AU2016265024A1

AU2016265024A1 - A truncated hydrometallurgical method for the removal of radionuclides from radioactive copper concentrates

Info

Publication number: AU2016265024A1
Application number: AU2016265024A
Authority: AU
Inventors: Peter John Bartsch; Grenvil Marquis Dunn; Stuart SAICH
Original assignee: ORWAY MINERAL CONSULTANTS WA Pty Ltd
Current assignee: ORWAY MINERAL CONSULTANTS (WA) Pty Ltd
Priority date: 2015-05-19
Filing date: 2016-05-13
Publication date: 2017-10-12
Also published as: BR112017023545A2; WO2016183611A1

Abstract

A simplified or truncated hydrometallurgical method for the removal of uranium, thorium, radium, lead, bismuth and polonium and/ or other radionuclides from a radioactive copper concentrate to produce an upgraded copper concentrate having lowered emission levels is described. The method involves subjecting the copper concentrate to an acidic leaching process (NONOX leach 120) using a copper sulfate and chloride salts containing lixiviant under lowered electrochemically conditions, to allow controlled removal of one or more of the radionuclides to produce the lowered emission level upgraded copper concentrate, and lower soluble copper losses in the discharge stream. The leaching process (120) is conducted to convert essentially all the copper in the lixiviant into the upgraded copper concentrate, and at elevated temperature and under pressure to suppress boiling in the leaching process.

Description

“A TRUNCATED HYDROMETALLURGICAL METHOD FOR THE REMOVAL OF RADIONUCLIDES FROM RADIOACTIVE COPPER CONCENTRATES”

Field of the Invention

The present invention relates to a truncated or simplified method for the hydrometallurgical removal of radionuclides from radioactive copper concentrates. The method relates particularly although not exclusively to the removal of the radionuclides uranium, thorium, radium, lead, bismuth and polonium. The method relates typically but not exclusively to the removal of radionuclides from copper concentrates being primary copper sulphide flotation concentrates and matte without pre-treatment or posttreatment. The method also relates to the efficient use of soluble copper in the non-oxidative leaching step for upgrade of copper concentrates and so minimize copper losses.

Background to the Invention

The dominant copper-containing minerals in most copper sulfide deposits are chalcopyrite, cubanite and bornite. Chalcocite, coveiite and in some cases enargite or tennantite are also present. The gangue mineral sulfides sometimes have pyrite and pyrrhotite present, many of these along with lesser quantities of host or gangue minerals report to the final flotation concentrate.

High-grade, copper sulfide concentrates, (typically greater than about 25% Cu weight/weight), are commonly treated by pyrometallurgical routes, whereas hydrometallurgical routes are generally favoured for the lower-grade, or impurity bearing copper concentrates. The economically and technically most favourable processing route can also be influenced by the concentration of minor amounts of valuable metals such as cobalt and nickel, or payable precious metals such as silver, gold, palladium and platinum, as well as contamination by radioactive elements such as uranium, thorium, radium, lead, bismuth or polonium and deleterious metals such as arsenic, present in the feed material. Hydrometallurgicai processing routes are generally more energy consuming than smelting, because the heat of combustion of the concentrates is not efficiently utilized.

The three dominant pyrometaliurgical routes for high-grade, copper sulfide concentrates are; a) smelting to a matte followed by converting to blister copper, b) direct to blister smelting and c) oxidative roasting.

The efficiency of the smelting technology is determined by, amongst other things, the Cu/S ratio and the concentration of slag forming components, especially iron, magnesium and silica. Conventional smelting processes are generally not applicable to lower grade copper concentrates. Not all of the copper content of the original feed is recovered as blister copper, with the remaining copper reporting to the slag and to the smelter dusts or fumes recovered from the smelter off-gases.

Roasting of copper concentrates requires the conversion of the copper content to a water-soluble or sulfate form, which is recovered from the roaster calcine by leaching, followed by solvent extraction and electrowinning. Roasting is often inefficient because copper-containing insoluble ferrite phases can form during the roasting stage and lock some copper and valuable by-products such as cobalt.

Many hydrometallurgicai processes have been described for treating copper-containing concentrates, for example;

Burkin A. R, Chemical Hydrometallurgy, 1952-1994, Trans. Inst Min. Metall., 103, 1994, C169-C176.

Dreisinger, D, Copper leaching from primary sulfides: Options for biological and chemical extraction of copper, HYDROMETALLURGY, 2006, 83, 10-20.

Few of the proposed processes have attained full-scale commercial development, and most give little or no attention to removal of impurity or penalty elements, including radionuclides, or disposal of these elements by environmentally benign methods. Hydrometailurgical processes for copper concentrates struggle to compete economically against pyrometailurgical steps such as smelting, for reasons including: a) effective removal of impurity or penalty elements, b) cost of power, c) environmentally acceptable disposal of residues, and d) difficulty in precious metal recovery.

Economic performance of the smelting routes in particular is improved if copper concentrates can be upgraded in their copper content or deleterious impurities can be removed before being fed to the smelting furnaces. But the source or production of copper sulfate solution for 'metathesis' reactions, and deportment of the impurity, radioactive or value elements, is not considered in most treatment routes, and nor is the disposal of residues and effluents.

Various means of hydrometailurgical upgrading of the copper content of a copper concentrate have been proposed, including 'metathesis' leaching which displaces the iron content of the concentrate with copper. The so-called 'metathesis' process, in which the chalcopyrite component of the concentrate is reacted with a copper sulfate solution to produce low-iron copper sulfide (e.g. digenite) and an acidic, ferrous sulfate solution, can be represented. 0)

A similar reaction also occurs for any bornite present in the copper concentrate. (2)

Similar reactions occur when cobait is recovered from a blend of cobaitite and chalcopyrite minerals or carrollite.

The descriptions of processes which involve metathesis reactions do not generally include the source of copper sulfate, the deportment of impurity, valuable and radioactive elements, or the treatment and disposal of residues and effluents.

One or both of these above reactions (1) and (2) are referred to directly or indirectly in US Patent Nos. 2 568 963, 2 662 009, 2 744 172 and 4 024 218, Canadian Patent No. 1 258 181, South African Patent No. 2007/01337, and WIPO Patent Publication No. WO 2004/106561. All of these patents propose to forward the upgraded copper sulfide concentrate, which typically contains above 50% Cu, to either a smelter or treatment by other means.

The flowsheets in these patent specifications contain several deficiencies, such as identifying an economic source of copper sulfate solution, incomplete separation of iron and copper in solution, the requirement of additional flotation steps, economic recovery of precious metals from the residues, or have difficulty removing other impurities such as radionuclides, including uranium and its decay elements, and the final destination or treatment route of residues and effluents which can be problematic.

More recently PCT/AU2014/000268 and PCT/AU2014/000269 aim to at least partially overcome some of these deficiencies. This prior art addresses more importantly the removal of uranium, along with the other radionuclides which are its decay elements, that would otherwise limit or penalise the processing of the concentrate in an off-shore or remote smelter, or prohibit or restrict the international trade of copper concentrate across international borders. Thus these inventions have successfully lowered the level of uranium and other radionuclides in a radioactive or ‘dirty’ copper concentrate to allow the concentrate to be smelted within the limits of national or international regulations. These recently published flowsheets have included pre- and/or post- treatment of the concentrates, which necessarily complicates the number, and type of process steps, as well as the control of the chemistry in each of the process steps. Such complexity adds to the operating and capital costs of implementing the flowsheet for the treatment of copper concentrates compared to the methods described by the present invention.

The present invention was developed with a view to providing a truncated flowsheet and simplified operation of facilities for hydrometallurgical removal of radionuclides from copper concentrates, which can be conducted to lower the soluble copper losses, and so enhance the economics of the treatment process. Overall capital and operating cost components of the total processing of concentrates are minimised, as well as ensuring residues may be disposed by means acceptable to regulatory authorities.

References to prior art in this specification are provided for illustrative purposes only and are not to be taken as an admission that such prior art is part of the common general knowledge in Australia or elsewhere.

Summary of the Invention

According to one aspect of the present invention there is provided a hydrometallurgical method for the removal of uranium, thorium, radium, lead, bismuth and polonium and/ or other radionuclides from a radioactive copper concentrate to produce an upgraded copper concentrate having lowered emission levels, the method comprising the step of: subjecting the copper concentrate to an acidic leaching process (NONOX leach) using a copper sulfate and chloride containing lixiviant under lowered electrochemical conditions, to allow controlled removal of one or more of the radionuclides to produce the lowered emission level upgraded copper concentrate, and lower soluble copper losses in the discharge stream, and wherein the leaching process is conducted: to convert essentially all the copper in the lixiviant into the upgraded copper concentrate, and employing elevated temperature and elevated pressure to suppress boiling in the leaching process.

Preferably the leaching process (NONOX leach) is conducted at an electrochemical potential of greater than 150mV (Ag/AgCI 3.8M KCI). More preferably the leaching process (NONOX leach) is conducted at an electrochemical potential in the range of between about 175mV and 450mV (Ag/AgCI 3.8M KCI).

Typically the electrochemical potential of the NONOX leach is controlled by the presence of, or the addition to the NONOX leach of, one or more of cupric sulfate, ferric ion, air, oxygen, sodium chlorate, pyrolusite, or hematite.

Preferably the sulfate and chloride containing lixiviant comprises at least copper sulfate and sodium chloride.

Preferably the radionuclides comprise one or more radionuclides selected from the group comprising U238, Th230, Ra226, Pb210, Po210 and Bi210.

Preferably the NONOX leach allows for the removal of about 20% to 99% of the uranium and/or thorium, and allows for lowering of one or more of radium, lead, bismuth and polonium levels to below 3Bq/g.

The copper sulfate and chloride containing lixiviant typically comprises one or more of the following solutes: sulfuric acid, hydrochloric acid, sodium chloride, chloride salt, potassium chloride, and copper sulfate, the sulfate and chloride containing lixiviant being obtained from within the NONOX leach, or from a source external to the NONOX leach.

Preferably each radionuclide in the lowered emission upgraded copper concentrate has a radioactive emission level of about 0.1 to 2.0Bq/g. The lowered emission upgraded copper concentrate may typically have a radioactive emission level of less than about 0.5Bq/g for U238 or Th230 and a radioactive emission level of less than about 2Bq/g for each of the other radionuclides. The copper sulfate and chloride containing lixiviant may comprise one or more of seawater, brines from seawater desalination, and fluorides, or other anions leached from the radioactive copper concentrate, or from process water. The chloride levels in the sulfate and chloride containing lixiviant are typically between 5 and 100g/L chloride.

Preferably the NONOX leach is conducted between about 100° C and 240° C. More preferably the NONOX leach is conducted between about 160° C and 240° C.

In the hydrometallurgical method of the invention, copper-iron-sulfides and cupric sulfide (covellite) are typically transformed in the NONOX leach, and more usually the NONOX leach is conducted under conditions to transform 50% to 99% of the copper-iron-sulfides to iron-depleted copper sulfides and covellite and to a sulfur-depleted variant (digenite/chalcocite). Similarly pyrite and pyrrhotite can be substantially altered under NONOX conditions to iron depleted copper-iron-sulfides.

Iron oxides that often host the value minerals can be altered under NON OX conditions.

Preferably the NONOX leach is conducted at a pressure in the range of 500-3500kPa.

Typically the retention time of the NONOX leach is between about 0.5 and 8 hours.

The method may further comprise feeding into the NONOX leach a copper sulfate slurry from a crystalliser or a copper sulfate leach liquor from a pressure oxidation reactor (PROX reactor), or copper sulfate solution from an external source, or a copper sulfate leach slurry direct from a PROX reactor. The sulfate slurry typically comprises copper sulfate, iron sulfate and sulfuric acid. Preferably the copper sulfate is added to more than one compartment in the NONOX leach reactor. Typically the copper aqueous concentration levels within the discharge liquor from the NONOX leach reactor are less than 5g/L and more typically less than 1g/L.

Throughout the specification, unless the context requires otherwise, the word “comprise” or variations such as “comprises” or “comprising”, will be understood to imply the inclusion of a stated integer or group of integers but not the exclusion of any other integer or group of integers. Likewise the word “preferably" or variations such as “preferred”, will be understood to imply that a stated integer or group of integers is desirable but not essential to the working of the invention.

Brief Description of the Drawings

The nature of the invention will be better understood from the following detailed description of preferred embodiments of the invention, given by way of example only, with reference to the accompanying drawing, in which:

Figure 1 is a flow diagram of a hydrometallurgical method for the removal of radionuclides from a radioactive or ‘dirty’ copper concentrate; and,

Figure 2 is a flow diagram of an embodiment of a simplified or truncated hydrometallurgical method according to the invention for the removal of radionuclides from a radioactive or ‘dirty’ copper concentrate.

Detailed Description of Preferred Embodiments

Figure 1 is a process flowsheet for a hydrometaliurgical method 80 for the removal of uranium, thorium, radium, lead, bismuth and polonium and/ or other radionuclides from a radioactive copper concentrate [92].

Copper concentrate [92] is produced by known methods of treating sulfide ore [91] such as milling and flotation [90]. Tailings [93] which contain the gangue minerals may be disposed directly to impoundment [169] or utilized for neutralization of effluent streams, or treatment of discharge liquors [180].

The “dirty” or radioactive copper flotation concentrate [92] containing primary and secondary copper sulfides, iron sulfides, primary uranium minerals and associated radionuclides is repulped [100] with process liquors that may contain copper, iron, sulfates, chlorides [94]. Typically the copper sulfide minerals include one or more of: chalcopyrite, covelite, bornite, chalcocite, cubanite, enargite, tennantite, tetrahedrite. Typically the iron sulfides may contain pyrite, arsenopyrite, pyrrhotite and the like. Typically the primary uranium minerals may contain brannerite, betafite, davidite, coffinite and uraninite. Generally, the decay products of uranium will be associated with the uranium minerals, but may be distributed with other minerals depending on the prior treatment of the ore and concentrate.

Repulped concentrate slurry [101] is advanced to a pre-treatment circuit [104] where steam [103] is employed to raise the slurry temperature to near atmospheric boil point. The concentrate in [104] can be employed to recover copper in a metathesis process from the repulp fluid [94] and then filtered [106] to reject unwanted soluble iron, etc. [105] to a tailings facility [169],

Optionally the pre-treated concentrate [107] can be slurried in process liquors and treated in an alkaline oxidation step [108], employing steam [123] and air or oxygen [109] and the hot slurry [126] is then pumped into the NONOX reactor for an acid leaching process (NONOX leach) [120],

An alternate variant to the upgrade of the dirty or radioactive copper concentrate has the dirty or radioactive copper concentrate [92] being fed to the first-stage radionuclide leach [140]. A further variant has the dirty copper concentrate [92] being fed direct to the NONOX reactor [120] (not shown in Figure 1).

The radioactive ('dirty’) copper concentrate is subjected to an acidic leaching process (NONOX leach) using a sulfate and chloride lixiviant (in which the chloride levels are between 5 and 100g/L chloride) under electrochemicaliy controlled conditions, to allow at least partial removal of one or more of the radionuclides to produce the lowered emission upgraded copper concentrate, wherein the leaching process is conducted at elevated temperature and under pressure to suppress boiling in the leaching process.

The upgraded copper concentrate is typically a concentrate which is upgraded in terms of copper metal.

The NONOX reactor [120] receives a copper sulfate, iron sulfate and sulfuric acid containing slurry [115] from the pressure oxidation (PROX) reactor [110]. Optionally stream [115] can be a copper, iron sulfate stream from which the pressure oxidation leach residue has been separated. Optionally stream [115] can be a copper sulfate crystal slurry. Optionally the copper sulfate addition to the NONOX reactor can be in two or more locations within the autoclave. Preferably the copper concentration in the NONOX reactor aqueous phase is maintained in excess of 1g/L and typically in excess of 5g/L. Another option generates stream [115] within the NONOX reactor from the products of the NONOX reaction from the dirty concentrates [92] or the pre-treated concentrates [102] or [126],

Preferably this hot slurry [115] from the PROX reactor [110] is fed directly without cooling into the NONOX reactor [120]. Steam [121] may be supplemented to maintain the reactor at the target temperature.

Temperatures below 240°C and typically below 210°C are employed in the NONOX leach. Typically the NONOX leach is conducted between about 100°C and 240°C, and more typically between about 160°C and 240°C. Nominally anaerobic leach conditions are maintained through the NONOX reactor. Small quantities of oxygen, air or an appropriate oxidant may be required to influence the overall chemistry within the reactor or to control the electro-chemical potential of the reactor above typically 150mV (Ag/AgCI; 3.8M KOI), and more typically in the range of between about 175mV and 450mV (Ag/AgCI 3.8M KOI). The electrochemical potential is chosen with a lower limit of just above that at which elemental copper could form and precipitate, and be lost from the aqueous phase. The inventors have found that it is possible to reduce the electro-chemical potential of the reactor to a potential of above 150mV (or above 175mV) instead of the higher ranges mentioned in the applicant’s previous application PCT/AU2014/000268. These lowered ranges are achieved, inter alia, at elevated temperatures and are supportive of improved radionuclide removal.

Conditions are employed such that a majority of the iron associated with the copper minerals is removed and a near stoichiometric amount of copper is precipitated in the concentrate.

Advantageously, conditions are employed in the reactor such that there is above 90% removal of uranium and thorium, and above 30% removal of radium, lead and polonium from the concentrate. Other base metals such as nickel, cobalt and zinc are also substantially removed from the dirty copper concentrate.

Copper solution transferred in stream [115] is lowered to concentration between 5-50 g/L in the exit stream [122] from the NONOX leach reactor.

The other components of the aqueous fraction of NONOX discharge [122] will vary with the mineral assemblage in concentrate, the carrier fluid comprising the slurry and the soluble content of feed stream [126] but include iron (major), sulfuric acid (major) and lesser quantities of dissolved uranium, lead, thorium, radium, polonium, bismuth, aluminium, selenium, magnesium, calcium, silicon, and other soluble metal anions such as sulfates, chlorides, etc. A NONOX leach retention time of between 0.5 and 8 hours is required and typically this could be between 1 to 3 hours.

Stream [122] is preferably cooled in a flash tank [125] where steam [123] and slurry [124] are discharged.

The flash steam [123] can be scrubbed and then used for preheat duties. Excess flash steam can be released to atmosphere or employed elsewhere in the flowsheet for heating.

The cooled slurry [124] can be further cooled and then thickened in decanter [130]. Flocculant [131] and recycle filtrate [133] and internal solution recycles are employed to aid slurry thickening. The thickener overflow [132] can be further clarified before optionally recovering uranium [186] and thereafter copper recovery [188].

Alternately, the thickener overflow liquor [132] is partially neutralised [180] by employing for example, alkaline filtrate [147], flotation tailings [93] and or limestone [182], The partial neutralisation step is employed to remove the acid from thickener overflow [132] and filtrate [157]. Additionally any radium, lead, bismuth and polonium in streams [132] and [157] are precipitated.

The partial neutralisation slurry [181] can be thickened and or filtered [183] to produce a slurry or cake [184],

The partial neutralisation filtrate [185] can be further processed in [186] for the recovery of uranium and the recovery of copper [188] which can be recycled [95] to repulp or concentrate [100], or marketed with clean concentrate [188A], The waste liquor containing primarily iron and lesser quantities of aluminium, magnesium, potassium, calcium, etc. [189] can be disposed of in the tailings storage facility [169].

The thickened slurry [134] is filtered [135] and the upgraded copper concentrate is then washed with water [136] before it is repulped in clean water [139] and transferred as a washed upgraded copper concentrate [137].

The washed concentrate is split [170] into two portions by employing known processes e.g. flotation, cycloning, screening, partitioning, etc. One part [111] is employed in the PROX leach [110], and the balance [138] is fed forward to an optional first-stage radionuclide leach [140],

The upgraded copper concentrate [111] that is fed to the PROX leach [110] may have an iron analysis of 3 to 5%, and in the PROX process this will be oxidised to a mixture of ferrous and ferric sulfates.

The PROX reactor [110] receives the upgraded copper concentrate slurry [111] along with sulfuric acid [112] and oxygen or air [114].

The PROX reactor operates: • at temperatures below 240°C and typically between 140 and 200°C; • at an oxygen partial pressure of between 200 and 1000 kPa, and • with a retention time of between 0.25 and 6 hours.

Sulfuric acid is required to stabilise copper (and iron) in solution. A free acid concentration of between 1 and 50 g/L is adequate for the (PROX) reactor [110],

Alternately the PROX reactor discharge [115] can be flashed as in the case of the NONOX reactor discharge [122] and the flashed underflow further separated into a solids fraction and a liquid fraction. The liquid fraction can then be fed to the NONOX reactor [120] and the solid fraction combined with stream [138] after the separation [170], Optionally the copper sulfate leach liquor from the PROX reactor [110] can be crystallised in a copper sulfate evaporator to produce a copper sulfate crystal slurry which can be fed directly to the NONOX reactor [120].

The PROX reactor discharge [115] can be flashed as in the case of the NONOX reactor discharge [122], and a flash tank underflow slurry pumped into the NONOX reactor [120]. Preferably, the PROX reactor [110] discharges directly into the NONOX reactor [120].

The latter option is more thermally efficient and will require less supplementary steam [121] to sustain the NONOX reactor [120] at its design operating temperature.

The repulped upgraded copper concentrate stream [138] is the feed to the optional first-stage radionuclide leach [140]. in the event that the PROX-NONOX steps (1) (3) (4) are not required to leach the uranium or radionuclides in the “dirty” copper concentrate [101], then a preheated repulped copper concentrate stream [102] can alternately be employed as the feed [102A] to the first-stage radionuclide leach [140].

The first and second-stages of radionuclide leaching [140] and [150] are only required if the radioactivity levels are “export limiting” in regard to marketing or regulation of upgraded copper concentrate. A sodium-based alkaline reagent, preferably sodium carbonate or sodium hydroxide [141], may be employed to remove sulfate chemical species from the NONOX upgraded copper concentrate [138], The first-stage leach liquor may also contain chlorides, should saline process waters be available.

The first-stage radionuclide leach [140] is conducted at temperatures below 100°C. Typically temperatures in the region of 40 to 95°C are adequate. Steam [142] is employed to provide the energy required to maintain the required temperatures in the leach [140]. A retention time of between 0.1 and 6 hours is required in the first-stage radionuclide leach [140].

The first-stage radionuclide leach [140] is conducted in a mildly oxidising electrochemical potential of the reactor above typically 250 mV, (Ag/AgCI; 3.8M KCI), to facilitate the dissolution of the electropositive radionuclides from the residue [138],

The first-stage radionuclide leach discharge [143] is filtered in a filter [145]. The residue is washed with water [144] to remove a majority of the soluble sulfate in the leach residue and the combined filtrate and wash filtrate [147] can be disposed of to the Tailings Partial Neutralisation Step [180] or the Tailings Storage Facility [169],

The first-stage leached concentrate [143] following the filter [145] is repulped in a repulp fluid [149] to produce a slurry [146] that is fed to the second-stage radionuclide leach [150].

The repulp fluid [149] can be a recycled acidic filtrate [157] or water.

The second-stage radionuclide leach [150] requires an acid [151] and optionally a chloride salt [152] to sequester and dissolve the radionuclide species and hence lixiviate them from the upgraded copper concentrate. Preferably the acid is hydrochloric acid [151] and optionally the salt is sodium chloride [152]. Mixtures of acids and salts or anions of the type nitrate, citrate, acetate etc. that enhance the dissolution of some of the radionuclides e.g. radium, lead, bismuth, polonium, etc. can also be employed in the second-stage radionuclide leach [150],

The second-stage radionuclide leach [150] is conducted at temperatures below 100°C. Typically temperatures in the region of 30 to 95°C are adequate. Steam [159] or heated repulp fluid [149] employed to sustain the required temperatures in the leach [150]. A retention time of between 0.1 to 6 hours is required in the second-stage radionuclide leach [150],

The second stage leach can be conducted as a filter-wash step. The acid stream [151] with or without the salt stream [152] can be applied to the washed first-stage residue in filter [145],

The second-stage radionuclide leach [150] is conducted under anaerobic to mildly aerobic conditions of the reactor above typically 250 mV, {Ag/AgCI; 3.8M KCl) to facilitate dissolution of the electro-positive radionuclides.

The second-stage radionuclide leach discharge slurry [153] is filtered in a filter [155], The residue in the filter is washed with desalinated water [154] to remove a majority of the chloride in the concentrate. The liquor filtrate and the wash filtrate [157] is transferred to the Tailings Partial Neutralisation step [180] or to the Tailings Storage Facility [169],

The washed residue [156] from the filter [155] is the 'clean', or upgrade, low emission, copper concentrate which is destined for the market without restrictions.

Alternately, the processes of Step (2), including first and second stage radio-active leaching, may be conducted prior to NONOX leach step {4). The sequence of the leach steps will be optimally determined following experimental tests on samples of the 'dirty' concentrate, and will depend on its distribution and content of the radioactive elements and their various responses.

Experiments have been conducted on various concentrates, with less process steps, i.e, using a 'truncated1 flowsheet. The present invention still includes the major step of non-oxidative (NONOX) leaching, and may also include pressure oxidative (PROX) leaching, along with ancillary steps of dewatering by thickening or filtering, as well as washing and splitting the concentrate stream that are well known from prior art. These experiments have discovered that by using controlled chemical conditions in the NONOX leaching step the radio-nuclides can be removed to low levels similar to that reached in the flowsheets as described in the prior art. The requirement for pre- and post- treatment of the concentrates to reach low levels of radionuclides can be avoided by the present invention, which allows lower capital and operating costs, as well as a simplified process operation. The tenor of soluble copper which discharges from the NONOX leach reactors is lowered to levels which minimize the losses of soluble copper.

The following description is a preferred embodiment of a simplified or truncated hydrometallurgical method 280 of the present invention and refers to the process flowsheet in Figure 2. Since the process flowsheet of Figure 2 is a simplified version of the process flowsheet of Figure 1, the same reference numerals will be used to identify the like process steps and components.

Copper concentrate [92] is produced by known methods of treating sulfide ore [91] such as milling and flotation [90], Tailings [93] which contain the gangue minerals may be disposed directly to impoundment [169] or utilized for neutralization of discharge liquors.

The “dirty” or radioactive copper flotation concentrate [92] containing primary and secondary copper sulfides, iron sulfides, primary uranium minerals and associated radionuclides is repulped [100] as a process slurry 101 that may contain copper, sulfates, iron hydroxides [115] and chlorides in the form of brine [102], Preheat can be applied to the repuip employing fresh or recycled steam [123]. Typically the copper sulfide minerals include one or more of: chalcopyrite, covellite, bornite, chalcocite, cubanite, enargite, tennantite, tetrahedrite. Typically the iron sulfides may contain pyrite, arsenopyrite, pyrrhotite and the like. Typically the primary uranium minerals may contain brannerite, betafite, davidite, coffinite and uraninite. Generally, the decay products of uranium will be associated with the uranium minerals, but may be distributed with other minerals depending on the prior treatment of the ore and concentrate.

Repulped concentrate slurry [101] is advanced to a non-oxidising leach (NONOX) reactor [120],

The radioactive (‘dirty’) copper concentrate is subjected to an acidic leaching process (NONOX leach) using a sulfate and chloride lixiviant (in which the chloride levels are between 5 and 10Og/L chloride) under electrochemically controlled conditions, to allow at least partial removal of one or more of the radionuclides to produce the lowered emission upgraded copper concentrate, wherein the leaching process is conducted at elevated temperature and under pressure to suppress boiling in the leaching process.

The repulp step [100] receives a copper sulfate, iron sulfate, iron hydroxides and sulfuric acid containing slurry [115] from the pressure oxidation (PROX) reactor [110]. Optionally stream [115] can be a copper, iron sulfate stream from which the pressure oxidation leach residue has been separated. Optionally stream [115] can be a copper sulfate crystal slurry or a concentrated copper solution. Optionally the copper sulfate addition to the NONOX reactor can be in two or more locations within the autoclave. Preferably the copper concentration in the NONOX reactor aqueous phase is discharged below 1 g/L, or typically below 5g/L. Another option generates stream [115] within the NONOX reactor from the products of the NONOX reaction and from the dirty concentrate [92].

Optionally this hot slurry [115] from the PROX reactor [110] is fed directly without cooling into the NONOX reactor [120]. Steam [121] may be supplemented to maintain the reactor at the target temperature.

Temperatures below 240°C and typically below 225°C are employed in the NONOX leach. Typically the NONOX leach is conducted between about 100°C and 240°C, and more typically between about 160°C and 240°C. Nominally anaerobic leach conditions are maintained through the NONOX reactor. Small quantities of oxygen, air or an appropriate oxidant may be required to influence the overall chemistry within the reactor or to control the electro-chemical potential of the reactor above typically 150mV (Ag/AgCI; 3.8M KCI), and more typically in the range of between about 175mV and 450mV (Ag/AgCI 3.8M KCI). The electrochemical potential is chosen with a lower limit of just above that at which elemental copper could form and precipitate, and be lost from the aqueous phase. The inventors have discovered that control of the electrochemical potential of the NONOX reactor to a potential of above 150mV (or above 175mV) is advantageous for leaching the radio-nuclides, instead of the higher ranges mentioned in the applicant’s previous application PCT/AU2014/000268. These lowered electrochemical ranges are achieved, inter alia, at elevated temperatures and are supportive of improved radionuclide removal.

Advantageously, conditions are employed in the reactor such that uranium extraction generally exceeds 70%, is typically above 80% and can exceed 90%. The removal of thorium, radium, lead and polonium from the concentrate is normally in excess of 30% and can exceed 90%. Other base metals such as nickel, cobalt and zinc are also substantially removed from the dirty copper concentrate.

Copper solution transferred in stream [115] is lowered to concentration between 0.1 and 5,0g/L in the exit stream [122] from the NONOX leach reactor.

The other components of the aqueous fraction of NONOX discharge [122] will vary with the mineral assemblage in concentrate, the carrier fluid comprising the slurry and the soluble content of feed stream [101] but include iron (major), sulfuric acid (minor) and lesser quantities of dissolved uranium, lead, thorium, radium, polonium, bismuth, aluminium, selenium, magnesium, calcium, silicon, and other soluble metal anions such as sulfates, chlorides, etc. A NONOX leach retention time of between 0.5 and 8 hours is required and typically this could be between 1 to 3 hours.

The cooled slurry [124] can be further cooled and then thickened in decanter [130]. Flocculant [131] and recycle filtrate [133] and internal solution recycles are employed to aid slurry thickening. The thickener overflow [132] can be further clarified before optionally recovering uranium [180] and thereafter the barren liquor [181] is disposed to a Tailings Storage Facility [169].

The barren liquor [181] containing primarily iron and lesser quantities of aluminium, magnesium, potassium, calcium, and some radionuclides can be disposed of in the tailings storage facility [169] and the aqueous fraction recycled as required to supplement the brine in [102],

The thickened slurry [134] is filtered [135] and the upgraded copper concentrate is then washed with water [136] before it is repulped in clean water [139] and transferred as a washed upgraded copper concentrate [137],

The washed concentrate is split [170] into two portions by employing known processes e.g. flotation, cycloning, screening, partitioning, etc. One part [111] is employed in the PROX leach [110], and the balance [171] is fed forward as the clean upgraded low emission copper concentrate to the market.

The upgraded copper concentrate [111] that is fed to the PROX leach [110] may have an iron analysis of 3 to 15%, and in the PROX process this will be oxidised to a mixture of ferrous and ferric sulfates.

The PROX reactor operates: • at temperatures below 240°C and typically between 140° and 200°C; • at an oxygen partial pressure of between 200 and 1000kPa, and • with a retention time of between 0.25 and 6 hours.

Sulfuric acid is required to stabilise copper (and iron) in solution. A free acid concentration of between 1 and 50g/L is adequate for the (PROX) reactor [110].

Alternately the PROX reactor discharge [115] can be flashed as in the case of the NON OX reactor discharge [122] and the flashed underflow further separated into a solids fraction and a liquid fraction. The liquid fraction can then be fed to the Dirty Concentrate Repulp [100] and the solid fraction combined with stream [171] after the separation [170], Optionally the solid fraction remains with the liquid and together as stream [115] is fed to the Dirty Concentrate Repulp [100]. Optionally the copper sulfate leach liquor from the PROX reactor [110] can be crystallised in a copper sulfate evaporator to produce a copper sulfate crystal slurry which can be fed directly to the Dirty Concentrate Repuip [100],

Example 1 A dirty or radioactive copper concentrate containing 43% copper, 20% iron, 25% total sulfur, consisting predominantly of chalcopyrite, bornite, and some chaicocite and pyrite was tested, employing the flowsheet in Figure 2. The uranium concentration was approximately 85ppm.

This concentrate was subjected to a non-oxidising (NONOX) leach at 20% solids where lixiviant concentration was 45g/L Copper as copper (2) sulfate, and 0.6g/L total soluble Iron in a brine lixiviant. The NONOX leach temperature was 210°C and the total pressure was approximately 2200kPa(g).

After a leach period of 2 hours the upgraded copper concentrate assayed: %

Cu 54.7

Fe 11.4 S 25.1 U 15 (PPM)

And the barren liquor assayed:

q/L

Cu 0.62

Fe 20.0

Free Acid 2.6

The present embodiment has a PROX slurry being fed to the NONOX reactor where the dirty copper concentrate described in Example 1 was also charged.

The radionuclide levels in the NONOX autoclave residue in Example 1 were:

Bq/g U238 0.18-0.20

Th330 <0.5

Ra226 <0.8

Pb210 < 0.4

Po210 < 2.0

Example 2 A further dirty and radioactive copper concentrate containing 28% copper, 31% iron and 35.6% sulfur and consisting primarily of chalcopyrite and some pyrite and covellite was tested employing the flowsheet in Figure 2. The uranium concentration was approximately 80ppm.

This concentrate was subjected to a non-oxidising (NONOX) leach at 10% solids where the lixiviant concentration was 42g/L copper as copper (2) sulfate and the chloride concentration was 50g/L The NONOX leach temperature was 210°C and the total pressure was 2300kPa(g).

The barren liquid after one hour of leaching assayed:

g/L

Cu 0.89

Fe 21.8

Free acid 15 (reflected as sulfuric acid) The residue after one hour assayed: %

Cu 51.2

Fe 11.4 S 34 U 9 (PPM)

Example 3

In a test to demonstrate the pressure oxidative (PROX) leach an upgraded copper concentrate with the assay below was employed in a continuous leach process: %

Cu 51.0

Fe 14.2 S (Sulfide) 24.2 U 16 (PPM)

The solids density in the PROX feed was 8% in a water diluted recycled PLS and the leach temperature was 140-141 °C with an oxygen partial pressure of 670 kPa.

The leachate composition was: OIL·

Cu 140

Fe 0.3 H2S04 ~3.0 pH 1.8

Eh +455 (Ag/AgCI @ 3.8 M KCI) and the residue from the autoclave assayed: %

Cu 3.8

Fe 31.5 S (Sulfide) 10.5 S (Sulfate) 0.8 U 18 (PPM)

Now that preferred embodiments of the hydrometaliurgical method for the removal of radionuclides from a radioactive copper concentrate have been described in detail, it will be apparent that the described embodiments provide a number of advantages over the prior art, including the following: (i) Reduced level of radioactive emission in the resulting copper concentrate after treatment in the NONOX reactor of the invention. (ii) Reduced level of radioactive emission allows the copper concentrate to be suitable for smelting. (iii) Reduced level of radioactive emission allows the copper concentrate to be suitable for transport and meeting legislative standards. (iv) Reduced levels of copper in the barren liquid emanating the NONOX autoclave, and hence lower soluble copper losses. (v) Truncated flowsheet with less process steps, i.e. elimination of pre-and post- treatment of concentrate, and hence simplified operation. (vi) Consequently lower capital and operating costs.

It will be readily apparent to persons skilled in the relevant arts that various modifications and improvements may be made to the foregoing embodiments, in addition to those already described, without departing from the basic inventive concepts of the present invention. Therefore, it will be appreciated that the scope of the invention is not limited to the specific embodiments described and is to be determined from the appended claims.

Claims

Claims

1. A hydrometallurgical method for the removal of uranium, thorium, radium, lead, bismuth and polonium and/ or other radionuclides from a radioactive copper concentrate to produce an upgraded copper concentrate having lowered emission levels, the method comprising the step of: subjecting the copper concentrate to an acidic leaching process {NONOX leach) using a copper sulfate and chloride containing iixiviant under lowered electrochemical conditions, to allow controlled removal of one or more of the radionuclides to produce the lowered emission level upgraded copper concentrate, and lower soluble copper losses in the discharge stream, and wherein the leaching process is conducted: to convert essentially all the copper in the Iixiviant into the upgraded copper concentrate, and employing elevated temperature and elevated pressure to suppress boiling in the leaching process.
2. A hydrometallurgical method as defined in claim 1, wherein the leaching process (NONOX leach) is conducted at an electrochemical potential of greater than 150mV (Ag/AgCI 3.8M KCI).
3. A hydrometallurgical method as defined in claim 2, wherein the leaching process {NONOX leach) is conducted at an electrochemical potential in the range of between about 175mV and 450mV (Ag/AgCI 3.8M KCI).
4. A hydrometallurgical method as defined in any one of claims 1 to 3, wherein the electrochemical potential of the NONOX leach is controlled by the presence of, or the addition to the NONOX leach of, one or more of cupric sulfate, ferric ion, air, oxygen, sodium chlorate, pyrolusite, or hematite.
5. A hydrometallurgical method as defined in claim 1, wherein the copper sulfate and chloride containing lixiviant comprises at least copper sulfate and sodium chloride.
6. A hydrometallurgical method as defined in claim 1, wherein the radionuclides comprise one or more radionuclides selected from the group comprising U238, Th230, Ra226, Pb210, Po210 and Bi210.
7. A hydrometallurgical method as defined in claim 1, wherein the NONOX leach allows for the removal of about 20% to 99% of the uranium and/or thorium, and allows for lowering of one or more of radium, lead, bismuth and polonium levels to below 3Bq/g.
8. A hydrometallurgical method as defined in claim 1, wherein the copper sulfate and chloride containing lixiviant comprises one or more of the following solutes: sulfuric acid, hydrochloric acid, sodium chloride, chloride salt, potassium chloride, and copper sulfate, the sulfate and chloride containing lixiviant being obtained from within the NONOX leach, or from a source external to the NON OX leach.
9. A hydrometallurgical method as defined in claim 1, wherein each radionuclide in the lowered emission upgraded copper concentrate has a radioactive emission level of about 0.1 to 2.0Bq/g.
10. A hydrometallurgical method as defined in claim 1, wherein the lowered emission upgraded copper concentrate has a radioactive emission level of less than about 0.5Bq/g for U238 or Th230, and a radioactive emission level of less than about 2Bq/g for each of the other radionuclides.
11. A hydrometallurgical method as defined in claim 1, wherein the copper sulfate and chloride containing lixiviant comprises one or more of seawater, brines from seawater desalination, and fluorides, or other anions leached from the radioactive copper concentrate, or from process water.
12. A hydrometallurgical method as defined in claim 1, wherein the chloride levels in the copper sulfate and chloride salts containing lixiviant are between 5 and 100g/L chloride.
13. A hydrometallurgical method as defined in claim 1, wherein the NONOX leach is conducted between about 100° C and 240° C.
14. A hydrometallurgical method as defined in claim 13, wherein the NONOX leach is conducted between about 160° C and 240° C.
15. A hydrometallurgical method as defined in claim 1, wherein copper-iron-sulfides and cupric sulfide (covellite) are transformed in the NONOX leach, and wherein the NONOX leach is conducted under conditions to transform 50% to 99% of the copper-iron-sulfides to iron-depleted copper sulfides and covellite and to a sulfur-depleted variant (digenite/chalcocite).
16. A hydrometallurgical method as defined in claim 1, wherein the NONOX leach is conducted at a pressure in the range of 500-3500kPa.
17. A hydrometallurgical method as defined in claim 1, wherein the retention time of the NONOX leach is between about 0,5 and 8 hours.
18. A hydrometallurgical method as defined in claim 1, further comprising feeding into the NONOX leach a copper sulfate slurry from a crystalliser or a concentrated copper sulfate leach liquor from a pressure oxidation reactor (PROX reactor), or copper sulfate solution from an externa! source, or a copper sulfate leach slurry direct from a PROX reactor.
19. A hydrometallurgical method as defined in claim 18, wherein the sulfate slurry or solution comprises copper sulfate, iron sulfate and sulfuric acid.
20. A hydrometallurgical method as defined in claim 18, wherein the copper sulfate is added to more than one compartment in the NONOX leach reactor.
21. A hydrometallurgical method as defined in claim 18, wherein the copper aqueous concentration levels within the discharge liquor from the NONOX reactor are less than 5g/L.
22. A hydrometallurgical method as defined in claim 21, wherein the copper aqueous concentration levels within the discharge liquor from the NONOX reactor are less than 1 g/L.