GB2370567A

GB2370567A - Process for extracting precious metal from waste material

Info

Publication number: GB2370567A
Application number: GB0026730A
Authority: GB
Inventors: Lee Fisher Robinson; Jack Bentley
Original assignee: Individual
Current assignee: Individual
Priority date: 2000-11-01
Filing date: 2000-11-01
Publication date: 2002-07-03
Anticipated expiration: 2020-11-01
Also published as: GB2370567B; GB0026730D0

Abstract

An extraction process for recovering precious metal and optionally valuable non-precious metal from base metal-containing material including recovery and reuse of a primary liquid reactant, which comprises treating the material with a primary liquid reactant capable of dissolving the base metal and optionally the non-precious metal but not the precious metal to a significant extent, forming a precious metal residue and a metal-containing used primary liquid reactant, separating the used primary liquid reactant from the residue, converting part of the used primary liquid reactant into a gaseous precursor for recycling and causing the precursor to contact liquid converting it to the primary liquid reactant, and treating the residue with a secondary liquid reactant to dissolve the precious metal. The precious metal may be gold, silver or a platinum group metal. The non-precious valuable metal may be cobalt, molybdenum, nickel, vanadium, tungsten, titanium, zirconium or manganese. The base metal may be iron, lead, copper, zinc or tin.

Description

EXTRACTION PROCESSES This invention is concerned with extraction processes. More particularly, the invention relates to processes for extracting at least one precious metal (e. g. one or more of gold, silver or a platinum group metal) from a feedstock material, typically waste, such material, containing such precious metal but which also contains a greater proportion of base and/or other non-precious metal (s) as valuable metal (s) as herein defined. material.

Specifically the invention is concerned with a methods of selectively recovering one or more precious metals as exemplified above from feedstock material containing significantly more base metal such as one or more of: iron, lead, copper, zinc or tin. The feedstock may also contain one or more valuable metal (s) as herein defined, one or more of which may also be recovered by extraction if desired.

In the metals industries, waste materials are produced which contain small but extrabtable proportions of precious or valuable metal and extraction thereof is economically viable. Waste materials are also produced in industrial processes outside of the metal industries and in the treatment of sewage, effluent and similar wastes.

Solids residues arising from these other industrial activities can contain precious and valuable metal.

Residues arising from the filtered, dried or calcined sludges can also be useful feedstock material for the prsent extraction processes they may contain some 1-10% by weight of valuable metal and/or some 5-500ppm precious metal.

For example waste materials in the nature of unwanted metal slags may contain small but significant proportions of precious metal e. g. silver, gold, platinum and palladium and/or valuable metal such as cobalt, molybdenum, nickel, vanadium, tungsten, titanium, zirconium and to a lesser extent manganese which is another nonprecious valuable metal may justify extraction from a feedstock containing it.

Some of these materials depending upon their origin and processing, can contain, for example up to about 260 g/ton of gold, up to about 300 g/ton of platinum or other platinum group metals, up to about 1,400 g/ton of silver and/or up to about 400 g/ton of cobalt. These levels of precious or valuable metal are in addition to the large excess of base metal present e. g. Fe, Cu, Zn, Pb or Sn as above in widely variable amounts. Such feedstock materials can be in residue or other solid form'and difficult to treat by conventional extraction processes to recover the precious or valuable metal content.

One stage in conventional such processes, for some solid waste materials having significant base metal content, is mechanical grinding to reduce the particle size to make liquid leaching processes more effective. However, mechanical grinding is economically undesirable in view of the energy inputs required and is preferably avoided.

There is a known process for extracting precious metal from metal ores, as distinct from waste materials, which avoids the use of cyanide. This process, known commercially as the HMCT process, was devised by Hydromet Mineral (UK) Ltd and has been described in European Patent 0 124 213 entitled"extraction process"and British Patent 2 158 734 entitled"improvements relating to pressure reactions".

It has, however, been the case that high base metalcontaining waste material such as the bars and slags above mentioned have not been suitable for the extraction process described in this European patent even with use of the coiled pressure reactor described in the British Patent.

This is because the base metal content and the physical size of the available waste material has been too high. As foreshadowed above, the costs associated with grinding such possible feedstock materials to an appropriate particulate size for liquid leaching would additionally render such precious metal extraction uneconomic. indeed it is technically disadvantageous to mechanically grind bars of high soft metal content, such as lead. Similarly, efforts to grind high iron-containing silica slags have been ineffective causing damage to the grinder while consuming excessive energy.

In the European Patent, an extraction process is disclosed which uses nitric acid in combination with a source of halide ions, such as chloride ions.

As a raw material, dilute nitric acid especially in impure or metal-contaminated form is relatively inexpensive and widely available. In some industries it is regarded as a disposable waste product. In other industries, impure dilute nitric acid is available in admixture also with sulphuric acid. Whilst such sulphuric acid can be regarded as an'impurity', it is also a potential catalyst or useful reactant within the presently proposed extraction processes.

Obviously commerical use of dilute impure nitric acid is economically more attractive than concentrated and/or pure nitric acid. If impure dilute nitric acid can be used in place, of or to supplement, concentrated and/or pure nitric acid then the operating costs of precious metal extraction processess requiring nitric acid become economically more favourable.

Industrial processes try to recover and/or reuse chemical reactants when technically and economically feasible. Thus a process requiring impure dilute nitric acid as a reactant which can be (at least partially) recycled has even greater economic attractions.

A new process for extracting precious metal from a high base metal feedstock material difficult to treat by known extraction processes wherein an economical reagent such as impure dilute nitric acid is recycled, would be highly desirable in the precious metals extraction industry and could be expected to lead to widespread commercial application. This is particularly so where the process includes options to recover valuable non-precious metal e. g.

Co/Ni from the difficult to treat feedstock.

We have now devised a process for extracting at least one precious metal from a waste solids feedstock of high base metal content and which can use and recycle impure dilute nitric acid.

According to this invention we provide an extraction process for recovering precious and optionally valuable nonprecious metal from base metal-containing material including recovery and re-use of a primary liquid reactant, the process comprising: (a) treating said material? at least once? with? the same or different? primary liquid reactant capable of dissolving said base metal and optionally said valuable non-precious but not said precious metal to a significant extent thereby forming precious metal containing residue and used liquid reactant containing dissolved said other metal and optionally if present or required said valuable non-precious metal.

(b) separating said used liquid reactant from said residue, (c) converting at least some of said used liquid reactant into gaseous precursor of said primary reactant, (d) treating said residue with secondary liquid reactant capable of dissolving precious metal contained therein, (e) causing the gaseous precursor to contact liquid capable of converting it into recovered such secondary liquid reactant, (f) supplying said secondary liquid reactant containing dissolved said metal to precious metal recovery means wherein dissolved said precious metal is extracted from the used second liquid reactant or otherwise treated to enable subsequent extraction of such precious metal, and (g) transferring recovered primary liquid reactant obtained in step (e) for reuse as primary liquid reactant in at least step (a) above.

The used liquid reactant can be treated separately, if required, to recover valuable non-precious metal by e. g. means which are known in the art.

Such a process can avoid the need for expensive mechanical grinding of the starting material with its inherent technical problems.

Preferred features of the above invention will now be described by way of non-limiting example only.

The starting material to be subjected to the process is preferably solid metal-containing feedstock material such as waste material as previously described which in addition to relatively minor proportions of at least one precious metal, also contains significantly greater proportions of at least one base metal such as iron, copper, zinc, tin and/or lead.

The base metal content may vary widely. Such material can be waste residues from sewage effluent treatments or industrial processes, or slags or melts typically slags or melts high in copper base metal content, e. g. from copper mines, which contain sufficient levels of the precious metals gold, silver, platinum or palladium which justifies extracting. Other suitable starting material includes slags and other residues from nickel and/or zinc processing or refining which typically contain zinc and/or lead as base metals together with valuable but non-precious metal, e. g. nickel and/or molybdenum either or both of which can also be recovered in addition to the precious metal content.

Additionally slags such as solid bars of high iron and lead base metal content can be used, even those which contain up to about 80% by weight of Fe and/or Pb.

The primary liquid reactant preferably comprises a major proportion of nitric acid, such as dilute nitric acid which need not be of the highest purity. Dilute nitric acid containing dissolved metal or in admixture with sulphuric acid can be used, for example. Such a reactant is used to dissolve the said other metal (e. g. base metal) content of the starting material such as whichever is present from, for example Fe, Cu, Zn, Pb and the like. Preferred primary liquid reactants should be compatible with (i. e. mix/dissolve in without adverse reaction) nitric and sulphuric acids and ionic halide salts but not capable of dissolving to a significant extent such precious metals as Pt, Pd or Au. Where H2SO4 is used as the or one or the primary reactants, it should be thoroughly washed out of the residue before contact with secondary reactant.

The primary liquid reactant may be, for example, spent nitric acid used for cleaning/etching in the electronic industry e. g. that of 20-40% concentration containing tens of ppm of metals such as Zn, Cu, Cd, Fe, Sn presently disposed of. It may comprise nitric/sulphuric acid as used for the surface treatment of super alloys or the production of gem stones, which acid mixture may contain Cr, Co, Ni and Fe by way of impurity.

The valuable non-precious metal present in the feedstock could can be, for example one or more of cobalt, manganese, nickel and molybdenum, all of which have a higher commercial value than any of the defined base metals.

In step (a) of the process, akin to a pre-leaching step, whether deployed once or more a solids residue is left still containing the precious metal desired for extraction but preferably most if not all of the other (i. e. base and any valuable but non-precious) metal in the starting material will have been dissolved by the primary liquid reactant in one stage (a) or a number of sequential stages using the same or different primary liquid reactant. Then the used i. e. spent primary liquid reactant (which may be dilute impure nitric acid containing relatively high proportions of dissolved metal) is separated from the precious metal-containing solids residue. Where the feedstock contains Pb or Sn, 2SO4 may be added to the spent primary liquid reactant to precipitate these metals as their sulphates and help HNO3 recovery.

In step (c) this used primary liquid reactant is preferably converted into oxide (s) of nitrogen (if dilute nitric acid is used as or a substantial component of the primary liquid reactant) by vapourising such spent reactant in e. g. a submerged combuster.

In step (d) the secondary liquid reactant is preferably a mixture of a strong oxidising agent and a source of halide ions acting as leaching agent for the precious metal (s) being extracted and recovered from the feedstock e. g. waste solids residue. For example such a secondary liquid reactant can comprise a mixture of nitric acid and soluble inorganic chloride such as sodium chlotide. The nitric acid is preferably strong, concentrated nitric acid although in this stage of the process and particularly during a continuous extraction process as opposed to batchprocessing, it is possible to use dilute nitric acid.

Herein, dilute nitric acid which has been recovered elsewhere in the process for reuse, can most advantageously be deployed. The secondary liquid reactant may comprise

nitric acid in admixture with one ormore of CuCl, HCl, Fecal3 and HF. Step (d) is preferably carried out in a tubular, coiled pressure reactor such as the type known as a 'Hydrocoil' (Trade Mark) unit as described in the above referred to British Patent.

The gaseous precursor from step (c) contacts liquid, such as water, to convert it back to primary liquid reactant, hence recycling of nitric acid component is achieved. When oxide (s) of nitrogen is (are) passed through water, dilute nitric acid is formed. It does not matter that nitrous acid may also be formed since the recovered material still comprises, in such a case, dilute nitric acid.

The precious metal recovery means can include any of the known absorbents for e. g. gold and similar precious metals, for example activated carbon recovery means.

Alternatives such as ion-exchange resins can be deployed.

The objective here is for the dissolved precious metal ionic species to be loaded onto a substrate from which they can be subsequently released, to complete the extraction process.

The dissolved precious metal content could become directly precipitated during this stage or absorbed onto such a substrate from which the precious metal can be subsequently released by techniques which are known in the art and need not be described here in detail, In the above process, primary liquid reactant preferably nitric acid or substantially containing nitric acid recovered at or adjusted to an appropriate optimum strength for reuse can be directed in a suitable line to be fed into the process step (a) for dissolution of at least the base metal content in the starting material, but additionally such recovered reactant could additionally be fed by suitable line into step (d) to aid in the dissolution of the precious metal (s) which is (are) desired to be extracted by the process.

In order that the invention and preferred features of it may be illustrated, more easily appreciated and readily carried into effect by those skilled in the art of valuable metal extraction, an embodiment of the process will now be described in detail purely by way of non-limiting example, in the drawings attached wherein: Figure 1 is a schematic flow diagram of an extraction process for recovering precious metal such as gold from a high base metal content slag or melt starting material; and' Figure 2 is a diagram of a coiled tubular pressure reaction vessel known as a'Hydrocoil' (Trade Mark) unit in which part of the process can be carried out.

Referring firstly to Figure 1 the vessels shown in the flow diagram are as follows: a plastic-lined stirred tank 1 for receiving the feedstock of slag or melt type high base metal starting material (feed line B) referred to as a ground feed and for receiving primary liquid reactant such as impure dilute nitric acid (feed line A). Reactor vessel 1 can incorporate a vent for transferring gaseous reaction product to vessel 5 described later. Vessel 2 contains a submerged combuster, autoclave, or other device for flashing off nitric acid and oxides of nitrogen, for producing gaseous precursor of primary liquid reactant (supplied by feed line C) containing spent secondary liquid reactant e. g. spent nitric acid liquor containing high levels of dissolved base or other non-precious metal. The gaseous precursor e. g. oxide (s) of nitrogen (NOX) is fed via line E to vessel 5, from vessel 2.

Device 3 is conveniently a'Hydrocoil'unit for receiving the solids residue from vessel 1 (via feed line F), a feed of strong saline solution (via feed line H), an initial charge of strong nitric acid optionally with sulphuric acid (via feed line G), and a continuous supply of recycled dilute nitric acid (via feed line M).

The spent liquor from reactor vessel 3 contains dissolved or otherwise solubilised precious metal species such as ions thereof in solution, and is transferred to vessel 4 being an (NOX) flash off and filtration unit.

Gaseous oxide (s) of nitrogen are transferred (via feed line K) into vessel 5 which is a standard acid recovery plant forconverting gaseous oxide (s) of nitrogen (NOX) into dilute nitric acid. It is interesting to observe at this point that the dilute nitric acid formed in vessel 5 can be substantially free of metal impurity so that the overall process not only recovers nitric acid for reuse but also purifies it by removal of substantially all dissolved metal.

The output of vessel 4 includes solids residues (undissolved) which are transferred (via feed line P) to a waste treatment line R, which couples with feed line D also containing base metal residues. Such solids residues can proceed to neutralisation and subsequent disposal (not shown).

Vessel 5 is supplied with not only NOIX from feed lines E and K but also with a dilute nitric acid wash from feed line S typically providing approximately 5% dilute nitric acid to aid in the nitric acid recovery for reuse.

The recovered such dilute nitric acid is supplied (via feed line M) both to the starting reactor vessel 1 to aid in dissolution of the base metal content in the starting material (via feed branch line 0) assuming an excess of recovered nitric acid is available, and to the vessel 3 wherein the precious metal is solubilised and/or otherwise dissolved (via feed branch line N).

Precious metal-containing leach liquor from vessel 4 is supplied (via feed line L) to one or more activated carbon recovery columns depicted by vessel 6. As an alternative to carbon recovery means, vessel 6 could be in the form of selective electrolysis apparatus.

Carbon is used as an adsorbent within vessel 6 and can be transferred (via supply line T) for precious metal desorbtion which is a technique by itself well known in the art.

Thereby precious metal such as one or more of gold, silver, platinum or palladium can be extracted from starting material containing relatively small proportions of such metal values in an efficient and economical manner.

Additionally, valuable but non-precious metal e. g. cobalt, nickel, molybdenum which is present in the feedstock solid material is also dissolved by the primary liquid reactant and can be recovered therefrom by techniques which are known in the art.

Stripped liquor from the recovery vessel 6 is supplied (via feed line U) to neutralisation and waste disposal means (not shown).

A carbon-based recovery system, if used as precious metal recovery means, such as activated carbon columns or carbon pulp, is preferably operated at very low pH i. e. high acid levels which not only improves the rate of precious metal retention but also decreases base metal loading and thereby enhances efficiency of the overall extraction process.

Referring to figure 2, the drawing shows an enlargement of vessel 3 in the form of a coiled, tubular pressure reactor. This includes a pipeline module 100 which comprises a tightly wound helical coil 200 of suitable comparatively cheap tubing such as low density polyethylene, with inlet and outlet means 300 for entry and exit of a reaction mixture. The module may be used singly or may be provided with means for linking two or more modules in series or parallel. Coil 200 is embedded in low cost pressure withstanding material 400 such as concrete, which may contain reinforcing material such as wire mesh. A lid 600 is provided at the top of the reinforcing material. The material 400 is surrounded by an insulating cylinder 700 of material such as expanded polystyrene which acts to prevent heat loss for reactions requiring elevated temperature. An insulating lid 800 is also provided. The insulating cylinder 700 and lid 800 enable the material 400 to act as a valuable heat store.

Of course, many variations of the above construction are possible. If the concrete 400 is replaced by tampered down foundry sand or clay, lid 600 will be of heavy material to keep the sand or clay in place. Instead of being constructed as a single block, to give better access, the coil 200 may be surrounded by a comparatively thin layer of tamped sand or clay, itself surrounded by a series of precast rings of reinforcing material.

To eliminate the possibility of air pockets surrounding the coil, for example of polythene, the tubing may be laid into a trough containing a fluid mix of cement, water and required additives. When set, the piping-containing block so formed can provide an inner ring for insertion in a mould so that a concrete aggregate steel reinforced block can be moulded around the inner ring. The resulting block can withstand high pressures, for example up to 1000 p. s. i. A heating coil of polythene may also be incorporated in the inner ring.

In the first vessel 1 the dissolution of the base metals by impure acid, the reaction will be speeded up by increased temperature, and such reaction may be exothermic.

Any such heat can be used to raise the temperature of the incoming feed in stage 1 This can also reduce the amount of heat to be supplied to the submerged combuster, in vessel 2 since spent liquor from feed line C will be at a higher temperature.

Since we provide embodiments which utilise strong acid containing electrolyte, in case of the slag or melt bar (starting material) dissolution thereof can be accelerated by passing an electric current through the liquid medium by means of a metallic cathode in conjunction with a platinum or carbon anode.

Claims

1. An extraction process for recovering precious and optionally valuable non-precious metal from base metal-containing material including recovery and re-use of a primary liquid reactant, the process comprising: (a) treating said material at least once with the same or different primary liquid reactant capable of dissolving said base metal and optionally said valuable nonprecious metal but not said precious metal to a significant extent, thereby forming precious metal-containing residue and used liquid reactant containing dissolved said other metal and optionally if present or required said valuable non-precious metal, (b) separating said used primary liquid reactant from said residue, (c) converting at least some of said used liquid reactant into gaseous precursor of said primary reactant, (d) treating said residue with secondary liquid reactant capable of dissolving precious metal contained therein, (e) causing the gaseous precursor to contact liquid capable of converting it into recovered the said primary liquid reactant, (f) supplying said secondary liquid reactant containing dissolved said metal to precious metal recovery means wherein dissolved said precious metal is extracted from the used second any liquid reactant or otherwise treated to enable subsequent

extraction of such precious metal, and t (g) transferring recovered primary liquid reactant obtained in step (e) for reuse as primary liquid reactant in at least step (a) above.

2. A process as claimed in claim 1 wherein the precious metal extracted is one or more of the following: gold, silver or platinum group metal

3. A process as claimed in either preceding claim wherein valuable nonprecious metal is also extracted and is one or more of the following : cobalt, molybdenum, nickel, vanadium, tungsten, titanium, zirconium and manganese.

4. A process as claimed in any preceding claim wherein the base metal is one or more of iron, lead, copper, zinc or tin.

5. A process as claimed in claim 4 in which the material is a feedstock of solid material.

6. A process as claimed in any preceding claim wherein the base metalcontaining material comprises waste, sewage, or effluent residue.

7. A process as claimed in claim 6 in which the solid material comprises a residue of waste, sewage or effluent sludge which has been filtered and/or dried and/or calcined before use in the treatment stage (a).

8. A process as claimed in claim 7 in which the residue before step (a) contains approximately 1-10% by weight of total metals content and approximately 5 to 500 ppm precious metal (s).

9. A process as claimed in claim 5 in which the feedstock comprises waste metal bars or slag such as a silica slag or metal of high iron and/or lead content, or comprises copper mining slag or melt or comprises nickel and/or zinc processing slag or other residue.

10. A process as claimed in claim 9 in which the slag contains up to about 260 g/ton of gold and/or up to about 300 g/ton of platinum or other platinum group metal, and/or up to about 1,400 g/ton of silver and/or up to about 400 g/ton of cobalt.

11. A process as claimed in any preceding claim wherein the or at least one of the primary liquid reactants comprises nitric acid.

12. A process as claimed in claim 11 in which the nitric acid is dilute and/or impure and/or metal-contaminated and/or admixed with sulphuric acid.

13. A process as claimed in claim 12 in which the or at least one said primary liquid reactant essentially consists of impure dilute nitric acid.

14. A process as claimed in any preceding claim in which step (a) is repeated at least once with the same or different primary liquid reactant before step (d).

15. A process as claimed in claim 14 in which steps (a) and (b) and optionally (c) are repeated at least once before step (d).

16. A process as claimed in any preceding claim wherein additionally, said used primary liquid reactant defined in step (b) is treated for recovery of valuable nonprecious metal (s).

17. A process as claimed in any one of claims 11 to 16 in which the primary liquid reactant comprises spent nitric acid of about 20%-40% concentration (by volume) containing tens of ppm of one or more of zinc, copper, cadmium, iron and tin.

18. A process as claimed in any one of claims 11 to 16 in which the primary liquid reactant comprises a mixture of dilute nitric and sulphuric acids containing one or more of chromium, cobalt, nickel and iron.

19. A process as claimed in any preceding claim, wherein the or each primary liquid reactant is substantially free of halide ions, and which is not capable of dissolving to any significant extent the precious metal (s) intended to be extracted.

20. A process as claimed in any preceding claim wherein the feedstock contains lead and/or tin as well as precious metal (s), and wherein during the process sulphuric acid is added to precipitate either or both of these metals as their insoluble sulphates.

21. A process as claimed in any preceding claim wherein during step (c) used

primary liquid reactant is converted into oxide (s) of nitrogen. t

22. A process as claimed in claim 21 in which conversion is carried out using a submerged combuster.

23. A process as claimed in any preceding claim wherein the secondary liquid reactant is a mixture of a strong oxidising agent and a source of halide ions to act as a leaching agent for the precious metal (s) being extracted and recovered.

24. A process as claimed in claim 23 in which the oxidising agent is nitricacid, in admixture with sodium chloride.

25. A process as claimed in claim 23 in which the oxidising agent is nitric acid in admixture with one or more of cupric chloride, ferric chloride, hydrochloric acid and hydrofluoric acid.

26. A process as claimed in any preceding claim in which step (d) is carried out in a tubular, coiled pressure reactor.