DESCRIPTION

Technical Field The present invention relates to a method for comminuting lumps of homogenous and/or heterogenous mineral material in an autogenous primary grinding system, with the aid of screening, crushing and grind¬ ing apparatus, in which the lumps of mineral material are crushed to a given largest fragment size and then divided into a given coarse ■ fraction, which forms the grinding mill charge of an autogenous primary grinding mill, and a given screened fragment size which is crushed to form a fine fraction.

The object of the present invention is to achieve maximum effeciency of comminution and minimum investment and operational costs in an integrate screeening, crushing and autogenous grinding system, in one or two stage

By mineral-material and material is meant here and in the following preferably ore minerals and industrial minerals:

Background Art

When processing a material, such as ore minerals and industrial minerals, in order to recover one or more of their valuable constituents, such as metal or industrial minerals etc, the material is normally disintegrated mechanically in an initial sub-operation. The main object of this initial mechanical disintegration is to liberate the valuable constituen from the material prior to subjection it to a subsequent separation process, in which the valuable constituents containe in the material can be separated in dependence upon differences in colour, shape, and density of differences in their surface active properties, magnetic properties or other properties.

Normally, the material is primarily disintegrated mechanically to a certain extent when it is blasted from the rock or cleft face, and then subjected to a series of further comminutin operations, which may take different forms. In the past, further crushing of the

of the maernal has normally been effected by crushing said material in a plurality of successive stages in jaw crushers and/or cone crushers, followed by fine grinding σf the material in rotary drums containing grinding media such as balls or rods, normally made of steel. Because of the hardness of the rock, however,, the grinding media are subjected to intense wear, with subsequent considerable costs.

In order to overcome this,, there has been developed over the years a technique in which thematerial itself forms the grinding media, this E τn:i-q_uebeing known as autogenous grinding.

The autogenous grinding technique has found wide use and is widely utilized the world over. Application of the autogenous grinding techniqu enables the extent to which the material is pri ariTy crushed to be imited to* a maximum lump size acceptable fm the aspect of transporta¬ tion. Consquently, the investment and operational costs of the crushers are relatively low. However, the absence of artificial grinding media having a high density in relation to the grinding ill charge, means that the specific grindability of the mill, expressed as grinding work/kWh* energy consumed, is decresed in comparison with commensurate mills in which grinding is effected with steel grinding media.

It is also known that the required power input of a drum mill when grind¬ ing, expressed in kW, is almost directly proportional to the density of the grinding mill charge media according to the relationship; p = k . *. q . n _c . L . D , where

p - power in kW

_ζ j - density of the grinding mill charge =? grinding media

k = mill constant

q = grinding charge, % by volume

n _c = relative mil l speed = actual mill speed criti cal mi l l speed

L = mill len h

It is axiomatic of the two latter factors (L, D) that the dimensions of the mill will be increased when the required power input increases, because of the increase in energy consumption, as compared with the case when grinding with highdensity grinding media; from which it will be seen that these factors increase the investment and operational costs of the autogenous grinding system.

In an autogenous grinding system, in which the grinding charge media is formed from the coarser and stronger parts of the actual material to be ground, the composition of the grinding charge formed is to¬ tally dependent on the properties of the material. Experience has shown that mineral deposits are seldom homogenous with respect to their structure and mechanical strength. Consequently, the hetero- genity of the material quite often causes the required input energy to vary, which in turn is greatly due to a naturally formed, unsuit¬ able particle-size distribution of the grinding mill charge. This is known to one skilled in the art as the "critical size" and it means on over-representation of certain particle-size fractions due to the incompetence of the material to create a satisfactory autogenous grinding mill charge.

It is also known to those skilled in this art that grinding of mate¬ rial in an autogenous grinding mill normally includes three com¬ minuting mechanisms, namely:

1. Impact grinding, which is highly effective from the energy aspect.

2. Attrition grinding, in which smaller pieces of material are squeezed apart between larger grinding media agents. Attrition is economical with respect to energy consumption.

3. Abrasive grindning, which although requiring more energy than 1) and 2) is of great significance to the process. In abrasive grindning fines are rubbed from the surfaces of the grinding media.

When approaching the "critical size, the impact phase of the grindning process, according to 1), no longer functions, and this phase transfer

to phase 3), thereby impairing the feed rate of a given mill. Thus, problems relating to "critical size" often require the grinding system to be excessively dimensioned, if a constant feed rate is to be maintained. Variations in the properties of the material to be ground also render it difficult to produce an autogenous grinding system of optimal design. Because of this, it often happens within the mining industry that autogenous grinding systems which have been especially planned and put into operation must later be converted to semi-auto¬ genous grinding systems using steel balls as grinding charge media . i.e. applying a semiautogenous technique.

As will be seen from the mi1*1-power formula above, when the feed rate of the material to be ground is constant, the power "p ^w and the charge volume "q" of the mill will change with varying grinding properties of the mill feed material i.e. there will be a change in the energy re¬ quired in kWh/tαn to effect grinding to a predetermined particle size distribution. It is known from the prior publication AU,B, 513,313 that the course taken by the grinding process is not only influenced by the physical properties of the material to be ground, but also by its mechanical composition, i.e. the particle size distribution of the feed.

DESCRIPTION OF THE PRESENT INVENTION

It has now been found possible to eliminate the great majority of the earlier disadvantages associated with autogenous grinding in primary mills, and also to provide the possibility of grinding material which has previously been considered incompetent for autogenous grinding. According to the present invention, the material to be ground is crushed and screened into two fractions; a coarse fraction for forming the grind ing mill charge, and a fine fraction comprising substantially the mill feed part, in which the relationship between the size of lumps at Kg--, whereby K _g5 denotes a point in the fraction distribution, where 95% by weight of the fraction is smaller than the given particle size; in the coarse fraction and the largest lump size of the fine fraction is char¬ acterized by the fact that the largest lump size of the fine fraction is limited by and determined by an intersection point of the tangents through the points of inflexion situated on each side of the "knee" on the size distribution graph of the grinding mill charge of said material when autogenously grinding the material; that feeding of the

coarse and fine fractions is regulated in a manner such that a) the amount of material charged to the mill is sufficient to maintain a given set-point value with regard to the required power input of the mill in question, or a given feed rate therethrougbi a ^{nα D} ) ^the primar ground mill discharge has been ground to a preselected degree in dependence upon firstly the extent in question to which the respective fractions have been crushed and secondly the mass distribution between the coarse and fine fractions in the material charged to the mill; and that the smallest particle size of the coarse fraction exceeds the lump size represented by the upper of said points of inflexion.

In conjunction with the present invention, it has surprisingly been found that a plurality of process parameters essential to the autogeno grinding process can be pre-determined and controlled. By grading the material to be ground and the grinding media in a pre-determined fashio in accordance-with the invention, the ground material leaving the autogenous grinding mill can be given a pre-determined particle size distribution, within wide limits, and the energy input, i.e. the grind¬ ing effeciency, can be considerable improved. Furthermore, in this way the. magnitudes of energy requirement (kWh/ton, fee rate (tph), and particle-size distribution in the mill discharge , these magnitudes normally varying greatly in conventional autogenous grinding processes, can be stabilized to a level which is extremely advantageous from the _. process aspect. With thought to the subsequent process steps of second- ary grinding and separation processes, it is extremely desirable to maintain uniform feed rate and particle size distribution.

Prior to the final grinding stage, which is often necessary in order to enable the subsequentseparation process to be carried out satisfactoril the primary grinding stage is normally followed by a further, so-called secondary grinding stage. In autogenous grinding processes, the seconda grinding stage is performed in a pebble mill in which the grinding char media comprises pebbles of suitable size fraction extracted from tire primary mill. The material to be ground is given its final pa t cle* stz distribution in the secondary grinding ^* stage; this stage being consider abl cheaper ^* to carry'out i.e. it can be effected to ^* 'higher grinding

U RE ^"

efficiency than the primary autogenous stage. Consequently, in order to achiea/e the lowest possible process costs it is important for the mill discharge of the primary autogenous grinding stage to obtain the coarsest possible particle-size distribution and, also to achieve a uniform feed rate.

The present invention enables an autogenous grinding system to be dimensioned anddesrgned right from the planning and pilot stages, form optimal utilization of the advantages afforded by autogenous grinding and to obtain, in operation, a communiting process which is highly superior to conventional crushing-grinding systems from a technical and cost aspect.

In this respect the invention relates to a method comprising the pre- treatment of a material precrushed to a largest lump size, in which the mater al is screened to form three fractions, the .coarsest fraction, possibly after being stored, being charged in the requisite amount to the mill as the grinding media and to form the grinding mill charge. The inteππidiate fraction of the aforesaid screened material is crushed to a given particle size in accordance with the invention, this particle size being rεfeinaiced ^* ICjg i.e. 95 % by weight of the fraction is smaller than the given particle size, and is mixed together with the third, fine fraction of said screened material , said fine fraction being screened to the same g ven Kgg particle size as the intermediate fraction. The fine fraction may be stored before being used.

The resultant coarse and fine fractions respectively, are autogenous grinding mill in a fixed ratio, normally 10-25% of the coarse fraction and 90-75% of the fine fraction. The ratio between the fractions is dependent upon the largest size of the lump material to be ground before the pre-chrushing operation, as well as the grinding properties of the material and pre-determined requirements with, respect to the mill dis¬ charge, said ratio being determined empirically with respect to said factors.

In accordance with the invention, in order to obtain maximum grindabilit and, furthermore, the desired degree of fineness of the mill discharge,

the pre-tre ted mixture of coarse and fine material fed to the mill is charged at a given ratio with respect to the properties of said materi and the desired final product from the primary autogenous grinding mil When grinding a given mineral material, pre-crushed to a selected parti size and having a naturally funned particle size distribution, 100 *<t in this way selected largest particle size, a certain particle size dis bution, of the grinding mill charge, is obtained at grinding in an auto nous grinding mill. A typical example of this is shown in Figures 1-2, which are size distribution graphs for mill charges to an autogenous gr ing mill. The graphs each show a part which is characteristic of screen ^w curves, namely the right, steep part of the curve having a continuous distribution towards finer fractions, down to a given particle size which in the illustrated case meet about a break point on the screenin graph which can be defined as a point in the screening graph where two tangents drawn through the inflexion points lying nearest the break po of the screening graph meet, namely an inflexion point located on the right of the steeply rising part, and one located on the next horizont left part of the screening curve shown in the graph. The points of in¬ flexion are situated on each side of the so called "knee" on the size distribution graph, (P.H. Fahlstrδm, 1974, Autogenous Grinding of Base Metal Ores at Boliden Aktiebolag, presented at the 75th Annual General Meeting of the ^* CIM, Vancouver, April 1973). The point at which the tangents intersect represents a point which can be defined as the break point of impact for the grinding mill charge in question. Said break point is a term used in grinding techniques, and can also define the particle size of the material produced by the impact grinding operation i.e. the largest particles are in such relationship to the average par¬ ticle size of the grinding mill charge that those particles belonging to the fine fraction, when entering the mill, are rapidly broken down by impact to particles smaller than, or equal to, the size represented by the left, more horizontal part of the screening curve, i.e. a par¬ ticle size of about 1 mm. In this respect it is ensured that the degree of the material (=Kg ₅ ) which is to be reached for the fine fraction of the material entering the grinding mill does not exceed this break poin The material discharged from the primary ^' autogenous grinding mill has now been preground to such an extent that it is well suited for final grinding in a secondary pebble mill, the grinding media of which can be taken, to advantage, from the primary grinding charge by means of pebbl

extraction described and illustrated in Swedish Patent Application 7909921-4. It will be understood, however, that a conventional bal l mill can be used instead of a secondare pebble mill .

As will be seen from Figure 1 , the break point can be moved in paral l on the screening graph, when pre-crushing of the coarse material is displaced. Fi ure Z illustrates the case where the materi l has been precrushed to a Kg ₅ particle size of about 150 and 300 mm respectivel In this case, the break point of iπpact, in respect of the same mater can be determined to Kg ₅ about 25, and 5Gπsn respecti vely, depending o the degree of crushing for the coarse fraction.

In the method according to the invention, however, the location of th given, break point is only critical upwardly. The fineness of the prim mill discharged can be controlled within wide limits, by a proper sel tion of the parameters relating to the quantity and size of the coars fraction relative to the fine fraction. In addition, an autogenous grinding ci rcuit comprising at least two stages can be controlled in a manner to utilize the circuit optimally and to achieve an optimum cost situation, substantially independent of the grinding properties the material , such as hardness, structure, ho ogenity. The smallest particle size of the coarse fraction exceeds at least the particle size represented- by the upper one of said inflexion poiηfe. The smalles particle size of the coarse fraction is -normally about 4 - 7 times the largest particle size of the fine fraction, while the lowest particle weight of the coarse fraction is 20 - 3 times the heaviest particle weight of the fine fraction. Thus, the method according to the inventio will always provide a better over al l economy- than conventional ^{• •} autoge ous, grinding techniques, besides affording particul ar advantages in th case of materials which are extremely uneconomical or technical ly in¬ competent for use with conventional autogenous grinding techniques.

As a typical example of the potential of the invention, two ores were selected and tested on a pilot scale. The first is il lustrated in Table which shows the result obtained with a coarse-grain quartzite, which also exhibits extremely good properties for conventional autogenous RE

grinding techniques. Table 2 shows the result obtained with a fine- grain complex tuffte, the properties of which render it unsuitable for autogenous grinding techniques.

5 Table 1

Conventional Technique

Autogenous according to

Grinding the invention

Feed rate, tph 4.1 6.9 + 68 %

10 Mill discharge . % "44 microns 29.0 eι..4 - 26 .%

Energy, kWh/t 9.5 5.4 - 44 %

15 Grindability

. kg/kWh <44 microns 26.1 33.1 + 27 %

Table 2

20 Feed rate, tph 1.60 3.46 + 116 %

Mill discharge . % <44 microns 64.4 42.1 - 35 %

25 Energy, kWh/t 36.6 15.8 - 57 %

Grindability

. kg/kWh <44 microns 17.0 24.2 ÷ 42 %

30 Thus, it will be seen from the Tables that, inter alia, the grinding effciency when- grinding i n accordance with the invention as compared with grinding using conventional autogenous grinding techniques is 27 % better for a material according to Table 1 and 42 % better for a material according to Table 2, and that the mill discharge contains

35 far less material <. 44 microns , which shows that the primary mil led product has contained the desired coarser fraction prior to the secondar grinding stage.

Preferred method of carrying out the invention

The invention will now be described in more detail with reference to the aforementioned drawings 1 - 3, and to a schematic flow diagram of a preferred method according to Figure 4.

The plant illustrated schematically in Figure 4 comprises firstly means for pre-treating the material, including a crusher 10, a screen¬ ing and crushing arrangement 11-12 and storage means for two separate fractions, a grinding plant comprising feeders 15, 16 which are program for control from a control unit 20, two belt weighers 17, 18, a primary and a secondary autogenous grinding mill 21 , 22,. a classifying (equipmen apparatus 23, and transducers 19 and 24.

The fragmented, large-! ump material is crushed to a given fragment size in the crusher 10, whereafter the material is divided into three fracti on a screening apparatus 11. The coarsest of the three fractions is deteππinded. by the predetermined coarsest fragment size from the crushe and by an undersize determined, inter alia, by the fraction .range suit- able for each particular ore ype. The intermediate fraction, which is determined downwardly in accordance with Appendix -1, is crushed in th& crusher 12 to the same Kg,- particle distribution as that of the fine fraction obtained from the screen 11 , and the charge of coarse and fine materials, respectively to the mill 21 is effected in accordance with a separate programmed process ^" model , from a microprocessor in the contro unit 20, the input data for said processor being obtained from the belt weighers 17,18 and the. transducer 19.

The energy input to the secondary-grinding process is regulated through the mill 22., the grinding mill charge of which is taken from the mill 2 with an automatically functioning grinding pebble extractor in accordan with ^* Swedish Patent Application 7909921-4, and is dependent upon the properties of the material in question.