GB2157669A - Exploiting metal deposits of low concentrations - Google Patents

Abstract

The metal to be recovered is maintained in an aqueous solution in dissolved or suspended form; biodegradable organic compounds and optionally nitrogen compounds and phosphorus compounds are added to the aqueous solution; thereupon, the aqueous solution is brought into contact with anaerobic sludge; and the anaerobic sludge, after having been charged with the metal, is dewatered and fed as product to a conventional metal recovery processing stage.

Description

SPECIFICATION Process for exploiting metal deposits of low metal concentrations The present invention relates to a process for exploiting metal deposits of low metal concentration.

Metals are obtained from natural deposits, ores, by various metallurgical methods, mostly multistage methods. Valuable and rare metals are also recovered from wastes, waste-water,, and sludges using specific processes involving recycling.

A prerequisite for economical metal recovery is the presence of an adequate metal concentration in the deposit in question; this concentration can be the lower, the more valuable and rare the metal to be recovered. Efforts are under way at the present time to achieve enrichment of metals from low-concentration solutions, such as, for example, seawater, with the aid of ion exchangers, adsorbents, chemical precipitants, and extraction methods. Such processes have been described, for example, in "Chemikerzeitung" 6183 : 177-189; and in German Patent Specification No. 2,930,110.

It is an object of the present invention to provide a process for enriching metals, especially valuable and rare metals, even in those deposits which have heretofore been unprofitable for exploitation because of too low a metal concentration and/or the expenditure required for enrichment purposes.

According to the invention, there is provided a process for exploiting metal deposits of low metal concentration, comprising the steps of maintaining the metal to be recovered in dissolved or suspended form in an aqueous solution; adding one or more biodegradable organic compounds to the aqueous solution; thereafter bringing the aqueous solution into contact with anaerobic sludge; and dewatering the anaerobic sludge, after it has been charged with the metal, and feeding the dewatered product, to a conventional metal recovery processing stage.

Optionally, one or more nitrogen and/or phosphorus compounds may also be added to the aqueous solution.

The invention is based on the realization that biologically active sludge can be usefully used as a reagent and sorbent especially for metals. Forthis purpose, an aqueous solution is obtained from the metal deposits, this solution containing the metal to be recovered. This step is not necessary if the deposit is already in the form of an aqueous solution. Thereafter biodegradable compounds, as well as nitrogen compounds and phosphorus compounds if required, are added to the aqueous solution, insofar as these are not already naturally contained in the solution to the required amount. As a further essential feature, the aqueous solution so prepared is brought into intimate contact in a reactor with biologically active anaerobic sludge which retains metals.The quantitative ratio in which the aqueous solution and the anaerobic sludge are brought into contact with each other depends on the metal to be recovered and on the concentration in which it is present. It will readily be apparent that more metal can be retained with larger amounts of sludge.

Various reactions, which can occur either in combination or individually, can be utilized as the retention mechanisms. These reactions include, interalia, precipitation in the form of oxides, hydroxides, and the like; complex linkages to functional groups of the organic sludge components; filtration action of the sludge, precipitation of the metals in a reducing medium, and sulphide precipitation by sulphur compounds converted into H2S. The anaerobic sludge is maintained in the biologically active state by the organic compounds and the inorganic nutrients, when present. After the sludge has been charged with metal and/or after exhaustion of its capacity for metal retention, the anaerobic sludge is dewatered, optionally dried and subjected to combustion, and fed as product to a processing operation in accordance with a standard method of metal recovery.

The process of this invention achieves a synergistic effect On the one hand, metal deposits of low concentration can be made accessible to economic exploitation and, on the other hand, biological wastewater sludge, the elimination of which as a waste product causes considerable expense, is made useful as a reagent and sorbent. Moreover, due to the metal enrichment, it is no longer necessary to provide expensive chemicals, such as complexing agents or precipitants, and sorbents, e.g. ion exchange resins, or extractants.

After dewatering, drying and subjection to combustion, the metal-charged sludges yield mineral products of high metal content, for example, in the processing of aqueous extraction solutions in waste dumps and underground leaching of, for example, gold, or for obtaining gold from the washing water of gravity separation in gold mining.

The above-mentioned metal retention capacity of the sludge is dependent on the metal to be recovered, its concentration, the amount of anaerobic sludge used, the temperature and the pH value. The residence time of the anaerobic sludge with the aqueous solution ranges (likewise depending on the metal) from several hours to several days.

Various possibilities exist for the aqueous solutions, likewise dependent on the type of metal in question.

In general, solutions of organic acids such as, for example, acetic acid or formic acid, have proved to be advantageous. It is also possible to add to the aqueous solution one or more mineral acids, such as, for example, HCI or H2SO4 and/or oxidizing agents, such as, for example, H202. It is advantageous to add one or more sulphur compounds, especially in case of metals forming sparingly soluble sulphides. This can be done, for instance, in the form of sulphate or thiosulphate. As mentioned above, the aqueous solution is combined with one or more organic compounds and optionally with one or more nitrogen compounds and phosphorus compounds.In this connection, it is advantageous to maintain a minimum organic compound content, expressed as COD (chemical oxygen demand), of 50 mug/1, especially 100 mail. Moreover, it is beneficial to maintain an average sludge content of 2-9% or, expressed differently, 2090 kg/m3. A higher sludge content can be provided, but ordinarily the range mentioned is sufficient for optimum metal retention. In case of lower average sludge contents, it can happen that the metal retention capacity of the sludge is exhausted within too short a time period, i.e. the sludge is charged with metals too quickly, or the retention mechanism is inadequate.

Experiments have shown that the process of the invention, i.e. contacting the aqueous solution with anaerobic sludge, can be performed especially well attemperatures in the range of 100 to 65 C, preferably 150 to 25 C. At lower temperatures, there is the possibility that the anaerobic sludge will no longer be sufficiently active, whilst at higher temperatures, the sludge likewise loses its activity because of its sensitivity to elevated temperatures. Here, the range of 150 to 250C corresponds to the average ambient temperature.

Furthermore, investigations have shown that the aqueous solution and the anaerobic sludge can be made to contact each other at a pH value of from 2.5 to 9.5. In contrast to conventional methods, the process of this invention can thus also be conducted in the acidic range (pH approximately 2.5), since no methanizing takes place which limits the conventional processes to the neutral region. On the other hand, results have proved that very good metal retention values can also be obtained in the alkaline region. It is clear that, in this connection, the pH value depends greatly on the metal to be recovered.

The process of this invention considerably expands the economical limit for many metallurgical methods, this limit being defined for metals by respective minimum concentrations in the deposits. However, the process is applicable, in principle, for any desired concentration of the metal to be recovered, although in its preferred embodiment it is intended for metals in low concentration. All kinds of metals can be enriched by the method of the invention, such as, for example, heavy metals, specific examples being Ag, Au, Cd, Cr, Cu, Hg, Ni, Pb and Zn.

The invention is illustrated by the following examples: Example 1 Silver Reco very from a Waste Dump The waste dump is subjected to percolation by a silver-dissolving agent, which is again collected at the bottom of the dump by way of a drainage system and is recirculated through a sludge-bed reactor.

Suitable silver-dissolving agents are aqueous solutions of organic acids, eg acetic acid, optionally with additions of mineral acids (HC1, H2SO4) and/or oxidizing agents, e.g. H202 or, respectively, complexing agents, e.g. thiosulphate.

The silver-containing eluate collected by the drainage system is brought into intimate contact with sludge in the anaerobic reactor, for example, an upward-current reactor. During this step, the silver is retained in the sludge bed by retention mechanisms such as: - sulphide precipitation by sulphur compounds converted into H2S; - precipitation in the form of oxide, hydroxide or the like; - complex linkages to functional groups of the organic sludge components.

The eluate of reduced silver content, is separated from the sludge, for example, by sedimentation and/or filtration, and pumped back to the waste dump for re-use.

A small portion of the organic ingredients is consumed by the vital processes of the sludge organisms and must therefore be replaced. Likewise, small amounts of nitrogen compounds and phosphorus compounds must be added as nutrients to the eluate before it enters the reactor.

Arithmetic Example Waste dump with 500,000 metric tons of rock Ag content: 0.005% = 50 ppm = 25,000 kg (mineable ores contain 0.01 - 0.3% Ag) Recoverable by means of the process of this invention: 50% = 12,500 kg Value, at present silver prices: 11.25 million DM (3 million) Reactor: upward-current reactor with 500 m3 useful volume average sludge content 20 kg dry solids/m3 total amountofsludge 10,000kg drysolidsAg Load attainable: 10,000ppm = 10,000mgAg/kg drysolids Ag Retention capacity: 100 kg Reactor operation: Residence time 12 h, liquid throughout 42 m3/h = 1,000 m3/day Agent selected: 10% strength aqueous acetic acid solution with additions of H2SO4 and H202 (pH 3.1 - 3.6) Ag Concentration of eluate prior to entering reactor: 3 mug/1 Ag Concentration of eluate upon leaving reactor: 1 mgil Ag enrichment in sludge: 2 kg/day Reactor service life with one sludge filling: 50 days.

A solid with 2.5% Ag can be obtained from the sludge (dewatering, drying, combustion, annealing loss 60%).

Example 2 Gold Recovery from Pit Waters The pit water of an ore mine contains small amounts of gold, in part dissolved, in part bound in complex form to humic acids and in part in the shape of extremely finely divided suspended matter.

The pit water is conducted through a settling tank to separate materials that can be sedimented. Prior to introduction into an anaerobic reactor, the organic chemicals, as well as nitrogen compounds and phosphorus compounds, required for the viability of the sludge organisms and for providing a reducing environment, are added in metered quantities.

In the anaerobic reactor, e.g. an upward-current reactor, the gold is retained in the sludge by retention mechanisms such as: - precipitation of gold and gold hydroxides in a reducing environment; - filtration effect of sludge; - complex binding to functional groups of the organic sludge components.

Gold-containing solid particles drifting out of the anaerobic reactor are held back in a fine filter connected downstream of the reactor. Charged sludge from the reactor and filtered-off solids are dewatered and passed on to further processing by a standard method of gold recovery.

Arithmetic Example Available pit water: 5,000 m3/day Au Content: 0.5 mg/1 = 2.5 kg/day Recoverable by process of invention: 50% ^ 1.25 kg Au/day Value at today's gold prices: about DM 40,000 (10,500) Reactor: upward-current reactor with 1,000 m3 useful volume average sludge content 20 kg dry solids/m3 total amountofsludge 20,000 kg residence time: about 5 h operating life with one sludge charge: about 80 days Au load attainable: 5,000 ppm =5,000 mg Au/kg dry solids Retention capacity of one sludge charge: 100 kg

Claims (12)

1. A process for exploiting metal deposits of low metal concentration, comprising the steps of maintaining the metal to be recovered in dissolved or suspended form in an aqueous solution; adding one or more biodegradable organic compounds to the aqueous solution; thereafter bringing the aqueous solution into contact with anaerobic sludge; and dewatering the anaerobic sludge, after it has been charged with the metal, and feeding the dewatered product, to a conventional metal recovery processing stage.

2. A process according to Claim 1, wherein one or more nitrogen compounds and/or phosphorus compounds, is or are added to said aqueous solution before it is brought into contact with the anaerobic sludge.

3. A process according to Claim 1 or Claim 2, wherein an aqueous organic acid is used as the aqueous solution.

4. A process according to any one of Claims 1 to 3, wherein one or more mineral acids and/or oxidizing agents is or are added to the aqueous solution.

5. A process according to any one of the preceding Claims, wherein a sulphur compound is added to the aqueous solution.

6. A process according to any one of the preceding Claims, wherein a COD content of at least 50 mg/1 is maintained.

7. A process according to Claim 6, wherein the COD content is at least 100 mg/1.

8. A process according to any one of the preceding Claims, wherein an average sludge content of 20~90 kg/m3 is maintained.

9. A process according to any one of the preceding Claims, wherein the aqueous solution is brought into contact with anaerobic sludge at a temperature in the range of 100 to 650 C.

10. A process according to Claim 9, wherein the aqueous solution is brought into contact with anaerobic sludge at a temperature in the range of 15 to 250 C.

11. A process according to any one of the preceding Claims, wherein the aqueous solution and the anaerobic sludge are brought into contact with each other at a pH value in the range of 2.5 to 9.5.

12. A process for exploiting metal deposits of low metal concentration, substantially as hereinbefore described with reference to Example 1, or Example 2.