GB2157975A - Comminuting mineral containing ore - Google Patents

Abstract

The apparatus consists of an autogenous mill (10), an ore sorter (18) and means (12, 20) for feeding pebbles from the mill to the ore sorter (18) and from the sorter (18) back to the mill (10). The method includes the steps of feeding the ore to be processed to the mill (10), grinding the ore in the mill (10), removing pebbles in a specific size range from the mill (10), sorting the removed pebbles into fractions above and below a predetermined mineral characteristic level by means of the ore sorter (18) and feeding the pebbles above the predetermined characteristic level back into the mill (10) for further grinding. The sorter (18) may comprise a radiation detector 28 operatively linked to an air blast valve 30. However the sorter may operate on the basis of conductivity, magneticity, colour or like characteristics of the pebbles. <IMAGE>

Description

SPECIFICATION Ore Processing This invention relates to a method of and apparatus for processing mineral containing ores.

In any ore dressing process the milling circuit is the most expensive step in the process in respect of both capital and operating costs. Autogenous milling is currently a popular method of milling. In this process the ore is fed into a mill where the smaller and more friable ore particles are comminuted by the larger hard rock particles. As the grinding operation in the mill progresses with the finely comminuted material being removed from and run of mine ore being introduced into the mill a substantial body of pebbles in a fairly narrow size range, which is specific to a mill of a particular size, builds up in the mill.

These critically sized pebbles consist predominantly of barren or low-grade material which adversely effect the productive capacity and so efficiency of the mill in terms of both energy requirement and grade. One method of dispensing with the pebbles is to introduce steel grinding balls into the mill. This method of removing the pebbles is, however, costly, as greater power is required to operate the mill and wear of the mill liners is increased. Yet further disadvantages to this method are that the steel balls wear and are expensive, the larger grinding particles in the mill are broken and the grade of the mill output is reduced by comminution of the predominantly waste critically sized pebbles.

It is the object of this invention to provide a method of and apparatus for processing the ore more efficiently than known methods.

A method of processing mineral containing ore according to the invention includes the steps of feeding the ore to be processed to a mill, grinding the ore in the mill, removing pebbles in a specific size range from the mill, sorting the removed pebbles into fractions above and below a predetermined mineral characteristic level, and further grinding the pebbles above the characteristic level.

In one form of the invention the predetermined mineral characteristic of the pebbles which have been removed from the mill is measured by detecting a physical characteristic such as the radioactivity, conductivity, magneticity, colour and the like of the mineral in the pebbles and separating those pebbles having a measure of the desired mineral characteristic above the predetermined level from the remainder of the pebbles.

Apparatus for processing mineral containing ore according to the invention includes a grinding mill, means to feed ore to the mill, means to exhaust pebbles in a predetermined size range from the mill, and an ore sorter for measuring a predetermined physical characteristic of the mineral in each pebble and means for sorting the pebbles which contain a measure of the characteristic above a predetermined level from the remainder of the pebbles.

The invention is now described by way of example with reference to the drawing which is a schematic diagram of a plant for exercising the process of the invention.

The apparatus of the invention is shown in the drawing to include an autogenous mill 10, a feed conveyor 12, a 25 mm vibrating screen 14, a 1,2 mm vibrating screen 16, a sump 17 which is located below the superimposed screens, a radiometric ore sorter 18, a bucket conveyor 20, a slurry pump 22 and a hydrocyclone 24.

The conventional discharge grates in the periphery of the mill are replaced by pebble discharge ports.

The ore sorter 18 is a radiometric sorter similar to those described in South African Patent No. 80/7438, British Patent No's. 982,763 and 855,678 and American Patent No. 4,194,634 which detect the ratio active omission of ore particles and sort the particles into accept and reject fractions above and below a predetermined radiation level.

The sorter 18 is shown in the drawing to consist of a conveyor 26, a radiation detector 28, an air blast valve 30 and accept and reject ore bins 32 and 34.

The ore to be processed in this embodiment of the invention is gold bearing and carries tracer elements of uranium.

In use, run-of-mine rock 38 is fed to the conveyor 12 and the mill 10 at a predetermined rate. The rock in the mill is comminuted autogenously in the conventional manner with the critically sized pebbles and smaller fines being discharged from the pebble ports onto the screen 14. The 25 mm+pebbles and particles are conveyed by the screen 14 and another suitable conveyor to the sorter 18. The 25 mm-particles gravitate through the screen 14 onto the screen 16 with the 1,2 mm-particles passing through the screen into the sump 17 from where they are pumped away either as final product or to the hydrocyclone if further classification should be required. The 1,2 mm+particles which are trapped by the screen 16 are fed to the bucket conveyor 20 for reintroduction to the mill via the hydrocyclone 24.

The bulk of the ore particles which are trapped by the screen 14 are predominantly barren hard rock pebbles which have been critically sized by the mill 10. The pebbles are fed by the conveyor 26 past the radiation detector 28 where pebbles which emit radiation above a predetermined level are identified and later blasted from the natural discharge stream from the conveyor 26 by the air valve or valves 30 into the accept bin 32. From the accept bin the mineral rich pebbles are fed to a crusher 40 where they are crushed from the critical size and fed with the particles from the screen 16 back to the mill via the bucket conveyor.

The reject fraction of the pebbles from the sorter 18 is fed to the reject bin 34 and from there to waste.

As mentioned above, the critically sized pebbles are composed largely of hard barren rock with only a small percentage containing sufficient uranium and consequently gold to justify crushing and further milling. This elimination of a large quantity of barren material from the mill significantly reduces downstream handling costs of the mill output and has shown in tests to provide an increase in mill capacity of nearly 50% and an energy saving in mill operation of as much as 23%.

The purpose of the tests was to evaluate the effect on milling efficiency by the removal of the predominantly barren critically sized pebbles from the mill during the milling operation. The tests were conducted with an Aerofall autogenous mill which had flat end liners, a diameter of 1,7 m and a length of 0,5 m. The plant was well instrumented for the testwork. Power was recorded continuously and spot checks were made by accurately timing the rate of rotation of the power meter disc. Temperatures, water flow rates and oil pressure of the mill bearings were constantly monitored. 12 test runs were made using a sample of ore from the Beisia mine in South Africa.

The first series of test runs (1 to 4) were made as a base test for the following tests. In the first series of tests the mill was operated normally with the mill discharge being through two peripheral discharge grates each having six 140 mm long slots which were 10 mm wide. A product of approximately 65%75,um was required and a 1,2 mm screen was used for classification.

The first run was used to determine an indication of the feed rate required to reach steady state at a reasonable power level.

For run 2 the feed rate was set at 350 kg/hr, and the mill was stopped when it was approaching steady operation. The mill load was dropped and sized, to show that the total mass of rock in the load was 908 kg., of which 68,4% was critical size material (25 to 60 mm).

Run 3 was a continuation of run 2.

In run 4 steady state was reached, and the mill load had settled down to 814 kg. rock, of which 63,6% was in the critical size range. It is evident from these runs that there is no difficulty regarding the build-up of critical size material.

For runs 5 and 6 one of the discharge grates was replaced with a single pebble port which had two 60 x 60 mm holes for the removal of pebbles. The classification set-up used in runs 1 to 4 was kept unaltered for direct comparison of the products. In run 5 a feed rate of 800 kg/hr proved too high-steady operation was reached in run 6 at a feed rate of 700 kg/hr. The mill load at steady state was 881 kg., and occupied 42% of the mill volume. The fraction of critical size material in the mill load was reduced to 48,6%, due to the removal of pebbles from the mill at a rate of about 140 kg/hr.The effect of this change in the size distribution of the load was to make the product considerably coarser: 30%--75 pm, compared to 58%--75 pm for run 4. In terms of energy consumption per ton of -75 pm material produced, the installation of a pebble port resulted in an increass from 30 kWh/ton-75pm (run 4) to 35 kWh/ton-75 pm for run 6. However, this was probably an unreasonable basis for comparison-the use of the expression "kWh/ton-75 pm produced" as a means of comparing milling efficiencies is only valid when products of a similar size are considered.The product obtained from run 6 was too coarse to be applicable to normal production scale operation, so this test was followed by attempts to produce a fine product.

For run 7 the feed rate was set at 450 kg/hr with a 100 mm hydrocyclone being used as the final classifier. Runs 8 and 9 were used to achieve steady state operation with this circuit. The product from the cyclone was somewhat finer than desired (92%-75 pm), but it was felt that this would be more relevant than the results obtained from run 6. At 450 kg/hr feed, the mill reached steady state at a load of 663 kg. rock (30,3% of the mil volume), and pebbles were produced at 78 kg/hr.

After the very fine product obtained from run 9, it was necessary to repeat the base test of run 4 (no pebble ports) using the same cyclone classification system as in run 9. Steady operation was reached in run 10 at a feed rate of 300 kg/hr, to generate a product very similar to that obtained from run 9 (92%--75 Cim).

The similarity of the product size meant that it was reasonable to compare directly the kWh/ton of -75 pm material produced. The tests showed that by altering the size distribution of the mill load by using one pebble port the efficiency of milling was increased from 22,1 kWh/ton--75 Cim material with no pebble ports, to 18,9 kWh/ton-75 pm with one pebble port.

As a final test of the effect of pebble ports on the autogenous milling characteristics (runs 11 and 12) both 10 mm discharge grates were replaced with pebble ports, so that the discharge arrangement in the mill consisted only of four 60x60 mm holes. The classification circuit was kept the same as in runs 9 and 10.

Steady operation was reached at 600 kg/hr feed, and pebbles were produced at 157 kg/hr. The product was a little coarser than in runs 9 and 10, at 82%-75 pm, but the production of -75 pm material was even more efficient than run 9, at 16,9 kWh/ton of -75 pm material produced.

The data from the above tests is summarised in the following table:-

Critical Feed l l | Circ. Size % Product kWhlton Run Rate | Pebbles | Product | Load of Load %-75 urn -75 urn (kg/hr) 10 300 0 300 1860 65,4 92,0 22,1 9 450 78 372 2250 55,2 92,4 18,9 12 600 157 ~L 443 2580 48,6 81,9 16,9 Run 10: 2x10 mm discharge grates only Run 9:1 discharge grate plus lx pebble port Run 12: 2xpebble ports only The table data shows the following trends: (i) The maximum throughput of the autogenous mill increases approximately linearly with the area of pebble ports available--in the case of the pilot tests the increase was from 300 kg/hr with no pebble ports to 600 kg/hr with two pebble ports.

(ii) The rate of production of pebbles increases linearly with the number of pebble ports.

(iii) The circulating load increases with the number of pebble ports. This is a consequence of the higher discharge rate of pebbles from the mill, which requires that more -25 mm material be returned to the mill.

(iv) The fraction of material in the critical size range in the mill load decreases with the number of pebble ports, as the removal of pebbles corresponds to the removal of critical size material.

(v) The installation of one pebble port did not seem to affect the product size, but when a second pebble port was added, the product became coarser. The changes in product fineness were not large, and in a production set-up this may be corrected by adjusting the classification circuit accordingly.

(vi) The energy consumption for the production of -75 pm material decreased as the number of pebble ports was increased-from 22,1 to 16,9 kWh/ton of -75 pm produced, over the range of the tests.

A final consideration is that the available discharge area, as a fraction of mill volume, will probably be smaller in a production mill than in the short pilot mill used for the above tests. This could present a limitation to the maximum possible discharge rate of pebbles. The pebble discharge area in a production scale mill should therefore be as large as possible, within the limitations of mechanical strength requirements.

From the above pilot tests there is no doubt that the removal of largely barren pebbles greatly improves autogenous milling efficiency which, in the method of the invention, will be only slightly negatively affected by the reintroduction of the small percentage (10% or less) of crushed mineral rich pebbles into the mill from the sorter 18 and crusher 40. The reintroduced pebbles obviously enhances the grade of the mill product and, as mentioned above, significantly reduces the downstream handling costs of the product.

The invention is not limited to the precise method as herein described. The sorter 18 could for example be adapted by means of neutron bombardment, x-ray, laser or any other suitable means to detect a specific physical characteristic of the mineral carried by the pebbles and then to sort the pebbles into accept and reject fractions in accordance with the quantity of the mineral which they carry.

Claims (9)

1. A method of processing mineral containing ore including the steps of feeding the ore to be processed to a mill, grinding the ore in the mill, removing pebbles in a specific size range from the mill, sorting the removed pebbles into fractions above and below a predetermined mineral characteristic level and feeding the pebbles above the predetermined characteristic level back into the mill for further grinding.

2. A method as claimed in claim 1 including the step of crushing the pebbles above the predetermined characteristic level prior to feeding them back into the mill.

3. A method as claimed in either one of claims 1 or 2 in which the characteristic of the pebbles which have been removed from the mill is measured by an ore sorter which is adapted to detect a physical characteristic such as the radioactivity, conductivity, magneticity, colour and the like of the mineral and further adapted to sort the pebbles into fractions above and below a predetermined level of the mineral characteristic.

4. Apparatus for processing mineral containing ore including a mill, means to feed ore to the mill, means to exhaust pebbles in a predetermined size range from the mill and an ore sorter including means for measuring a predetermined characteristic of the mineral in each pebble and means for sorting the pebbles which contain a measure of the mineral characteristic above a predetermined level from the remainder of the pebbles which have been exhausted from the mill, means for feeding the pebbles from the mill to the ore sorter and means forfeeding the pebbles which incl ude the mineral characteristic above the predetermined level from the ore sorter back to the mill.

5. Apparatus as claimed in claim 4 in which the mill is an autogenous mill which includes discharge ports in its periphery.

6. Apparatus as claimed in either one of claims 4 or 5 including a crusher for crushing the pebbles in the feed means between the ore sorter and mill.

7. Apparatus as claimed in any one of claims 4to 6 including at least one screen onto which the product from the mill is exhausted and from which the pebbles are fed to the feed means to the ore sorter.

8. A method of processing mineral containing ore substantially as herein described with reference to the drawing.

9. Apparatus for processing mineral containing ore substantially as herein described with reference to the drawing.