WO2006024886A1

WO2006024886A1 - Method for processing a value bearing feed material

Info

Publication number: WO2006024886A1
Application number: PCT/IB2004/002812
Authority: WO
Inventors: Ian Smit; Jan Tjeerd Smit; Robert Vincent Crosbie; Vilim Ser
Original assignee: Anglo Operations Pty Ltd
Current assignee: Anglo Operations Pty Ltd
Priority date: 2004-08-31
Filing date: 2004-08-31
Publication date: 2006-03-09
Anticipated expiration: 2007-02-28

Abstract

The invention provides a method of processing heterogeneous value bearing material. In a first step, the method includes crushing the value bearing material in a bed of particles while maintaining a bed voidage value of at least 20%. The crushed material is then subjected to a value recovery process downstream of the crushing, and performance data is obtained from the value recovery process. In a further step, the method includes using the value recovery performance data to control the crushing of the value bearing material, thereby to liberate the value preferentially from the value bearing material.

Description

METHOD FOR PROCESSING A VALUE BEARING FEED MATERIAL

BACKGROUND OF THE INVENTION

THIS invention relates to the processing of value bearing feed materials such as metal ores. More specifically, the invention relates to a method of recovering value from a heterogeneous value bearing material.

Comminution processes in the mining industry generally consist of crushing and milling. The milling often takes place in a rotating drum, such as in ball milling in which steel balls are added to the drum to facilitate size reduction of the feed material, or autogenous milling in which size reduction is effected by the tumbling action of the feed material upon itself. Generally, such size reduction operations take place with the addition of water, which permits the various process streams to be pumped, and which allows for the use of simple, high throughput hydrocyclones for size separation.

Although such comminution processes are capable of comminuting very large tonnages of ore in a consistent manner, these processes suffer from several drawbacks. Typically, large amounts of ultrafine particles are generated, beyond the degree of size reduction required for value liberation or for suspendability of the comminuted material in a pulp. This causes unnecessarily high reagent requirements in downstream processing, and dilution of concentrates prepared from such materials due to entrainment of the fines with process water. Furthermore, these comminution processes suffer from relatively high energy consumption, at least partly due to the unnecessary generation of ultrafines. Thus, although these comminution processes are effective in size reduction, this is achieved without sufficient consideration for downstream processes.

Comminution processes in which the product size distribution is limited are known. For example, U.S. Patent No. 6,508,421 discloses a process in which the size distribution of a comminution product is limited to maximize the generation of material in a size range suitable for downstream value recovery, and minimize the formation of fines. Generally, in these methods the size range of comminuted material, which is typical for a given material, at a given pulp density, using a given value recovery method, is determined upfront and hence is not sensitive to variations in real-time value recovery.

It is an object of the present invention to control the size distribution of comminuted material, continuously or at least on a regular basis, to account for variations in downstream, real-time value recovery and thereby enhance the overall efficiency of the value recovery.

SUMMARY OF THE INVENTION

According to one aspect of the invention there is provided a method of processing heterogeneous value bearing material, the method comprising the steps of: crushing the value bearing material in a bed of particles while maintaining a bed voidage value of at least 20%; subjecting the crushed material to a value recovery process downstream of the crushing; obtaining performance data from the value recovery process; and using the value recovery performance data to control the crushing of the value bearing material, thereby to liberate the value preferentially from the value bearing material. By "bed voidage value" is meant the difference between the density of the material making up the particle bed, and the density of the particle bed as a whole. A bed voidage value of 20% means that the particle bed comprises 80% feed material and 20% voids.

Preferably, the method includes classifying the value bearing material after at least one cycle of crushing, and recycling a coarse fraction of the classified material for further comminution.

The method typically includes removing comminuted feed material having particles smaller than a predetermined size prior to overgrinding thereof to a smaller size. To this end, the comminuted feed material is preferably classified after no more than three compression cycles, and most preferably after each compression cycle.

Preferably, the reduction ratio over the comminution step is below 3000. For example, a feed with a top size of say 15 centimeters is not reduced beyond a production top size of 50 micrometers.

The classification may be carried out by a cyclone, a dynamic or static air classifier, a screw classifier, or a screening apparatus, with or without the addition of water.

The value recovery process may comprise, for example, froth flotation, vat leaching or gravity separation.

In one embodiment of the invention, the value recovery performance data is obtained continuously.

In another embodiment, the value recovery performance data is obtained intermittently.

The value recovery performance data may be obtained by sampling the feed, product and/or tailings streams of the value recovery process, and subsequently subjecting the samples to chemical analysis, X-ray analysis, or fire assay.

Preferably, the method includes the step of subjecting the samples to on¬ line analysis, for example by using on-line XRD analysis, on-line XRF analysis, or automated sampling followed by automated, robotic laboratory procedures.

The value recovery performance data may be computed manually or automatically. In the latter case, the results may be computed locally, for example by means of a programmable logic controller (PLC), or centrally by means of a plant information system.

The step of using the value recovery performance data to control the crushing of the value bearing material may be effected with the aid of an algorithm, such as an adaptable control algorithm. In one embodiment of the invention, the algorithm is designed to effect coarsening of the comminution product in response to low value grades, and further comminution in response to low value recovery.

The value bearing material may be crushed in a vertical roller mill, a horizontal roller mill, an inertial cone crusher or a high pressure grinding roll, for example.

The crushing of the value bearing material may be controlled by changing variables such as the particle bed compression pressure within the crusher; the feed rate, feed size, and/or product size of comminuted material; the classification size set point; the air volume, air velocity, and/or air pressure differentials over the crusher; the intensity of friction between the particle bed and a comminution tool; the particle bed depth within the crusher; the re-circulation rate between the crusher and the classification device; and/or the comminution tool rotational speed. The value bearing materials may include ores of precious metals, base metals, industrial minerals or diamonds.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in more detail, by way of example only, with reference to the accompanying drawings in which:

Figure 1 illustrates, schematically, apparatus for processing heterogeneous value bearing material in accordance with the present invention; and

Figure 2 is a graph illustrating the relationship between particle size and value recovery.

DETAILED DESCRIPTION OF THE INVENTION

The present invention has particular application in the processing of heterogeneous value bearing materials such as base metal ores, gold ores, platinum ores, diamond ores, metalliferous slags, etc. Typically, in these applications, value bearing feed material is crushed or milled to liberate the value from the feed material, and the value is then recovered from the comminution product in a downstream value recovery process.

It has been established that compression breakage in a bed of particles, also known as inter-particle comminution, results in preferential cracking of particles along grain boundaries. This liberates valuable minerals from the heterogeneous materials, with minimum breakage of the gangue constituents, which tend to be present in the material as discrete grains or pebbles. The formation of ultrafines takes place mainly in the damage zones on points of particle contact. The resulting fragments may fracture further if the compression event is extended in time and there is less open space into which these fragments may fall. In such a case, the formation of ultrafines is increased, with commensurate energy consumption during comminution and significant limitations on the efficiency of downstream value recovery operations.

The present invention relies on value recovery performance data to control the comminution process and hence value liberation. For example, by monitoring the value recovery, the value grade or the reagent consumption at a downstream value recovery process, it can be established when the comminution product fed to the value recovery process is too fine or too course for a given purpose. The comminution process may then be controlled to effect a desired change in the size distribution of the comminution product.

With reference to Figure 1 of the drawings, the method of the invention may be used to process a heterogeneous value bearing material, for example to recover copper from a mined ore. In this embodiment of the invention, the mined ore is fed to a grinder or crusher 10 which may be a conventional crusher such as, for example, a Rhodax inertial cone crusher, a high pressure grinding roll (HPGR), a vertical roller mill, a horizontal roller mill, or another comminution device which operates by compressing a bed of particles, in order to reduce the size of the particles in the particle bed, by inter-particle comminution. Since it is desirable for the crusher 10 to respond relatively quickly to required changes in operating parameters, such as compressive stress, conventional large tumbling mills are not suitable for comminution in the process according to the invention.

To avoid excessive comminution and consequential material agglomeration which adversely affects subsequent value recovery, it is desirable to maintain the bed voidage value of the comminuted material at 20% or greater, and to maintain the reduction ratio over the comminution step, i.e. the ratio between the top size of the feed and the top size of the product, below 3000. For example, a feed with a top size of 15 centimeters should not be reduced beyond a product top size of 50 micrometers. A bed voidage value of 20% or greater may be maintained by suitable classification of the feed material prior to compression thereof and after each (or a limited number of) compression cycles. As representatively illustrated in Figure 1 , the crusher 10 may be operated in a closed circuit with an external classifier 12 which screens or classifies the comminuted particles frequently, at least after every three compression cycles of the crusher 10, and preferably after every compression cycle. This minimizes the production of ultrafines and assists in maximizing the particle bed voidage value. The classifier 12 may be a mechanical screening device, an air or hydrocyclone, a spiral classifier, an air classifier, or any other suitable classification device. Classifiers which minimize energy consumption are preferred for cost reasons. For high intensity compression/classification comminution, an air drafted vertical roller mill may be operated with an air classifier, or a HPGR may be operated with a separate classifier such as a screen.

Comminuted material which passes the classifier 12 is fed to a product collection site 14, from where it is fed to a value recovery installation, such as a froth flotation plant 16. Froth flotation plants are well-known in the art and need not be described in detail for a full understanding of the invention. Generally, however, froth flotation plants include a plurality of flotation cells 16.1 to 16.n which are connected to one another in series. Each cell includes a tank with an inlet for allowing slurry into the tank, an agitator for agitating the slurry and developing a floating froth, a first outlet in an upper region of the tank for allowing the floating froth out of the tank, and a second outlet in a lower region of the tank for allowing the slurry out of the tank. Copper within the slurry is selectively carried with the froth and thus is separated by flotation from the rest of the slurry. In other embodiments and/or applications of the invention, instead of a froth flotation plant the comminuted material may be subjected to a leaching process, for example, by contacting the comminuted material with a leaching solution in stirred vats, pachuca tanks, or an autoclave. The leaching solution typically comprises an aqueous solution of cyanide, sulfuric acid, ammonia, or other commonly used lixiviants. Alternatively, a gravity concentration process could be used for value extraction, such as commonly executed by arrangements of jigs, heavy media cyclones, or enhanced gravity concentrators, such as Knelson or Falcon concentrators. It is desirable that the performance of the value recovery installation be established continuously, or at least on a regular basis, and accordingly processes such as heap or dump leaching, the performance of which cannot be established reliably, and only after considerable effort and extensive time delay, are not suitable.

In accordance with the invention, the value recovery performance is used to control comminution in the crusher 10 so as to achieve preferential value liberation for enhanced value recovery and/or improved economic performance. For this purpose, particular performance characteristics of, for example, the value recovery, the value grade and/or the reagent consumption, may be selected, and the operation of the crusher 10 may be controlled by adjusting operating parameters of the crusher in response to changes in value recovery performance.

Figure 2 illustrates the relationship between the particle sizes of the comminuted material and value recovery. As can be seen, for a given particle size distribution, the value recovery is optimized. A drop in value recovery generally indicates a need for a finer grind, whereas a drop in value grade indicates overgrinding of the gangue. By monitoring the value recovery performance, it is possible to determine whether the particle size distribution of the comminuted material is within a desired range or requires adjustment.

The operating parameters of the crusher 10 which may be adjusted in response to changes in the value recovery performance include the particle bed compression pressure within the crusher; the feed rate, feed size, and/or product size of comminuted material; the classification size set point; the air volume, air velocity, and/or air pressure differentials over the crusher; the intensity of friction between the particle bed and a comminution tool; the particle bed depth within the crusher; the re-circulation rate between the crusher and the classification device; and/or the comminution tool rotational speed.

In the illustrated embodiment of the invention, a conventional sampling device 18, such as, for example, a pressure pipe sampler, a cross current sampler or a shark fin sampler, is used to sample the flotation concentrate, either on a continuous or intermittent basis, and the samples are subjected to chemical analysis, X-ray analysis or fire assay to determine the value recovery performance. Sampling may also occur on the feed or tailings streams, either in addition to or as an alternative to sampling of the flotation concentrate. The sampling may be automated, for example by using on¬ line analyzers such as on-line XRD analyzers or on-line XRF analyzers, and the automated sampling may be followed by automated, robotic laboratory procedures. Value recovery performance data may be computed manually or automatically, for example by a programmable logic controller (PLC) or by a plant information system.

The step of using the value recovery performance data to control the crushing of the value bearing material typically is carried out with the aid of an algorithm, such as an adaptable control algorithm. For example, the algorithm may be designed to effect coarsening of the comminution product in response to low value grades, and to effect further comminution in response to low value recovery.

EXAMPLE 1

A nickel-PGM ore was comminuted in a vertical roller mill (VRM) and subsequently subjected to flotation in accordance with the method of the present invention. Test work was done at a continuous, integrated pilot plant scale, at a feed rate averaging 300 kg/h. For comparative purposes the nickel-PGM ore was also comminuted conventionally in a ball mill followed by flotation. Results were follows:

As can be seen, the value grade and value recovery results obtained with the process according to the present invention were substantially higher than those obtained in the conventional process.

In addition, the flotation depressant reagent consumption in the process according to the invention was approximately 40% of that required in the conventional process.

EXAMPLE 2

A zinc/lead ore was comminuted in a vertical roller mill (VRM) and subsequently subjected to flotation in accordance with the method of the present invention. Test work was done at a continuous, integrated pilot plant scale, at a feed rate averaging 500 kg/h. For comparative purposes the zinc/lead ore was also comminuted conventionally in a ball mill followed by flotation. The objective in each case was to produce a concentrate containing 48.0% zinc. Results were follows:

As can be seen, for a given value grade, the value recovery obtained with the process according to the present invention was substantially higher than that obtained in the conventional process.

It was therefore clearly demonstrated that the process according to the invention was considerably more effective in treating these materials than conventional ball mill and subsequent froth flotation processing.

Claims

1. A method of processing heterogeneous value bearing material, the method comprising the steps of: crushing the value bearing material in a bed of particles while maintaining a bed voidage value of at least 20%; subjecting the crushed material to a value recovery process downstream of the crushing; obtaining performance data from the value recovery process; and using the value recovery performance data to control the crushing of the value bearing material, thereby to liberate the value preferentially from the value bearing material.

2. A method according to claim 1 , including the step of classifying the value bearing material after at least one cycle of crushing, and recycling a coarse fraction of the classified material for further comminution.

3. A method according to claim 2, wherein the comminuted material is classified after each compression cycle.

4. A method according to any one of the preceding claims, wherein the reduction ratio over the comminution step is below 3000.

5. A method according to any one of the preceding claims, wherein the value recovery process comprises froth flotation, vat leaching or gravity separation.

6. A method according to any one of the preceding claims, wherein the value recovery performance data is obtained continuously.

7. A method according to any one of claims 1 to 5, wherein the value recovery performance data is obtained intermittently.

8. A method according to any one of the preceding claims, wherein the value recovery performance data is obtained by sampling the feed, product and/or tailings streams of the value recovery process, and subsequently subjecting the samples to chemical analysis, X-ray analysis, or fire assay.

9. A method according to any one of claims 1 to 7, wherein the value recovery performance data is obtained by sampling the feed, product and/or tailings streams of the value recovery process, and subjecting the samples to on-line analysis.

10. A method according to claim 9, wherein the samples are subjected to on-line XRD analysis or on-line XRF analysis.

11. A method according to any one of claims 1 to 7, wherein the value recovery performance data is obtained by automated sampling of the feed, product and/or tailings streams of the value recovery process, followed by automated, robotic laboratory procedures.

12. A method according to any one of the preceding claims, wherein the value recovery performance data is computed automatically.

13. A method according to claim 12, wherein the value recovery performance data is computed by means of a programmable logic controller (PLC) or by means of a plant information system.

14. A method according to any one of the preceding claims, wherein the step of using the value recovery performance data to control the crushing of the value bearing material is effected with the aid of an algorithm.

15. A method according to claim 14, wherein the algorithm is an adaptable control algorithm.

16. A method according to any one of the preceding claims, wherein the crushing of the value bearing material is controlled by changing the particle bed compression pressure within the crusher; the feed rate, feed size, and/or product size of comminuted material; the classification size set point; the air volume, air velocity, and/or air pressure differentials over the crusher; the intensity of friction between the particle bed and a comminution tool; the particle bed depth within the crusher; the re-circulation rate between the crusher and the classification device; and/or the comminution tool rotational speed.