NZ206461A - Recovering dissolved metals by passing pulp through a bed of particles - Google Patents

Description

New Zealand Paient Spedficaiion for Paient Number £06461 20646f HO DWfiSWM Priority Date(s): . &~l?:7^^ Complete Specification Filed: '.7 Class: ..

Publication Date: ... 03 SEP 1985 P-O. Journal, No: ..,..

N.Z.No. : NEW ZEALAND Patents Act 3953 COMPLETE SPECIFICATION "RECOVERY OF METAL VALUES FROM A PULP CONTAINING SUCH VALUES IN DISSOLVED FORM." We, ANGLO AMERICAN CORPORATION OF SOUTH AFRICA LIMITED of 44 Main Street. Johannesburg. Transvaal, South Africa, a company registered according to the laws of the Republic of South Africa do hereby declare the invention, for which we pray that a Patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement : - 2 064 BACKGROUND OF THE INVENTION This invention relates to the recovery of metal values from a pulp containing such values in dissolved form.

In mining operations, the mined ore is generally crushed and milled 5 and is then subjected to a leaching step with a suitable chemical reagent in order to solubilise the metal values of interest in the ore, the resultant product being called a pulp. Such a pulp may contain from about 1 to about 60 percent by weight solids, but typically contains 30 to 50 percent be weight solids. Such pulps 10 are commonly subjected to a liquid-solid separation stage prior to the recovery of the metal values from the resulting clarified or partially clarified solution. Typically such a liquid-solid separation stage may take the form of thickening, followed by filtration. Cycloning, centrifugation and counter-current 15 decantation may also be used in such a liquid-solid separation stage.

Prior art methods of dealing with this clarified or partially clarified solution resulting from a liquid-solid separation stage as described above, cover a wide spectrum of metallurgical routes.

Thus, for example, the use of ion exchange resins and solvents, 5 adsorbents such as activated carbon, together with the chemical precipitation and electrowinning of metal values, are conventional techniques well known in the art of metal recovery.

The pulp may also be treated with ion exchange resins or adsorbents such as activated carbon in the so-called "in-pulp" processes, in 10 which no prior liquid solid separation is carried out. One of the most successful methods currently being used is the carbon-in-pulp process for the recovery of gold and silver values. This method involves contacting the pulp with granular activated carbon. The gold and other valuable metal values are adsorbed on to the 15 activated carbon from whence they may be recovered by a variety of known techniques. This method is usually carried out by contacting the leached pulp with the activated carbon in a counter-current fashion involving a series of contacting stages. In each stage the activated carbon is agitated in the leached pulp in a suitable 20 vessel. Average pulp residence times in the process are commonly in the region of four to eight hours while average carbon residence times may be ten to thirty days. A high degree of interstage carbon mixing is common to the process by virtue of the methods used in transferring carbon from one stage to another. This carbon 25 mixing has a detrimental effect on the metallurgical efficiency of the process.

While the carbon-in-pulp process achieves excellent recoveries of gold and silver values, it requires large contacting vessels and is energy intensive. The large contacting vessels are required 30 because low carbon concentrations are used to minimise mechanical problems, e.g. inter-stage screening and breakdown of the carbon. 20646 As the adsorption of gold and silver are critically affected by the carbon concentration, relatively slow carbon and pulp movement rates are common to the process. Such slow movement rates result in low gold loadings on the carbon going forward to the elution circuit as 5 well as a high degree of metal value lock-up in the adsorption circuit. This lock-up of metal values in a carbon-in-pulp circuit may constitute a major indirect operating cost due to the value of the contained metals. Further, the interstage screening of carbon at each contacting stage of the process may also present a major 10 mechanical problem particularly on large plants where relative screen areas are limited due to the large size of the contacting vessels.

SUMMARY OF THE INVENTION According the present invention, there is provided a method of 15 recovering metal values from a pulp containing such values in dissolved form including the steps of: (a) forming a bed comprising a random mass of particles of a material capable of removing the metal values from the pulp, in a column having a feed end and a discharge end; and (b) passing a stream of the pulp through the column from the feed end to the discharge end so that the particles of the bed are suspended in a substantially stationary non-fluidised state between the feed end and the discharge end of the column.

// DETAILED DESCRIPTION OF THE INVENTION The method of the invention will now be described in more detail.

Initially the particles in the bed are at rest. Once the stream of the pulp begins to be passed through the bed, the particles move 5 towards the discharge end of the column and become suspended in a substantially stationary non-fluidised state between the feed end and the discharge end of the column. As the stream of the pulp _ passes through the bed, the metal values in the pulp are captured by the particles, and a concentration or loading gradient is 10 established in the bed, with the particles closest to the feed end having higher metal value loadings than the particles near the discharge end. It is important that this concentration gradient be maintained in the bed so that periodically, the particles having the highest metal value loadings, may be removed from the bed, and 15 replaced by fresh particles. This is achieved by ensuring that the particles are suspended in a substantially stationary non-fluidised state so that there is no mixing or agitation of the particles in the bed.

The column for use in the method of the invention is preferably 20 inclined upwardly, with the feed end being below the discharge end, and more preferably, the column is substantially vertical. In this case, the stream of the pulp will be passed in an up-flow mode through the bed. The bed of the particles may be confined within the column by means of suitable screens which will allow the flow of 25 pulp therethrough, but which will prevent the passage of the particles therethrough. When there is an upward flow of the pulp through the bed, the flow velocity and the viscosity of the pulp will generally be sufficient to lift and suspend the particles of the bed in a substantially stationary non-fluidised state.

The method of the invention preferably includes the step of periodically stopping the passing of the stream of the pulp through the column so that the particles of the bed move back towards the feed end of the column while remaining in a substantially non-5 fluidised state. This relaxation of the bed reduces any tendency for clogging or choking of the bed to occur.

The method of the invention preferably also includes the step of periodically removing the particles in a selected zone of the bed from the column. Preferably, the particles having the highest 10 metal value loadings, i.e. the particles in the zone nearest the feed end of the column, are removed from the bed. The removed particles may then be further treated to recover the metal values therefrom.

Generally these removed particles will be replaced with a similar 15 volume of fresh particles introduced into the column from the discharge end of the column.

The method of the invention may be applied to a plurality of beds in which each bed is discrete, or in which the beds form part of a continuous bed. Under normal operation, the passage of the pulp 20 through the system is counter-current to that of the particles of the material forming the bed in order to maximise metallurgical efficiency. The movement of the particles of the bed in such an operation may be carried out either continuously or intermittently.

The material capable of removing the metal values from the pulp may 25 be a suitable ion exchange resin, a suitable chelating resin, or an adsorbent such as activated carbon. 2 06461 7 The rate of adsorption of metal values from solution must be such that the inventory of particles and the loading of metal values on the particles are within acceptable limits. Generally this implies the use of relatively small particles which have a relatively large 5 surface area per unit volume.

Further, the particles of the material must be of a size and shape as to allow for the passage of the pulp through the bed of the particles. Thus, the particles may be elongate in shape and may be solid or may have an elongate hole extending partially or wholly therethrough. For example, when the material is activated carbon, the particles may comprise rod-like pieces having a substantially circular cross-section, with a diameter of 2 to 5 mm, which are made by an extrusion process. However, other shapes and sizes of particles are also suitable for the method of the invention, e.g. irregular shaped pieces with a maximum dimension of 2 to 20 mm.

Essential to the method of the invention is that the metal values are recovered from a pulp and not a clarified solution. The pulp, which may be formed by leaching a crushed and milled ore with a suitable chemical reagent, will generally have a solids content of iJ^tq 60 percent by weight, more typically 30 to 50 percent by weight. The solids in such pulps are usually in fine particulate form, e.g. having a typical particle size of less than 300 microns.

The method of the invention has particular application to the recovery of gold and silver values from a pulp containing such 25 values in dissolved form. In this case, it is preferred to use particles of activated carbon in the bed.

J2.0&4-S f Examples illustrating the method of the invention will now be given. EXAMPLE 1 This example illustrates the treatment of a gold plant residue pulp taken from the President Steyn Gold Mine according to the method of 5 the invention.

The pulp, resulting from a conventional gold leach operation, contained approximately 48 percent by weight solids. Approximately 80 percent of the solids had a particle size of less than 74 microns. As a considerable quantity of wood fibre was present in 10 the pulp, this was screened off using a 425 micron vibrating screen prior to use. The pulp was contained in a 200 litre vessel fitted with a suitable stirrer from where it was circulated through a vertical column. The column was 2 metres long and had an internal diameter of 200 mm. A wedge wire screen with a 2 mm slit gap was 15 positioned at the top of the column in order to confine the bed in the column. Approximately 30 kg of calcined coconut shell activated carbon, pre-screened to 3 to 15 mm in size, was contained in the column, leaving a freeboard depth of about 15 mm. The pulp was circulated through the column at a flow rate of 15 to 20 litres 20 per minute. The gauge pressure drop across the column during this operation measured 80 to 90 kilopascals. No choking of the column with coarse solids was indicated following continuous operation lasting five hours.

EXAMPLE 2 This example illustrates the treatment of a gold plant residue pulp similar to that used in Example 1 according to the method of the i nvention. ? f): 4 * The same column as used in Example 1 was used again, but the column contained approximately 30 kilograms of extruded activated carbon. The extruded activated carbon was tube-like in shape having a diameter of 3 mm with particle sizes varying from 5 to 15 mm long. 5 The leached pulp was contained in a 200 litre vessel fitted with a suitable stirrer from where it was circulated through the column at a flow rate of 30 litres per minute. The gauge pressure drop across the column during this operation measured 80 kilopascals. No choking of the column with coarse solids was indicated following 10 continuous operation lasting several days.

EXAMPLE 3 This example illustrates the gold recovery obtainable utilising the method of the invention.

Leached pulp containing 6 to 11 milligrams .per litre of gold in 15 solution was taken from the existing gold plant circuit at the Vaal Reefs Exploration and Mining. Company (South Division) and pumped in an up-flow mode into a column containing activated carbon. The pulp resulting from a conventional gold leach using cyanide, contained approximately 40 to 50 percent solids by weight having an 20 average particle size analysis as follows: 2,5 percent + 150 microns; 26,4 percent + 74 microns and 71,1 percent - 74 microns. The leached pulp also contained approximately 200 milligrams per litre cyanide as NaCN, and had a pH in the range 10 to 11,5. A considerable quantity of wood fibre was also present in the pulp and 25 this was screened off using a 425 micron vibrating screen prior to use. The column was 10 metres long and had an internal diameter of 185 mm. A wedge wire screen with a 2 mm slit gap was positioned at the top of the column in order to confine the activated carbon in the column. Approximately 150 kilograms of 3 mm diameter extruded 30 or molded activated carbon was contained in the column leaving a 2 06461 free-board bed depth of about 300 mm. The pulp was pumped into the column at a flow rate of 15 litres per minute which corresponded to a superficial flow velocity in the column of 0,56 metre per minute.

The counter-current movement of carbon in the column was carried out 5 on an eight hour shift basis. Loaded carbon was discharged from the bottom of the column, while a similar volume of virgin carbon was added to the top of the column. During normal column operation the line pressure to the column averaged 300 to 500 kilopascals (gauge pressure). The counter-current movement of carbon as 10 described above had a most beneficial effect on reducing the line pressure to the column.

The column was run in the mode described for a period of 32 days with operational time including carbon movement approaching 97 percent. During this time the recovery of soluble gold in the pulp 15 averaged 99 percent with the soluble feed to the column averaging 9,6 milligrams per litre of gold. When carbon was moved at a rate corresponding in volume to a bed depth of one third of a metre per eight hour shift, loaded carbon from the column contained 10 to 11 kilograms of gold per tonne of carbon. Likewise, when carbon was 20 moved at twice this rate, the loaded carbon contained approximately 5 kilograms of gold per tonne of carbon.

EXAMPLE 4 Residue pulp containing 0,1 to 0,4 milligrams per litre of gold in solution was taken from the existing gold plant circuit at the Vaal 25 Reefs Exploration and Mining Company (South Division) and pumped into a column as described in Example 3. a In the first test of 9 days duration, using a 10 metre long column, the gold in solution was reduced to 0,001 milligrams per litre. In a second test of 15 days duration, where the column was shortened to 5 metres, the gold in solution was reduced to 0,005 milligrams per 5 litre. The respective gold recoveries were 99 percent and 95 percent. The pulp conditions in both these tests were similar to those described in Example 3. During normal operation on the 5 metre column, the line pressure to the column averaged 100 to 200 kilopascals (gauge pressure). In both tests the carbon movement 10 rate was maintained at one third of a metre per 8 hour shift. The carbon removed was loaded with 150 to 200 grammes of gold per tonne of carbon. Any reduction in the carbon movement rate would result in higher gold loadings on the carbon.

The method of the invention has several advantages. Firstly, using 15 the method of the invention, much lower pulp residence times in the column are obtainable, when compared with the carbon-in-pulp method. For example, pulp residence times for the method of the invention are a matter of minutes compared with residence times of several hours for the conventional carbon-in-pulp method. Secondly, it is 20 possible to obtain higher metal value loadings on the metal-capturing particles. // ■ , $ i

Claims (5)

1.WHAT WE CLAIM IS: 1 .

2.A method of recovering metal values from a pulp containing such metal values in dissolved form includes the steps of: 5 (a) forming a bed comprising a random mass of particles of a material capable of removing the metal values from the pulp, in a column having a feed end and a discharge end; and i (b ) passing a stream of the pulp through the column from the feed end to the discharge end so that the particles of the bed are 10 suspended in a substantially stationary non-fluidised state between the feed end and the discharge end of the column. ,2.

3.A method according to claim 1 wherein the column is upwardly inclined, with the feed end being below the discharge end. 15 3.v A method according to claim 2 wherein the column is substantially vertical.

4. A method according to any one of claims 1 to 3 which includes the 20 step of periodically stopping the passing of the stream of the pulp through the column so that the particles of the bed move back toward the feed end of the column while remaining in a substantially non-fluidised state. •■7 13 - . 5. A method according to any one of claims 1 to 4 which includes the step of periodically removing the particles in a selected zone of the bed from the column. 5 6. A method according to claim 5 wherein the particles in the zone of the bed nearest the feed end of the column are removed from the bed. 7. A method according to claim 5 or claim 6 wherein the particles 10 removed from the bed are replaced with a similar volume of particles introduced into the column from the discharge end of the column. 8. A method according to any one of the preceding claims wherein the particles have a relatively large surface area per unit volume. 15 9. A method according to any one of the preceding claims wherein the material capable of removing the metal values from the pulp comprises a suitable ion exchange resin or a suitable chelating resin. 20 10. A method according to any one of claims 1 to 8 wherein the material capable of removing the metal values from the pulp comprises activated carbon. 11- 25 A method according to any one of claims 1 to 10 wherein the particles are elongate in shape. - 14 - 12. A method according to any one of claims 1 to 11 wherein the pulp has a solids content of 10 to 60 percent by weight. 13. 5 A method according to claim 12 wherein the pulp has a solids content of 30 to 50 percent by weight. 14. A method according to any one of the preceding claims for the recovery of gold and silver values from a pulp containing such 10 values in dissolved form. !5. A method of recovering metal values from a pulp containing such values in dissolved form includes the steps of: (a) forming a bed comprising a random mass of particles of a 15 material capable of removing the metal value from the pulp, in a column having a feed end and a discharge end; (b) passing a stream of the pulp through the column from the feed end to the discharge end so that the particles of the bed are suspended in a substantially stationary non-fluidised state 20 between the feed end and the discharge end of the column; (c) periodically stopping the passing of the stream of the pulp through the column so that the particles of the bed move towards the feed end of the column while remaining in a substantially non-fluidised state; and 25 (d) periodically removing the particles in a selected zone of the bed from the column. - 15 - 16. A method according to claim 15 wherein the column is substantially vertical. 17.

5.A method according to claim 15 or claim 16 which includes the step of replacing the particles removed from the bed with a similar volume of particles introduced into the column from the discharge end of the column. 18. 10 A method according to any one of claims 15 to 17 for the recovery of gold and silver values from a pulp containing such values in dissolved form. 19- A method of recovering metal values from a pulp containing such 15 values in dissolved form substantially as herein described with reference to any one of Examples 1 to 4. ANGLO AMERICAN CORPORATION OF SOUTH AFRICA LIMITED. By Their Attorneys HENRY HUGHES LIMITED By: ■ - J