” grinding. The small pebbles along with the original fines in the feed and the abrasion products from the two first levels are attrition floor between the bigger pebbles before being swept out of the mill. The attrition product may have a distribution just like the standard ball and pebble mill products. It can amount to 20 – 50% of the combined mill product relying upon grinding circumstances.

The steady-state abrasion fee for like surfaces beneath autogenous grinding circumstances will mainly rely upon following elements:

  1. Hardness, cleavability, plasticity of mineral grains in ore.
  2. Measurement and shape of mineral grains.
  3. Bond power between grains.
  4. Bulk elasticity of ore.
  5. Topography and curvature of contact areas.
  6. Strain at contact areas.
  7. Differential tangential velocity for surfaces in contact.
  8. Medium present in contact area (Air, water, pulp, a.o.).

The elements mentioned above will decide the (tangential) frictional forces between the contact surfaces and thus the power obtainable for performing the abrasion work.

For aircraft surfaces of elastic rock materials composed of equalsized exhausting mineral grains in a weaker bond matrix, the abrasion price is ideally proportional to contact drive Fc and differential contact velocity Vc and inversely to contact area ac.

∝ = Okay · Vc · Fc/ac = Okay · Vc · Laptop……………………………………(1)

where the typical contact strain laptop equals Fc/ac. The idea is that the surfaces make contact at quite a few discrete factors and that the number of contact points improve linearily with strain.

Aside from ore or rock characteristics, the kinetics of the mill cost are most essential for the abrasion price in autogenous mills. The movements of the charge in a tumbling mill depend primarily upon mill velocity, diameter and cost quantity. Liner profile, measurement distribution of mill charge, and inner friction in charge are secondary elements. In the first strategy we disregard them and assume a no-slip liner, evensized spherical pebbles much smaller than mill diameter and fixed frictional circumstances. The mill cost might be expressed as “charge number” M indicating the fraction of the mill volume occupied by the charge. For regular operations M shall be in the area zero,3 – 0,four (30 – 40%).

Both concept and experiments have shown that the charge movement pattern is decided by a dimensionless “speed number” C which equals the sq. root of the quotient between centripetal and gravitational forces appearing on the pebbles on the mill shell:

the place n = mill velocity in rpm, D = inside mill diameter, and “f” is the slip factor indicating the lag between mill charge and shell. For the no-slip case f equals zero. 100 x C is then the “critical speed” of the mill in %. For autogenous mills C is often in the range zero,6¬0,9 (60 – 90%).

For C values as much as 0,70 the motion of the pebbles will probably be “cascading” that is when reaching the high aspect of the mill, the pebbles roll and slide downwards whereas retaining in contact with the rest of the cost.

When C will increase past 0,70 the charge will begin “cataracting” with an growing number of pebbles thrown away from the remainder of the charge for some part of the cycle. Throughout this time clearly no abrasion can take place, and the kinetic power is expended as influence when the pebble once more hits the charge or the shell.

The expressions which were derived to date have been based mostly on a cost assumed to include equalsized pebbles This is not the case in actual autogenous grinding, and subsequent we must contemplate the impact of a cost of combined pebble sizes. Quite a few equilibrium pebble (and ball) costs have been investigated and the outcomes point out that the linear abrasion charges shall be roughly the same for all the pebbles in a given cost apart from the surplus put on in the rounding-off interval. The dimensions distribution of the equilibrium pebble cost over a lot of the measurement vary then shall be conform to and linearly displaced from the dimensions distribution of the mill feed. Within the advantageous measurement vary the cost will show a robust depletion as soon as the third stage attrition grinding takes impact. The overall result is a preponderance of pebbles in the dimensions comparable to 50 – 80% passing for the feed and each the contact velocity vc and the contact strain laptop ought to primarily depend on the pebbles in this measurement range. As an approximation the d50 for the feed might be taken because the relevant pebble measurement for mixed-size expenses, and used as pebble diameter d in the expressions developed for vc and laptop for evensized pebbles.

One other factor that have to be taken in account is the segregation in the mill charge. If all pebble sizes have been distributed randomly inside the charge the contact forces between pebbles also needs to be distributed randomly around a worth decided by the typical pebble measurement.

For a segregated cost the totally different pebble sizes will probably be kind of concentrated in certain areas, and the contact forces will mainly depend on the typical pebble measurement in the world. In autogenous mills the coarse pebbles normally segregates radially in the direction of the periphery leaving an excess of effective pebbles in the higher central part of the charge. The forces and pressures right here will are likely to develop into very low because of the mixed results of small pebbles and shallow depths.

The segregation no matter the place and how it takes place also could have a unfavourable effect on the abrasion grinding by inflicting a selective depletion of the bigger pebbles in the charge.

Impression cracking in tumbling mills has two causes, either the blow of the protruding elements of the mill liner towards the ore items in the charge or the influence brought on by flying balls or pebbles when hitting the cost or the shell liner. The first operates each at cascading and cataracting speeds, the second only at cataracting.

The subsequent query that arises is how the cracking incidence is said to the impression power delivered to a pebble. The velocities might be in the vary from 1-5 m/second, and it is recognized from influence crushers that velocities in the vary from 10 – 50 m/sec. are needed for environment friendly crushing of ordinary ores and rocks. Items with inner flaws might crack extra easily, but fairly competent and homogenous materials should not be damaged by single blows of this depth. Repeated blows are another matter and as it’s recognized that brittle supplies often have very low fatique strengths, cumulative blow results are in all probability the primary reason for cracking. The blow frequency is in the order of 1-5 blows/min. and as residence occasions for individual pebbles are 20 minutes and upward to several hours, the whole blow number may be appreciable.

Operational Features

The outcomes obtained from operating and testing autogenous grinding installations strongly indicates that abrasion grinding is the primary operative mechanism, and that the important thing to success in this area is to create circumstances favouring abrasion.

With typical crushing and grinding gear literally any rock or ore materials may be decreased to any desired fineness.

The elemental limitation of the autogenous grinding process is that the structural characteristics of the material to be floor are the overriding elements determining both the general applicability of the method and the range of product finenesses it could actually furnish. In depth testing and careful design is subsequently needed to guarantee successful autogenous operations.

The perfect feed to an autogenous grinding operation is an ore with a maximum amount of coarse pieces around 75-150 mm three″- 5″) measurement. The ore construction should include exhausting isometric grains bonded collectively by a weaker matrix material. Nevertheless, the power of the matrix ought to be enough to face up to the cumulative effect of influence blows in the mill for even the most important ore items present. An ore with such characteristics ought to put on down by abrasion till the pebbles are small enough to be decreased by attrition.

Adjustment of an autogenous grinding system to a finer product than the optimum may be completed through the use of circumstances favouring attrition. Grinding in closed circuit with cyclones or screens will work in this path, and the product measurement will often decrease with growing circulating load. But at the similar time both mill capacity and grinding efficiency will drop resulting from reducing abrasion action. At very high circulating masses the amount of residing pulp might get so excessive that it reduces both the abrasion and attrition action. Grinding in closed circuit subsequently have a limited effect on product fineness, and grind effectivity are likely to drop when circulating masses improve past 100%. Probably the most successful autogenous operations often restricts the closed circuit to scalping off and returning only a small amount of stray oversize.

The cracking phenomenon is the main problem one has to deal with in autogenous grinding, however it is seldom that the state of affairs gets out of hand, especially if the problem has been thought-about in the course of the design of the circuit. The primary trigger for hassle of this type is ores with extreme slaty or fractured buildings.

Giant variations in operating results as a result of modifications in mill feed high quality is often probably the most significant issue in autogenous grinding. In some instances the difficulties are as a consequence of geologic options of the orebody. Altering mineral composition and grain measurement is one disturbing issue, one other is variations resulting from oxidation of a part of the orebody. The last influence has been very disturbing for some installations in tropic or subtropic climates. Comparable difficulties could also be traced to the mining circumstances the place the mode of breaking and dealing with the ore can result in variations in the dimensions distribution and the power of the ore delivered to the mill.

Whatever the blending steps taken, the mill feed will all the time show qualitative short-term fluctuations, which in some instances might cause the working outcomes to fall outdoors the allowed limits. Some sort of mill circuit management then shall be mandatory, and numerous totally different regulating schemes are in operation or have been tried. The requirements of the purchasers or the following processes on the product are often such that a secure fineness of grind in the circuit is one objective. Depending upon circumstances the subsequent control goal is to acquire either fixed throughput always or the maximal throughput that the circumstances at every time will permit.

At larger costs the relation to power departs from straight proportionality. With growing mill cost the facility will go through a most, often at M between zero,45 and zero,50,and then decrease once more, and the grinding capacity will comply with the facility. The cause for this conduct is that the cost movements are impeded when the mill fills up. This impact will additional lower the abrasion price and in this manner enforce the charge swelling brought on by a lower in ore grindability. The mill will subsequently unfastened its self-regulating, property and turn out to be instable in this area. The tactic of setting a hard and fast feed fee and depart the remaining to the mill subsequently solely will work out nicely under the mill’s maximal capability. One other management technique that steadily is used for autogenous mills, is to manage the feed so to keep a continuing energy load, as near the maximum as is deemed protected with respect to the ore modifications. The charge is then stored within a slender range which provides better utilization of mill capability, however the feed fee and to some extent the product measurement may fluctuate appreciably.

Probably the most promising attack on the regulating drawback is to incorporate the feed measurement distribution as a managed variable. This technique has been tried at three or 4 crops, partly for regulating and partly for blending purposes, however often have been discarded on account of operational troubles. Probably the most profitable software is at Cobar Mine in Australia the place the ore is fed in two measurement fractions at managed charges. On this method they have been capable of even out both the whole feed fee and the mill energy draft and thus stabilize the operation.

Wet autogenous grinding is crucial comminution improvement for mineral dressing purposes because the breakthrough of the ball mill. The elemental mechanisms of the method are imperfectly recognized and research in this course will definitely additional using the tactic materially. The results from working mills and pilot exams strongly factors to abrasion grinding as crucial and favourable mechanism, while impression cracking is a detrimental impact and in instances a limiting factor for the appliance.

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wet autogenous grinding in tumbling mills