CA2017079C - The leaching of finely milled materials - Google Patents

Abstract

A process is provided in which fine milling of source material containing leachable values is carried out whilst the source material is in contact with the leach liquor and any necessary oxidants. The leach liquor and source material are preferably fed upwardly, as a slurry, from the bottom of the mill to the top. The mill has grinding media particles therein having a density of at least 1Kg/~ greater than that of the slurry passing through the mill and an impeller having radiating arms operating at an arm tip speed and power density selected to achieve the desired results. The process is especially useful for the milling and leaching of gold bearing materials such as pyrite, pyrrhotite and marcasite.

Description

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THE LEACHING OF FINELY MILLED MATERIALS
FIELD OF THE INVENTION
This invention relates to a leaching process involving the simultaneous ~ine milling and leaching of subdivided source materials to recover a required component of the source material.
BACKGROUND TO THE INVENTION
The objective of carrying out fine milling is generally the recovery of metal values, or simply the production of a milled product, from a source thereof.
The source material may be in the form of a primary ore or mineral, a concentrate, a calcine, or reclaimed tailings (herein referred to collectively as "the source material"). In such fine milling techniques the particle size of such "source material" is substantially reduced in order to enhance dissolution of the metal values required in the particles of source material, or to render such source material suitable for some other purpose, such as the production of a sinter, or ceramic material.
It is well known that dissolution of metal values, in particular gold, from certain types of source materials such as pyrite, arsenopyrite, silica and other refractory types of materials, is enhanced by milling the material to smaller particle sizes, followed by subsequent leaching using an appropriate leach liquor, in the case of gold, usually a cyanide leach liquor. This is due to the fact that a portion of the metal, particularly gold, which is physically encapsulated, is released during fine milling, and made available for leaching. The finer the source material is milled, the more of such metal is released for leaching. Clearly, there comes a point where, due to the cost of milling, it is uneconomical to mill the source material any finer.
In spite of the fact that simultaneous grinding and leaching has been proposed previously (see U.S. Patent No. 4,269,808 to Kawabata) it has not been possible heretofore to economically treat certain materials. This applies in particular to certain precious metal containing materials, where grinding them to sizes I:

20~7p7g substantially finer than 20wm has met with little or no success. This is evidenced by the large amounts of material worldwide that have been discarded from conventional treatment plants and that could form economically viable feedstock for a further suitable recovery process.
It has now surprisingly been found that if milling and simultaneous leaching is effected under certain conditions, enhanced results are achieved.
SUMMARY OF THE INVENTION
In accordance with this invention there is provided a process for the leaching during simultaneous fine milling of a sub-divided source material (as herein defined) wherein the source material is fed, as a slurry, substantially continuously, together with a leach solution, to an inlet region of a stirred mill having an impeller with outwardly extending stirring arms, each of which has a tip at its free end, with said impeller rotating in a grinding chamber containing grinding media composed of a multitude of media particles, and withdrawing finely milled source material together with leach solution from an outlet region of the stirred mill spaced from the inlet region, the stirred mill being operated at a power density (being the power input in kilowatts per cubic metre of grinding media) of at least 100 kW/m', at a minimum impeller speed chosen to produce a product of the arm tip speed and the individual mean media particle mass (such product being referred to herein as the media particle momentum) of at least 0.15 gram-metres per second, and wherein further, the density of the grinding media is greater than the density of the slurry by at least 1Kg/~.
Further features of the invention provide for the media particle momentum to be at least about 0.20gm/s; for the liquid of the slurry being milled to embody any required oxidizing medium for assisting in leaching or oxidizing metal values and other constituents in the source material; for the grinding media particles to be spherical, cylindrical, polygonal or irregular shaped elements made either of steel or ceramic, in particular zirconia, materials; for oxygen or other gases required for v oxidizing or leaching to be introduced together with leach liquor at the first region of the stirred mill; and for the metal to be recovered to include gold in which case the leach solution is preferably a cyanide leach liquor.
The impeller may be rotatable either about a vertical or a horizontal axis and, in either case, the leach solution and subdivided source material are preferably introduced in a lower region and withdrawn from an upper region. This counteracts the tendency of the solids to sink to the lower region.
Preferably the process is carried out such that the particle size of the source material is reduced to 90% minus 20wm, preferably 100% minus 20~m and most preferably 90% minus 5 to 10~.m. Clearly a large proportion of particles will have considerably smaller sizes and applicants believe that particle sizes of 3wm or less may be achieved.
The flowrate of the leach solution will clearly be chosen to provide a desired residence time of the subdivided source material within the mill in order to achieve a required leaching of the metal values.
The fineness of both source materials and products will, hereinafter, be discussed with reference to the term dX where x is the percentage of the particles of the material which are smaller than a stated size. Thus a rating of dso =
20~.m means that 50% of the mass of the sample has a particle size of 20~,m or less and a rating of d9o = 20~.m means that 90% of the mass of the sample has a particle size of 20~m or less.
It is envisaged that the process of this invention will be particularly suitable for the recovery of gold from highly refractory ores and concentrates as well as from tailings from earlier extraction processes.

The invention can also be advantageously applied to processes wherein the material being milled is simultaneously subjected to the action of other chemicals, in particular oxidants during the milling process. One such application is to gold bearing pyrite, pyrrhotite or marcasite concentrates where milling releases iron and sulphur into solution. This would consume cyanide and, accordingly, the material is simultaneously oxidized, conveniently by the action of air or oxygen at the alkaline pH of the cyanide solution.
It is believed that the advantageous effects of the invention are achieved in consequence of rapidly changing the torsional and compressive stresses that act upon the particles of source material whilst they are resident in the mill, and in this way, the particles are fractured rather than broken. The fractures range in size from large permanent fractures to small transient fractures which disappear when the stresses applied by the media are removed. This phenomenon is called the "Rehbinder Effect". The fact that the intraparticle fractures are within a state of flux while the particle is resident in the mill has an advantageous effect on the rate of leaching, and the overall recovery of the precious or other metals, because, as the fractures open and close, there is the effect of "pumping" the lixiviant into and out of the particle, thus enabling the metal particles still trapped within the host particles to be dissolved.
For the above-mentioned effect to occur, the operative power density Kw per cubic meter of chamber volume has to be above the threshold level indicated above.
Thus the liberation of the metals from the host particle by the process is by a combination of size reduction and permanent and transient fracturing whilst the material is resident in the energy field of the mill. Also part of the metal is dissolved without actually being fully released.
The invention, and some embodiments thereof, will now be further described with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic illustration of a stirred mill to be employed according to the invention;
FIG. 2 is a schematic flow sheet of a pilot mill assembly used for test purposes; and, FIG. 3 is a graph showing the comparative results obtained with three different mill operating conditions.
DETAILED DESCRIPTION WITH REFERENCE TO THE DRAWINGS
As illustrated in Fig. 1 a stirred mill 1 comprises a grinding chamber 2 in which a substantially vertical shaft 3 of an impeller 4 having a series of outwardly directed stirring arms 5 is rotatable by means of a motor and gearbox 6. The grinding chamber is filled with grinding media 7 which may assume the form of spheres, cylinders, polygonal shapes or irregular shaped steel or ceramic grinding elements. In tests carried out to date, both steel and ceramic (zirconia) grinding balls (spheres) have been employed.
A slurry inlet 8 is provided at the bottom of the grinding chamber either through the bottom or through the sidewall thereof, the inlet being fed from a feed pump 9 with slurry, in use. If required, a gas inlet 10 can be provided, for example for oxygen, air, or any other gas, to assist in the dissolution process or assist in the reduction of the consumption of cyanide, or both.
The upper end wall of the grinding chamber is provided with a retaining screen 11 extending therearound for retaining the grinding media whilst allowing the milled slurry to leave the grinding chamber simply as an overflow. A
holding screen 12 is provided across the inlet 8 at the bottom of the chamber to retain the grinding media.
In tests carried out on a pilot scale grinding mill, the grinding chamber volume was 150 litres, the chamber being 450mm in diameter and 900mm high.

The holding screen 12 was provided with 2mm diameter holes and the retaining screen 11 had slots of l.2mm x 20mm in size. The grinding media was steel balls (although some batch tests were also carried out using zirconia balls) having a Rockwell hardness of 58C and a size distribution of 89% < 8.5mm diameter 9% < 3.97mm diameter and 2% < 2.38mm diameter.
There were fifteen pairs of diametrically opposite, radially extending arms of 400mm length offset angularly by 90° relative to adjacent arms along the length of the vertical shaft 3. Rotation was effected at a speed giving a tip speed of 2.6m/s.
The media momentum of the grinding media particles was 2.6gm/sec. The power density employed was 170Kw/m' and the density difference between the media and slurry was 6.45Kg/~. The mill contained 497Kg of balls having a bulk volume of 120 litres giving a net void volume of 49 litres.
For the tests carried out on certain gold bearing source materials, cyanide leach solution was used and Ca(OH)1 was used to maintain an adequately high pH.
For these tests the arrangement of Fig. 2 was employed wherein calcine (which had previously been leached with cyanide i.e. there was no freely teachable gold remaining) was fed from a hopper to a mixing vessel 14 and sodium cyanide, lime and water were simultaneously added. The slurry which had a solids content of 50% by mass was continuously fed to the stirred mill 15 described above.
Residence times of 15.7; 9.0 and 6.3 minutes were tested. The operating conditions were as follows:
Residence time, nominal, minutes Solids Water Pulp pH NaCn Kg/t Kg/min. P/min. B/min.
15.7 2.41 2.41 3.12 11.0 5 9.0 4.23 4.33 5.45 11.0 5 6.3 6.04 6.04 7.78 11.0 5 ,a~

20~~0~9 _7_ The leaching was continued for 24 hours after milling had been affected and after dilution of the milled product to 35% by mass solids.
The tests showed that from a calcine having a dgo, dso, and d,o of 76.0; 20.8 and 5.9.~,m respectively, the products had the size distributions- shown in the following table:
Nominal Gold dissolution Gold dissolution Res time Size distribution in mill after mill + 24hr d9o dso duo mins. mill ~.m % % leach 15.7 9.0 3.2 0.8 47.2 50.0 9.0 15.1 4.4 0.9 36.7 40.0 6.3 19.4 5.3 1.0 28.4 32.5 It will be seen that gold dissolution increases with residence time as would be expected. Also, good recoveries are made from a source material generally regarded as highly refractory, by far the vast majority of dissolution taking place within the mill in the relatively very short period of time for which the slurry was present in the mill.
The invention can be similarly used to fine mill existing dumps of treated ore and concentrates/products from treatment of such dumps to recover further values therein. In so doing about 50% of the residual gold has been recovered.
In the case of a coarse calcine, 73% of the gold therein was recovered by leaching during the grinding process.
In general, as indicated above, the particle size of the product is at least 95%
less than 20~.m with, as usual, a large proportion of the particles having a substantially smaller Y', - ~ size.
4'e~ s _g_ It has also been found that increased dissolutions are achieved when zirconia balls replace the steel balls described above. Zirconia balls have further advantages in that they do not consume cyanide (in the case of gold leaching) and are substantially more wear resistant than steel.
Simply as a comparative exercise, three different tests were conducted, the results of these showing the most significant advantages of the invention.
The tests are, in this description, labelled as Tests A, B and C.
Test A was carried out on a vibrating type of mill in which the grinding media was alumina balls having a mean diameter of 12.5mm. This mill was operated at a power density of 20kW/m' and the median particle mass was 5.22grams (in this case the impeller tip speed is irrelevant as no impeller is present).
The dissolutions obtained at four different residence times, at which four different media product sizes were obtained, are illustrated by Graph "A" in Fig. 3. It will be noted that a maximum dissolution which was under 35% was obtained at a median product size of somewhat over 5~,m. The difference in density of the grinding media and the slurry was, in this case 1.65Kg/~.
In Test B a stirred mill was employed using a grinding media of alumina balls having a mean diameter of about l.3mm. The impeller tip speed was, in this case, 11.73m/s and the media particle mass 0.0048, thus giving a media particle momentum of 0.05gm/s. The results of this test are shown by Graph "B" in Fig.

from which it will be noted that, in spite of the fact that a power density of 838kW/m' was used, very poor dissolutions were obtained in view of the extremely small median product size of between 1 and 2wm that were achieved in the one case.
In spite of such a small product size, only 40% dissolution was achieved. In this case the difference in density between the grinding media and the slurry was 1.65Kg/P.

In contradistinction, Test C was carried out using steel balls of an average diameter of 3.Omm and at a power density of 114kW/m3. In this case the impeller tip speed was 2.2m/s and the media particle mass was 0.1 grams thus giving a media particle momentum of 0.24g.m/s. The density of the grinding medium was greater than that of the slurry by 6.45Kg/P. In this case the dissolutions achieved were extremely satisfactory as shown by Graph "C" in Fig. 3.
Additional tests carried out in the same manner as Test C, except for the fact that the power density was changed, gave a precious metal dissolution of only 14%
at a median product size of 1.9~m and power density of 92kW/m3; a precious metal dissolution of 44% at a median product size of 1.9~cm but a power density of 148kW/m' and 51% dissolution at a median product size of 1.6~.m and a power density of 200kW/m'. This illustrates the distinct advantage achieved by employing a satisfactory power density coupled with the other stated constraints.
It is to be understood that the optimum conditions for any particular sub-divided source material may well vary quite widely and, accordingly, it is for individual requirements to be assessed through tests carried out within the scope for which applicant seeks protection.
The invention therefore provides a simple yet effective process for enhancing the recovery of metal values from source materials and, in particular, of gold from pyrite and other materials from which it is generally difficult to liberate.
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Claims (13)

1. A process for leaching and simultaneous fine milling of a subdivided source material wherein the source material is fed, as a slurry, substantially continuously, together with a leach solution, to an inlet region of a stirred mill having an impeller with outwardly extending stirring arms, each of which has a tip at its free end, with said impeller rotating in a grinding chamber containing grinding media composed of a multitude of media particles, and withdrawing finely milled source material together with leach solution from an outlet region of the stirred mill spaced from the inlet region, the stirred mill being operated at power density (being the power input in kilowatts per cubic metre of grinding media) of at least 100kW/m3, at a minimum impeller speed chosen to produce a product of the arm tip speed and the individual mean media particle mass (such product being referred to herein as the media particle momentum) of at least 0.15 gram-metres per second, and wherein further, the density of the grinding media is greater than the density of the slurry by at least 1Kg/~.

2. A process as in claim 1 in which the media particle momentum is at least 0.20 gram-metres per second.

3. A process as in either claims 1 or 2 in which the slurry being milled comprises an oxidizing medium.

4. A process as in any one of claims 1- 3 in which the grinding media particles are any one or more of spherical, cylindrical, polygonal or irregular in shape.

5. A process as in any one of claims 1 - 4 in which the grinding media particles are of steel or ceramic material.

6. A process as in any one of claims 1 - 4 in which the grinding media particles are made of zirconia materials.

7. A process as in any one of claims 1 - 6 in which oxygen required for oxidizing or leaching is introduced into the leach solution at the inlet region.

8. A process as in any one of claims 1 - 7 in which the subdivided source material contains gold and the leach solution is a cyanide leach liquor.

9. A process as in claim 8 in which the source material is one or more of pyrite, pyrrhotite or marcasite.

10. A process as in any one of claims 1 - 9 in which the impeller is rotatable about a vertical axis and the subdivided source material and leach solution are introduced to the mill in a lower region thereof and withdrawn from an upper region.

11. A process as in any one of claims 1 -10 in which milling is carried out such that the particle size of source material is reduced to 90% less than 20 µm.

12. A process as in claim 11 in which the particle size of the source material is reduced to 100% less than 20 µm.

13. A process as in any one of claims 11 or 12 in which the particle size of the source material is reduced to 90% less than 10 µm.