Abstract:
The invention provides processes for recovering copper metal from sulphide minerals via basic copper sulphate which is produced by pressure oxidation in chloride spiked sulphate solutions. Chloride contaminants in the basic copper sulphate obtained from the pressure oxidation reaction are removed in a reaction with an alkali, such as CaO. Thereafter, the basic copper sulphate is leached with a weak acid, and the resultant pregnant copper solution contains chloride ions at low concentrations, which allow copper electrolysis to operate efficiently.

Description:
FIELD OF THE INVENTION  
         [0001]    The invention is in the field of hydrometallurgical treatment of metal ores or concentrates. More particularly the invention is in the field hydrometallurgical treatment of copper sulphide ores or concentrates in the presence of halide ions.  
         BACKGROUND OF THE INVENTION  
         [0002]    Hydrometallurgical methods for the extraction of copper metal values from sulphide ores or concentrates have been developed as an alternative to conventional pyrometallurgical methods. This is at least in part due to the fact that conventional pyrometallurgical methods are energetically intensive, typically operating at temperatures exceeding 1000° C., and environmentally damaging, often releasing sulphur dioxide as a pollutant gas into the environment. In contrast, hydrometallurgical processes typically employ aqueous systems at much lower temperatures (&lt;250° C.), with the result that the sulphide is oxidized to sulphate or elemental sulphur. However, the commercial exploitation of hydrometallurgical treatments of copper sulphide minerals is limited at present. This may in part be attributable to the refractory nature of chalcopyrite ores, the most common copper-bearing mineral.  
           [0003]    Less aggressive sulphate solution systems often result in incomplete dissolution of copper sulphide minerals. Addition of small amounts of halide ions to the sulphate solution systems was found to enhance the dissolution of copper sulphide minerals. For example, hydrometallurgical treatment of chalcopyrite ores using solutions having high concentrations of chloride ions may facilitate the recovery of copper values from the mineral under relatively mild operating conditions. In such processes, the copper metal product is often obtained in powder form from the chloride solutions. However, high concentrations of chloride ions may cause corrosion of construction materials, requiring the use of more expensive corrosion resistant materials. For example, U.S. Pat. Nos. 4,039,406 and No. 4,338,168 describe a process for using a sulphate solution system spiked with chloride ions to pressure-leach copper sulphide minerals at 120° C. to 160° C. under oxygen pressure. Under these conditions, the copper values present in the sulphide ore or concentrate are typically converted to a basic copper sulphate solid, while iron in the chalcopyrite forms hematite (a very stable iron oxide) and most of the sulphide is oxidized to elemental sulphur. As the process is disclosed in this patent, the basic copper sulphate bearing residue may be separated from the pressure oxidation leach solution and washed with water to remove halide ions. As taught by the patent, the washed copper sulphate residue may then be subjected to a selective dissolution of copper in an ammoniacal ammonium sulphate solution followed by solvent extraction, stripping with sulphuric acid and electrowinning. Alternatively the basic copper sulphate bearing residue may be subjected to a weakly acidic leaching using dilute sulphuric acid, and the copper values recovered by conventional electrolysis from a pregnant leach solution which has been subjected to a solution purification step.  
           [0004]    In the processes described in U.S. Pat. Nos. 4,039,406 and No. 4,338.168, some of the chlorides may report, in dissolved or solid forms, to the basic copper sulphate solids. These chlorides in the basic copper sulphate may dissolve when spent electrolyte is used to leach the basic copper sulphate, and small amounts of chloride ions in the copper electrolyte may in fact be beneficial for improving the quality of the copper metal deposited at the cathode in electrowinning. However, higher concentrations of chloride ions in the electrolyte may cause corrosion of the stainless steel cathode sheet. To address this problem, U.S. Pat. Nos. 4,039,406 and No. 4,338,168 teach the use of copper metal powder to remove chloride ions from the pregnant weak acid leach solution, as solid cuprous chloride. However, the cost of the copper powder used in such a step may be relatively high, mitigating against the economic viability of the whole hydrometallurgical process for treating chalcopyrite ores.  
           [0005]    One approach to controlling chloride concentrations is for example described in U.S. Pat. No. 5,431,788, which teaches the use of solvent extraction technology to control chloride deportment to the electrolyte after a weak acid leaching of basic copper sulphate (as disclosed in U.S. Pat. No. 4,039,406). There remains a need for alternative methods of ameliorating the effects of halide ions in hydrometallurgical process for treating copper ores.  
         SUMMARY OF THE INVENTION  
         [0006]    In various aspects, the present invention relates to processes for recovery of copper metal from sulphide minerals via a basic copper sulphate residue that is produced by pressure oxidation in chloride spiked sulphate solutions. In selected embodiments of the invention, chloride contaminants in the basic copper sulphate residue may be removed in an alkali treatment step before the basic copper sulphate residue is leached with spent electrolyte. These embodiments of the processes of the invention may in some cases be adapted so that the resultant pregnant copper solution produced by the weak acid leach contains chloride ions at low concentrations. The low chloride concentrations may facilitate the efficient operation of a subsequent copper electrolysis step. In some embodiments, the chloride ions removed from the basic copper sulphate may be precipitated as cuprous chloride and recycled back to the pressure oxidation as calcium chloride, to improve the efficiency of the whole process of hydrometallurgical treatment of copper ores.  
           [0007]    In some embodiments, for example, copper sulphide concentrates or ores may be pressure oxidized under conditions as disclosed in U.S. Pat. No. 4,039,406. The discharged slurry may be subjected to a solid-liquid separation step. The solid product of this separation step may contain basic copper sulphate, hematite, elemental sulphur and unreacted sulphide minerals. This solid residue may for example be briefly rinsed with recycled process liquor. The filtered solution from the solid-liquid separation step may for example be recycled to the pressure oxidation stage.  
           [0008]    In accordance with the present invention, the separated solids containing basic copper sulphate may be processed to remove chlorides from the solids. In this chloride wash step, the wet solids may be mixed with recycled process liquor and the slurry pH raised to 11-12 by the addition of an alkali, such as sodium hydroxide or lime. In some embodiments, the slurry temperature in this step may be maintained below a selected maximum temperature, such as about 40° C., to reduce undesirable side reactions (discussed in more detail below).  
           [0009]    The resulting slurry from the chloride wash step may be filtered to separate the solid components from the liquid. Recycled solution may for example be used to rinse the filtered solids. The solution recovered from the solids may be processed to recover chlorides. In preferred embodiments, the solids will now be largely free or relatively low in chloride concentration, and the solids may be subjected to the next leaching step to dissolve the copper component. In the subsequent leaching step, spent electrolyte recycled from electrolysis may for example be used. The pregnant copper rich solution produced by the leach may for example be sent to an electrolytic plant to recover copper metal at cathodes.  
           [0010]    In some embodiments, the solution produced in the above chloride wash stage may be discarded, when the recovery of the chloride ions is not economically warranted and its disposal does not cause environmental concern. In some embodiments, the solution produced in the above chloride wash stage may be treated in a circuit to recover the chloride ions to produce a low-chloride solution which can be returned to the chloride wash stage. This chloride recovery process may for example be effected by the addition of cuprous oxide to the chloride-containing solution to precipitate insoluble cuprous chloride. The produced cuprous chloride solids may be separated in a solid-liquid separation step. Recovered cuprous chloride may then be reacted with lime in a step that converts the cuprous chloride to cuprous oxide and calcium chloride solution. The cuprous oxide may be reused in the chloride recovery step and the calcium chloride may be recycled to the pressure oxidation step. If there is a need to remove chloride ions from pregnant copper electrolyte, a second chloride recovery step may for example be implemented. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]    [0011]FIG. 1 is a flow chart illustrating a hydrometallurgical process for recovering copper from sulphide ores or concentrates in accordance with alternative aspects of the invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0012]    Alternative embodiments of the present invention are described below with cross-references to numerals identifying the steps shown in the flow chart of FIG. 1.  
       Pressure Oxidation (“PO” 10)  
       [0013]    Copper sulphide concentrates or ores may be oxidized to convert copper sulphide components to basic copper sulphate, CuSO 4 *2Cu(OH) 2 , in the presence of chloride ions. Oxidation may for example be carried out under conditions as described in U.S. Pat. No. 4,039,406 (incorporated herein by reference). In some embodiments, the chloride ions may for example be present at 10-20 g/L. Under such conditions, iron sulphides in the concentrate or ore may form hematite, Fe 2 O 3 . Under such conditions, a small percentage of the sulphur in the mineral sulphides may be oxidized to sulphate, while most of the sulphur is converted to elemental sulphur.  
         [0014]    In some embodiments, it may be advantageous for concentrates or ores to be reground to finer-than-average grain sizes prior to the oxidation process, to facilitate higher reaction rates. The concentrate or ore slurry containing copper sulphides may for example be heated at 120° C. to 160° C. under an oxygen pressure of about 200 psi. After 1 to 2 hours, the reactor slurry may be cooled for discharge. The discharged slurry may be filtered to separate liquids  14  from the solid components  12  (which may be referred to as the “PO solids”). The recovered liquor may be recycled (stream  16 ) to the Pressure Oxidation step.  
         [0015]    In some embodiments, metallic elements in the feed concentrate and ores, such as zinc and nickel, may dissolve and remain in the aqueous phase, accumulating in the recycled Pressure Oxidation solution. In such embodiments, a portion of the recycled solution may be treated to remove these elements using standard processes such as sulphidation.  
         [0016]    In some embodiments, a calcium chloride solution  78  derived from the CuCl Conversion step  70  (see below) may be recycled (stream  78 ) and added to the Pressure Oxidation step  10 . In the Pressure Oxidation step, the presence of sulphate ions in the solution will typically immediately precipitate the calcium ions as calcium sulphate. The recycled chloride ions may thereby be made to serve as effective active ingredients in improving the efficiency of the Pressure Oxidations step.  
         [0017]    In summary, a major chemical reaction in this step may include the following: 
         3CuFeS 2 ( s )+5.25O 2  ( g )+2H 2 O( l )=CuSO 4 *2Cu(OH) 2 ( s )+1.5Fe 2 O 3 ( s )+5S°( s ) 
       Chloride Wash (20)  
       [0018]    In this step, the PO solids  12  from the Pressure Oxidation step  10  may be treated to produce a feed material which is substantially free of chloride for the Copper Leach step  30  below. The soluble chloride concentration may for example be reduced so that it is below about 0.004% (or in alternative embodiments below about 0.010%, 0.009%, 0.008%, 0.007%, 0.006%, 0.005%, 0.003%, 0.002% or 0.001%). Since the PO solids  12  are typically washed with a limited volume of wash water during the Pressure Oxidation step, significant amounts of chloride ions will generally remain in the entrained liquor in the wet solids. In addition, during the Pressure Oxidation step, various solid compounds containing chloride ions also form. In the process of the present invention, these chloride ions in the liquid and solid forms may be removed in Chloride Wash step  20 .  
         [0019]    In Chloride Wash  20 , filtered PO solids  12  from the Pressure Oxidation step may first be slurried in a large volume of process water to dissolve soluble chlorides. For this to be effective, the dissolved chloride concentration in the process solution should be lower than the chloride concentration in the filtered PO solids  12 . An alkali such as calcium oxide or lime (CaO) may then be added so as to convert solid chlorides to oxides and alkali chlorides. In alternative embodiments, other alkalis such as sodium hydroxide may also be used. The calcium chloride formed upon addition of lime will typically dissolve readily in the process water.  
         [0020]    The lime or other alkali may also be added so as to convert the basic copper sulphate solids to copper hydroxide and an alkali sulphate. Chloride ions trapped within the basic copper sulphate crystals may thereby be released into the solution during this solid transformation. The conversion of the basic copper sulphate to copper hydroxide and alkali sulphate in this step may be a convenient way to prevent excess sulphate ions from passing into the copper electrolyte to be used for electrowinning. In some cases, if a surplus of sulphate ions occurs in the electrolyte, a step may be necessary to reduce the sulphate concentration prior to electrowinning.  
         [0021]    In this step, chemical reaction rates are in general promoted by increased temperatures. However, the added alkali and the elemental sulphur produced during Pressure Oxidation may react to form alkali polysulphide compounds, and the rate of these reactions may be greatly accelerated by higher temperatures. Alkali polysulphides may in turn react with oxides and hydroxides of copper, forming copper sulphides which are less soluble in the following Copper Leach step  30  (described below). These undesirable side reactions may therefore result in the loss of copper in the leach residue of the Copper Leach step. In alternative embodiments, an upper temperature limit for this step may therefore be adjusted to minimize the formation of the alkali polysulphides. This upper temperature limit will typically depend in part on the feed material used in the process of the invention. In selected embodiments, a limit in the neighborhood of 40° C. to 45° C. may be desired. In alternative embodiments, upper limits may be selected from any value between 30° C. and 55° C., such as 35° C. or 50° C.  
         [0022]    In preferred embodiments, adequate amounts of alkali may be used to advantageously effect the performance of Chloride Wash step  20 . In exemplary embodiments, these quantities were found to be in the range of 5 to 10 weight percent of the dry PO residue weight. In alternative embodiments, these quantities may for example be any value between 1 and 25 weight percent of the dry cake weight (such as 2, 3, 4, 6, 7, 8, 9, 11, 12, 13 or 14 weight percent).  
         [0023]    The reacted slurry from the Chloride Wash step  20  may be filtered to separate solids  26  from liquid  27 . The separated solids  26  may be washed with process water to remove dissolved chloride ions. In some embodiments, at least part of filtrate  25  from this solid-liquid separation step may be treated in the Chloride Recovery I step  60  (described below) and another portion of the solution  24  may be used as the process solution in the Chloride Wash step. If the recovery of the chloride ions in filtrate  25  is economically less important and disposal of filtrate  25  may be undertaken without causing undue environmental concern, filtrate  25  may be discarded.  
         [0024]    In summary, major reactions in this step may include the following: 
         CuCl 2 ( s )+CaO( s )+H 2 O( l )=Cu(OH) 2 ( s )+CaCl 2  ( l ) 
         CuSO 4 *2Cu(OH) 2  ( s )+CaO( s )+3H 2 O( l )=3Cu(OH) 2 ( s )+CaSO 4 *2H 2 O ( s ) 
       Copper Leach (30)  
       [0025]    In some embodiments, the filtered solid cake  26  from the Chloride Wash step  20  may be leached with acidic electrolyte  44  returning from the Electrowinning step  40  (described below). Copper hydroxide formed in the previous Chloride Wash step may for example be dissolved by sulphuric acid in the electrolyte. In some embodiments, slurry pH may be maintained so as to limit the dissolution of iron and other impurities from the solids, for example at any value from about 2 to about 2.5.  
         [0026]    The reaction temperature in Copper Leach  30  may for example be maintained in a range of from about 30° C. to about 50° C. The total residence time for the reactions of Copper Leach  30  may be adjusted to optimize dissolution of the copper from the solid phase. For example, residence time may be from one half to two hours, with one hour being adequate is some embodiments.  
         [0027]    In selected embodiments, the leach slurry may be subjected to a solid-liquid separation step to obtain a solution  36  suitable for the Electrowinning step. Solid residues  34  from this solid-liquid separation will generally consist of hematite, elemental sulphur, unreacted sulphides, gangue minerals and gypsum. This residue may for example be discarded, or it may be treated for precious metals recovery using standard approaches.  
         [0028]    In summary, a major reaction in this step may be expressed as follows: 
         Cu(OH) 2 ( s )+H 2 SO 4 ( l )=CuSO 4 ( l )+2H 2 ( l ) 
       Electrowinning (40)  
       [0029]    An electrowinning step  40  may be employed to recover copper metal from copper sulphate electrolyte. For example, conventional copper electrowinning technology may be employed with modifications to the chemical composition of the electrolyte made in order to optimize the efficiency of the Copper Leach step. Conventional electrowinning technology employs stainless steel cathode sheets on which copper metal is deposited, and although some chloride ions are needed to control the metal deposit quality, higher concentrations of chloride should be avoided to minimize corrosion of the stainless steel cathode sheet. For this reason, in accordance with the present invention the chloride ion transfer from the Pressure Oxidation solution and solids to the electrolyte may be minimized. Electrowinning may be carried out to produce electrolytic copper metal, and spent electrolyte  44  comprising sulphuric acid that may be reused in the Copper Leach step.  
         [0030]    In summary, major reactions in this step may include the following: 
         At cathode: CuSO 4 ( l )+2e=Cu°(metal)+SO 4   2−(   l ) 
         At anode: H 2 O( l )=0.5O 2 ( g )+2H + ( l )+2 e   
       Chloride Recovery I (60)  
       [0031]    In some embodiments, the Chloride Wash step  20  may be a closed circuit in which the solution is continually recycled. In such embodiments, the chloride ions that are dissolved from the solids  12  of the Pressure Oxidation step  10  will accumulate in the circuit solution. The chloride ions may then be recovered by treating a chloride-containing liquid in at least a part of this circulating solution.  
         [0032]    In one embodiment, the Chloride Recovery I process is carried out by adding cuprous oxide to a chloride-containing liquid in the circulating Chloride Wash solution. The cuprous oxide may for example be regenerated in the CuCl Conversion step  70  (described below). An acid, such as sulphuric acid, may be used to bring down the pH of the solution  25  from the Chloride Wash step  20  so as to maintain the slurry pH in an optimum range, such as from about 2 to 2.5. In some embodiments, a relatively small amount of copper ions may also be required to obtain a low final chloride level in the final solution from this Chloride Recovery I step. For example, a concentration of 0.2% may be adequate (or in alternative embodiments 0.1 g/L, 0.3 g/L, 0.4 g/L or 0.5 g/L. In some cases, spent electrolyte  44  may be a good source for the sulphuric acid and the copper ions to be used in the Chloride Recovery I process.  
         [0033]    In some embodiments, the Chloride Recovery I reaction may take place at ambient temperature so that no heating is required, for example at any value of from about 15° C. to about 40° C. In exemplary embodiments, the reaction rate for cuprous chloride formation is very high, requiring a reaction time of as little as 10-15 minutes. In alternative embodiments, chloride ion concentration may be lowered to about 0.1 g/L. In some embodiments, cuprous oxide additions of approximately 200% of the theoretical requirement may be employed. The cuprous chloride product  74  may optionally be filtered and recycled to regenerate cuprous oxide in the Cuprous Chloride Conversion step  70 . For cuprous chloride recycling, gypsum that forms as a solid contaminant may be removed, for example by a gravity separation technique. The chloride-reduced solution  76  may for example be recycled to the Chloride Wash step  20 .  
         [0034]    In summary, a major reaction in this step may be written as the following: 
         Cu 2 O( s )+CaCl 2  ( l )+H 2 SO 4  ( l )+H 2 O( l )=CaSO 4 *2H 2  O( s )+2CuCl ( s ) 
       Chloride Recovery II (80)  
       [0035]    In some embodiments, a second chloride recovery step may optionally be implemented, depending for example on the need to control the chloride ion concentration in the Copper Leach—Electrowinning circuit. This may be particularly useful if accidental chloride ion transfer to the circuit solution occurs, whereupon this step may be used to remove chloride ions from the copper rich solutions. The transfer of trace amounts of chloride ion to the electrolyte may in some cases result in accumulation of the chloride ions to an unacceptable level in the closed circuit of the Copper Leach—Electrowinning steps. This may be addressed by treating at least a portion of the pregnant copper solution with a Chloride Recovery II step  80 . The volume of the solution to be treated in the Chloride Recovery II step  80  may for example depend upon factors such as chloride transfer rate, chloride level in electrolyte and electrolyte bleed rate. In some embodiments, at least a part of the cuprous oxide  79  produced in the Cuprous Chloride Conversion step may be diverted to this Chloride Recovery II step. The solids produced  84  in the Chloride Recovery II step may for example be separated from the solution and used in the Chloride Recovery I step  60 , to utilize the unreacted cuprous oxide. As for the Chloride Recovery I, solution pH may for example be maintained at about 2 to 2.5 with sulphuric acid. A reaction time of less than 30 minutes may be required in some embodiments to achieve chloride ion concentrations of 30-40 mg/L.  
         [0036]    In summary, a major reaction in this step may be written as the following: 
         [0037]    Cu 2 O ( s )+CuCl 2 ( l )+H 2 SO 4 (1)=2CuCl ( s )+CuSO 4  ( l )+H 2 O( l ) 
       Cuprous Chloride Conversion (70)  
       [0038]    In some embodiments, cuprous chloride produced in the Chloride Recovery I and II steps may be treated with lime  71  so as to produce cuprous oxide  79  and calcium chloride  78  solution. The calcium chloride solution may then be returned to the Pressure Oxidation step  10  to recycle chloride ions. The solid product from the Cuprous Chloride Conversion step will generally consist of cuprous oxide which may in turn be reused in the Chloride Recovery I and Chloride Recovery II steps in order to remove chloride ions from solution.  
         [0039]    In summary, a major reaction in this step may be written as the following: 
         2CuCl ( l )+CaO ( s )=Cu 2 O( s )+CaCl 2 ( l ) 
       Process Optimization Summary  
       [0040]    In alternative embodiments, the hydrometallurgical copper recovery processes of the present invention may be optimized to provide particular advantages when recovering copper metal from basic copper sulphate. The Chloride Wash step may for example be adapted to remove chloride ions from the PO solids so as to minimize transfer of the chloride ions to the electrowinning circuit. In some embodiments, sulphate ions in the basic copper sulphate in the PO solids may be fixed as insoluble gypsum prior to Copper Leach (so as to reduce the need to remove the sulphate ions from the Copper Leach circuit). In some embodiments, chloride ions escaping the Pressure Oxidation stage entrained in the PO solids may be recovered as CuCl in the downstream process steps. The recovered CuCl may then be converted to CaCl 2  to be returned to the Pressure Oxidation stage as a chloride ion source.  
         [0041]    In some embodiments, the process of the invention may be adapted so that the primary material to be discharged from the process is the solid residue from the Copper Leach step. This residue will typically contain iron oxide (hematite), unreacted sulphides, gypsum, elemental sulphur and precious metals. In some embodiments, the precious metals may be recovered from this residue by a separate recovery processes. Alternatively, the residue may be discarded to a conventional tailings disposal facility.  
         [0042]    Various aspects of the present invention are illustrated in the following examples. In some cases, process steps and parameters from alternative examples may be interchanged, and the illustration of selected embodiments does not limit the scope of the invention nor imply that the selected process steps or parameters must always be combined. EXAMPLE 1  
       Pressure Oxidation and Chloride Wash  
       [0043]    In this example, three types of copper concentrates commercially produced in the Province of British Columbia, Canada are subjected to pressure oxidation to provide samples for alternative Chloride Wash embodiments. Copper grades of these concentrates are in a range of between 20% to 40%.  
         [0044]    200 g dry weight of concentrate and one liter of leach solution, which consisted of 13 g/L HCl and 23 g/L Cu ++  as copper sulphate, were placed in a 2L capacity titanium autoclave. The pressure oxidation conditions used were 140° C., 200 psi oxygen total pressure and a residence time of 60 minutes. Several pressure oxidation tests were repeated with each concentrate to collect composite samples of pressure oxidation residue. Each batch of the composite samples was re-slurried with plenty of tap water to remove most of the soluble chloride from the residues. After filtration the wet cake was dried in an oven.  
         [0045]    Table 1 summarizes the chemical compositions of the feed concentrates and the resultant pressure oxidation residues. As described above, the main constituent copper sulphides and iron sulphides in the feed concentrates are converted to their respective solid compounds, i.e. basic copper sulphate for copper, iron oxide (hematite ) for iron and elemental sulphur for sulphur. At the same time, most of the minor elements in the concentrates report to the solid residues. As a result, the weights of the product solids from the pressure oxidation increase by approximately 30%, thus decreasing the grades of copper and iron in the product solids.  
         [0046]    Although chloride analyses were not performed for the materials listed in Table 1, the chloride contents in the PO residues generally varied in a range from 0.1 to 0.5%. Since the chloride contents are attributed to both soluble and insoluble forms, the degree of washing of the PO residues influenced the final chloride concentrations. During laboratory test investigation, it was found that all the chloride in the PO residues is not soluble in the following Copper Leach. Thus, the chloride concentration in the PO residue is not always indicative of the dissolution rate of the chloride ions in the pregnant Copper Leach solution.  
                                                                                                                             TABLE 1                           Summary of Chemical Analysis of Copper Concentrates and Their Pressure       Oxidation Residues            Concentrate   Chemical Analysis (%, g/T for Au &amp; Ag)            or PO residue   Cu   Fe   St   SiO 2     Zn   Pb   As   Sb   Bi   Au   Ag                    Concentrate-A   26.6   26.3   29.8   10.8   0.02   0.01   &lt;0.01   &lt;0.01   0.01   2.56   65.8       PO Residue-A   19.2   20.8   23.7   6.66   0.01   0.01   &lt;0.01   &lt;0.01   0.01   1.86   46.8       Concentrate-B   39.0   18.7   26.5   7.66   0.09   0.01   0.01   &lt;0.01   &lt;0.01   0.84   144       PO Residue-B   28.5   17.8   20.6   7.62   0.01   0.01   0.06   &lt;0.01   &lt;0.01   0.94   102       Concentrate-C   25.5   22.7   24.7   15.5   0.07   0.02   &lt;0.01   &lt;0.01   &lt;0.01   44.3   30.5       PO Residue-C   20.0   17.3   19.8   11.8   0.01   0.02   &lt;0.01   &lt;0.01   &lt;0.01   26.2   19.1                  
 
         [0047]    A laboratory test method, which simulated the Copper Leach step, was used to estimate the chloride contamination in the PO residues. This method consisted of mixing 100 g (dry weight) of the PO residue in 500-600 ml of distilled water and adding concentrated sulphuric acid to maintain the pH of the slurry mixture at 2-2.5 at 40° C. After one hour, the slurry was filtered and the cake was washed with warm water. The recovered filtrate was analyzed for chloride ions. The well washed cake was dried and analyzed for copper. Leach rates of copper from 100 grams of PO residue were estimated by the dry weight and copper concentration of the Copper Leach residue.  
         [0048]    To evaluate the effectiveness of the Chloride Wash step, 100 g (dry weight) of PO residues were subjected to various Chloride Wash tests which were followed by the simulated Copper Leach. The above prepared PO residues in Table 1 were subjected to the Chloride Wash tests. The test conditions and results are listed in Table 2. 100 g of dried solid residue were mixed with 300-700 mL of distilled water. Weighed amounts of alkalis such as calcium oxide, sodium carbonate and sodium hydroxide were added to the slurry mixture at set temperature for 60-120 minutes. After the tests, the solids were separated from the solution by filtration. 300 mL of distilled water was twice used to rinse the solids. The rinsed solids were immediately reslurried with a total of 500-600 mL distilled water including the moisture content in the filtered solids to perform the simulated Copper Leach. The slurry was then heated to 40° C. and its acidity was maintained at pH values of 2-2.5 to dissolve the copper value. After one hour, the slurry was filtered in a 15 cm diameter ceramic filter with No. 1 Whatman filter paper. The time required to filter the slurry was recorded. The filter cake was rinsed twice with 500 mL of warm water. Chlorine analysis was performed for the filtrates recovered from the Chloride Wash and Copper Leach stages. By this test method, more useful information could be gathered to evaluate the effectiveness of the Chloride Wash stage.  
                                                                                                                                   TABLE 2                       Chloride Wash Test Results                                    Chloride Wash (CW)                    PO                                           PO   Residue   Water       Reagent   Test           Filtration   [Cl − ] in       Test   Residue   Weight   Added   Reagent   Weight   Time           Time   Filtrate       Number   type   (g)   (mL)   Used   (g)   (min)   Temp° C.   Final pH   (m:sec)   (mg/L)               Test 1   A   100   —   —   —   —   —   —   —   —       Test 2   A   100   300   None   0.0   60   Amb.   4.6   3:31   9.9       Test 3   A   100   300   Na 2 CO 3     11.0   60   Amb.   10.3   2:50   25.5       Test 4   A   100   400   NaOH   4.2   60   Amb.   11.2   5:30   52.9       Test 5   A   100   700   NaOH   8.4   60   Amb.   11.5   13:30    37.1       Test 6   A   100   300   CaO   3.0   60   Amb.   12.6   2:02   63.8       Test 7   A   100   400   CaO   6.0   60   Amb.   12.7   1:50   85.5       Test 8   A   100   300   CaO   7.0   60   65   10.6   0:23   72.8       Test 9   A   100   300   CaO   7.0   120   56   5.9   0:42   61.7       Test 10   A   100   300   CaO   7.0   120   65   5.5   0:32   126.4       Test 11   B   100   —   —   —   —   —   —   —   —       Test 12   B   100   300   None   0   60   Amb.   4.8   3:45   15.7       Test 13   B   100   300   CaO   9.3   120   42   12.2   1:03   86.1       Test 14   B   100   360   CaO   10   120   50   11.6   0:55   57.1       Test 15   C   100   —   —   —   —   —   —   —   —       Test 16   C   100   300   None   0   60   Amb.   4.6   2:15   11.9       Test 17   C   100   300   CaO   7.8   120   45   12.0   0:46   92.3                        Copper Leach (CL)   Cl Deportment                        Fil-                   Total Cl −     Total Cl −                 Water   H 2 SO 4     tration   Dry   Cu in   Cu   [Cl − ] in   in CW   in CL   Cl −         Test   Added   used   Time   Residue   Residue   Leached   Filtrate   Filtrate   Filtrate   Removal       Number   (mL)   (g)   (m:sec)   Wt. (g)   (%)   (%)   (mg/L)   (mg)   (mg)   Rate (%)               Test 1   600   23.6   —   62.2   1.9   94   22.3   —   13.4   0       Test 2   554   20.3   11:35   65.0   2.0   93   17.5   3.0   9.7   28       Test 3   600   24.1   11:30   64.3   1.7   94   14.6   7.7   8.8   35       Test 4   600   25.0    9:13   64.1   2.1   93   11.1   21.2   6.7   50       Test 5   600   30.6   11:30   62.8   1.6   95   8.1   26.0   4.9   64       Test 6   600   24.7    7:12   66.0   2.0   93   14.6   19.1   8.8   35       Test 7   494   29.1    2:50   73.1   2.1   92   7.9   34.2   3.9   71       Test 8   600   27.8    3:41   74.7   2.5   90   6.5   21.8   3.9   71       Test 9   600   33.4    6:39   73.7   2.0   92   12.2   18.5   7.3   46       Test 10   600   26.4    5:10   75.5   4.2   84   16.2   37.9   9.7   28       Test 11   600   29.4   —   46.0   1.6   97   30.4   —   18.2   0       Test 12   600   30.9   25:34   45.7   1.6   97   24.4   4.7   14.6   20       Test 13   600   45.5    4:24   65.9   1.7   96   9.0   25.8   5.4   70       Test 14   600   44.9    3:33   69.0   3.7   91   8.8   20.6   5.3   71       Test 15   600   21.1   —   61.4   1.2   96   28.4   —   17.0   0       Test 16   600   20.2    7:59   61.9   1.2   96   23.5   3.6   14.1   17       Test 17   600   33.9    5:25   78.3   1.7   93   6.7   27.7   4.0   76                  
 
         [0049]    The results of seventeen tests are summarized in Table 2. In Tests No. 1, No. 11 and No. 15, the PO residues were subjected directly to the Copper Leach test without the Chloride Wash step to determine the chloride contamination in the pregnant Copper Leach solution. The resultant Copper Leach solution contained 22.3 mg/L Cl −  ions for the PO residue of Concentrate A (Test 1), 30.4 mg/L Cl −  for the same residue of Concentrate B (Test 11), and 28.4 mg/L Cl −  for the same residue of Concentrate C (Test 15). These chloride ion concentrations are too high for the solution, which is used in closed circuit, to be suitable for electrowinning.  
         [0050]    In Tests No. 2, No. 12 and No. 16, the first Chloride Wash step was carried out without addition of alkali during the Chloride Wash step to examine the extent of chloride ion dissolution in water. This water leach was followed by the regular Copper Leach test. Though the initial feed materials were well washed with water, there was additional dissolution of chloride from the solid phase. The reduced chloride ion concentrations in the Copper Leach solution are seen in Table 2. The extent of the decrease in the chloride ion concentration of the Copper Leach solution is expressed as “Chloride Removal Rate” in the far right column of the table. The Cl −  Removal Rate is estimated by comparing the quantities of Cl −  dissolved between the test without the Chloride Wash stage (Tests 1, No. 11 and No. 15) and those with it. As being shown in Table 2, 17-28% of the chloride was removed by the water leach alone.  
         [0051]    In the same table, when the Chloride Wash stage was carried out with addition of sufficient amounts of alkali, increased amounts of chloride ions dissolved in the Chloride Wash stage, reducing the quantity of chloride ions dissolved in the Copper Leach. Accordingly, in some embodiments, the copper leach solution may be used for electrowinning without the need for additional treatment to remove chloride. As an example of inferences that may be drawn from various process parameters by one of skill in the art, for example to adapt or optimize the process of the invention, the following conclusions could be drawn from the results listed in Table 2 with respect to various parameters of the Chloride Wash:  
         [0052]    A) Alkali type: Sodium carbonate may not be sufficiently active to be effective with 35% of Cl −  Removal Rate in Test 3. Sodium hydroxide and calcium oxide appear to be suitable when adequate amounts of these reagents are used. 4.2 g of NaOH in Test 4 and 3.0 g of CaO in Test 6 were apparently not adequate, achieving only 50% of Cl-Removal Rate for NaOH and 35% of Cl −  removal Rate for CaO. In Tests 5 and 7, these quantities were increased to 8.4 g for NaOH and 6.0 g for CaO with increased Cl −  Removal Rate to 64 and 71%, respectively. Between NaOH and CaO, the use of NaOH introduces Na+ions into the Chloride Wash circuit, which is a closed circuit, and Na +  ions accumulate in the closed circuit, requiring removal of Na +  ions from the circuit. The removal of Na+ions from aqueous streams is generally more difficult. On the other hand, Ca ++  ions form gypsum in aqueous sulphate systems, facilitating easy removal of the ions. For this reason, the use of CaO or Ca(OH) 2  may be generally more desirable for the Chloride Wash step. Also, reagent cost of CaO may be much lower than that of NaOH. In addition, when CaO was used as an alkali, the filtration of the Copper Leach slurries was much quicker than that of other cases as shown in Table 2. The formation of gypsum, which forms much larger crystal grains, is believed to reduce the slurry filtration times.  
         [0053]    B) Retention time: Residence time was varied between 60 and 120 minutes in the tests. For most of the materials tested, 60 minutes were long enough to complete the reactions.  
         [0054]    C) Temperature: Chloride Wash was carried out at an ambient temperature. In Tests 2-7 for PO Residue type A, elevated temperatures of 56 and 65° C. were used in Tests 8-10. When the reaction temperature increased to 65° C. in Test 8, the chloride removal was good at 71% but the copper leach rate decreased to 90%. In Tests 9 and 10, the test period was increased to 120 minutes from the 60 minutes used in previous tests. In Test 9, the copper leach rate was not affected by the temperature of 56° C. and the longer test period of 120 minutes, but the chloride removal was lowered to 46%. In Test 10, the higher temperature of 65° C. and the longer reaction time of 120 minutes caused lower chloride removal and lower copper leach rates. In Test 13, for PO residue type B, at a reaction temperature of 42° C., a high chloride removal rate and a high copper leach rate were realized after 120 minutes of reaction time. When the reaction temperature was raised to 50° C. in Test 14, although a similar chloride removal rate was obtained, the copper leach rate was declined to 91% from 96%. For PO residue type C, in Test 17, the Chloride Wash was carried out at 45° C. for 120 minutes. A high chloride removal rate of 76% was achieved, but the copper extraction rate declined to 93% from 96% without the Chloride Wash treatment.  
       EXAMPLE 2  
     Copper Leach  
       [0055]    Copper Leach tests were performed on a PO residue sample which had been treated by the Chloride Wash process. The purposes of the Copper Leach tests were to find optimum conditions for the Copper Leach and to evaluate the effectiveness of the Chloride Wash treatment under various Copper Leach conditions.  
         [0056]    A composite sample of the pressure oxidation residue was prepared by performing several pressure oxidation runs as described in Example 1. Concentrate type-A described in Table 1 was used in these pressure oxidation runs. The composite sample consisted of 20.0% Cu, 20.2% Fe and 23.0% sulphur total. A small portion of this composite sample was subjected to the standard Copper Leach test to evaluate the extent of chloride contamination. In Test 21, 100 g dry weight of the sample were mixed with 600 mL of distilled water and the mixture was brought to 40° C. on a hot plate. While the mixture was well agitated, sulphuric acid was added to the mixture to maintain the slurry pH between 2 and 2.5. After 60 minutes of reaction, the mixture was filtered. The filter cake was washed with plenty of warm water to remove dissolved elements in the wet cake before drying in an oven. The dried cake was analyzed for residual copper to estimate the extraction of copper from the original pressure oxidation residue. Whereas the recovered filtrate was analyzed for copper, iron and chloride to estimate dissolution of iron and chloride. The results of this test is listed in Table 3 as Test -21.  
                                                                                                                                                                                                                                                               TABLE 3                       Copper Leach Test Results                                            Acid                               Dry       Source:   Mix-           Dry               Solids   Solution   (H 2 SO 4 )   ing           Residue   Cu in       Test   Weight   Added   (g)/SE*   Time   Temp       Weight   Residue       Number   (g)   (mL)   (mL)   (min)   (° C.)   Final pH   (g)   (%)                    PO Residue            Test 21   100   600   (21.2)   60   38   2.3   64.0   1.8            PO Residue Treated by Chloride Wash Process            Test 22   58   297   (17.2)   60   39   2.1   41.6   1.5       Test 23   58   42   347   60   43   2.3   40.1   1.7       Test 24   58   42   343   60   36   2.5   41.3   2.3       Test 25   58   42   374   60   42   1.7   39.5   1.4       Test 26   58   42   351   30   41   2.2   40.4   1.7       Test 27   58   42   356   60   31   2.3   40.8   1.6       Test 28   58   42   355   120   39   2.2   40.5   1.7       Test 29   58   42   334   60   65   2.1   39.5   2                                            Total Cl −                                     Leached/                               100 g   Chloride           Cu   Filtrate   Filtrate   Fe   Filtrate   PO   Removal       Test   Extraction   Cu ++     Fe total   Extraction   Cl −     Residue   Rate       Number   (%)   (g/L)   (mg/L)   (%)   (mg/L)   (mg)   (%)                    PO Residue            Test 21   94   31   413   1.3   33.2   20.3   0            PO Residue Treated by Chloride Wash Process            Test 22   94   29.6   377   1.2   6.2   3.7   82       Test 23   93   58.3   228   0.9   5.6   3.6   82       Test 24   91   58.0   110   0.4   5.2   3.3   84       Test 25   95   57.8   549   2.3   7.5   5.4   73       Test 26   93   57.0   290   0.1   9.5   6.6   67       Test 27   94   58.3   282   1.1   5.0   3.3   84       Test 28   93   58.0   277   1.1   6.3   4.2   79       Test 29   92   64.0   172   0.6   5.8   3.6   82                          
 
         [0057]    21.2g of sulphuric acid was consumed during the reaction time of 60 minutes to maintain the slurry pH at 2.3. After filtration, washing and drying of the reacted solids, 64.0 grams of residue were recovered. The residue consisted of 1.8% copper, resulting in a copper extraction rate of 94%. The leach filtrate consisted of 413 mg/L iron and 33.2 mg/L chloride. 1.3% of the iron in the PO residue dissolved during the Copper Leach. 20.3 mg of chloride dissolved from 100 grams of the PO residue composite sample.  
         [0058]    600 g dry weight of the composite sample were treated by the Chloride Wash process and the resultant wet solid product was subjected to Copper Leach tests. 600 g of the solids were mixed with 1.8 L of distilled water. The slurry was maintained at 40° C. and its pH was kept at 11-12 with 54 g of calcium oxide for 120 minutes. The slurry was filtered and rinsed with 4 liters of water. The filter cake contained 42% moisture and was used for the Copper Leach tests without drying. A small portion of this wet filter cake was dried and the dried solids were analyzed for copper, iron and sulphur total. The solids consisted of 17.6% Cu, 17.4% Fe and 19.2% sulphur total. A synthetic spent copper electrolyte, which consisted of 50 g/L H 2 SO 4  and 37.5 g/L Cu ++ , was prepared from distilled water, concentrated sulphuric acid and chemical reagent grade copper sulphate crystals. Chloride contamination in the copper sulphate used resulted in 0.9 mg/L dissolved chloride in the electrolyte.  
         [0059]    The filter cake prepared by the method described above was divided into 100 g fractions which were reslurried with either distilled water or the above prepared synthetic copper electrolyte. The slurry was kept at 30-65° C. and its pH value was maintained at a targeted range of 1.5-2.5 for 30-120 minutes by adding sulphuric acid or the synthetic spent copper electrolyte. After the leach, the slurry was filtered and the filter cake was well rinsed with tap water. The dried filter cake was analyzed for copper and the filtrate for copper, iron and chloride.  
         [0060]    The results of eight tests are summarized in Table 3. In this test series, the effectiveness of the Chloride Wash stage and the effects of various Copper Leach parameters on the leach performance were examined. In the second last column in Table 3, the amounts of chloride dissolved per 100 g dry weight of the pressure oxidation residue were estimated from the test data. As in Table 2 of Example 1, the values of Chloride Removal Rate in the last column of Table 3 were calculated by comparing the amounts of chloride dissolved between the solids without the Chloride Wash stage in Test 21 and the solids, that were treated by the Chloride Wash stage as in Test 22 and the other tests. As an example of inferences that may be drawn from various process parameters by one of skill in the art, for example to adapt or optimize the process of the invention, the results in Table 3 may allow the following conclusions to be drawn:  
         [0061]    A) Without the Chloride Wash treatment in Test 21, 20.3 mg of chlorine dissolved from 100 g of the composite pressure oxidation residue.  
         [0062]    B) After treatment by the Chloride Wash step, the similar Copper Leach test in Test 22 reduced the amount of chloride contamination to 3.7 mg per 100 g of PO solids, achieving a Chloride Removal rate of 82%. In Test 22, a copper extraction rate of 94% was obtained with an iron leach rate of 1.2%.  
         [0063]    C) In a comparison made between Tests 22 and 23, similar Copper Leach test results are produced by the use of either concentrated sulphuric acid or the acid contained in the synthetic copper electrolyte. These two tests resulted in a copper extraction of 93-94%, iron extractions of 0.9-1.2% and a chloride removal rate of 82%.  
         [0064]    D) In Tests 23, 24 and 25, the synthetic copper electrolyte was used to leach the copper value at the slurry pH ranging from 1.7 to 2.5. In Test 24 with the final slurry pH=2.5, the copper extraction rate decreased from 93 to 91% with the lowest iron dissolution rate of 0.4%. When the final slurry pH was decreased to 1.7 in Test 25, the copper extraction increased to 95% but with a high iron dissolution rate of 2.3%.  
         [0065]    E) In Tests 23, 26 and 28, a comparison could be made to examine the effect of mixing time while the final slurry pH was maintained at 2.2-2.3 and the reaction temperature was kept at 39-43° C. Though the extraction rates of copper and iron in those tests were unchanged at 93%, the mixing time of 30 minutes in Test 26 resulted in a lower Chloride Removal Rate of 67%. Mixing time of 60 minutes or longer is beneficial to maintain higher Chloride Removal Rates of 79-83% under those pH and reaction temperature conditions.  
         [0066]    F) In Tests 23, 27 and 29, the effect of the leach temperature was investigated, which the slurry pH was maintained at 2.1-2.3 for the leach duration of 60 minutes. High chloride removal rates of 82-84% were realized at these temperatures. Though the lowest iron extraction rate of 0.6% was obtained at the leach temperature of 65° C., acceptable rates of 1.1 and 0.9% were also realized at 31 and 43° C., respectively. The copper extraction of 92% was lower at 65° C. than 93-94% obtained at the lower temperatures.  
         [0067]    G) The results presented in Table 3 indicated that the Chloride Wash step could reduce the amount of chloride ions reporting in the pregnant Copper Leach solution by 82-84% when the conditions of Copper Leach were in the optimum range. While the chloride removal from the pressure oxidation residue is maintained in a low range, high copper extraction and low iron dissolution from the solids treated by the Chloride Wash step were obtained.  
         [0068]    H) No test results are produced to verify the suitability of the Copper Leach pregnant solution produced in Example 2 for electrowinning. However, its suitability can easily be verified by those familiar with the art of copper electrowinning, based on the chemical composition of the solution.  
       EXAMPLE 3  
     Chloride Recovery I  
       [0069]    Cuprous oxide is easily oxidized by air during storage and is not readily available in its pure form. Therefore, synthetic cuprous oxide was produced in a laboratory test. Laboratory tests of this sort may be useful in optimizing or adapting alternative embodiments of the invention.  
         [0070]    A solution containing 50 g/L Cu ++  and 55 g/L Cl −  was prepared by dissolving copper sulphate and calcium chloride crystals in distilled water. Solution pH was maintained at around 2 with hydrochloric acid. 25 g of fine zinc metal powder were added to one liter of the above solution. After agitating the solution with zinc powder for one hour, the cuprous chloride precipitate formed was filtered. The filter cake was then reslurried with 300 mL of distilled water and 25 g of calcium oxide were added to the slurry to convert the cuprous chloride to cuprous oxide. After mixing the slurry for one hour, the cuprous oxide solids were filtered. The filter cake was immediately stored in a freezer to minimize air oxidation. The wet filter cake was found to contain 35% copper in the cuprous form.  
         [0071]    Using the cuprous oxide prepared in the method described above, the adaptations of the Chloride Recovery I stage may be evaluated. Two types of CaCl 2  solution containing 811 and 1600 mg/L Cl −  were prepared by dissolving CaCl 2  crystals in distilled water. The initial pH of the solutions were adjusted at either 2 or 2.5 with the addition of sulphuric acid. Also added to the solution were either 0.25 or 2 g/L of copper ions as copper sulphate. While 500 mL of the solution was mixed in a beaker with a lid, various amounts of the wet cuprous oxide were added and solution samples were taken at appropriate time intervals to monitor chloride ion concentrations. Although some tests were carried out at different temperatures, most tests were performed at ambient temperature of 18-20° C. Table 4 summarizes the results of these tests.  
                                                                                                                 TABLE 4                           Summary of Chloride Recovery I Tests                        *Wet Cu 2 O           Test       Initial Solution   Addition   Chloride Analysis (mg/L Cl − )            Number   Temp ° C.   pH   Cu 2+ (g/L)   (g/L)   0 min   5 min   15 min   30 min                    Test 31   Ambient   2.0   0.25   4.0   811   —   257   405       Test 32   Ambient   2.0   0.25   8.0   811   —   128   188       Test 33   Ambient   2.0   2   16.0   811   —   92   88       Test 34   Ambient   2.0   0.25   16.0   1600   122   98   112       Test 35   Ambient   2.5   0.25   16.0   1600   130   100   114       Test 36   13   2.0   0.25   16.0   1600    88   76   95       Test 37   38   2.0   0.25   16.0   1600   220   140   140                          
 
         [0072]    For the solution containing 811 mg/L Cl −  and 0.25 g/L Cu ++  in Test 31, the addition of wet cuprous oxide at 4 g/L was insufficient for an effective removal of chloride ions. When the cuprous oxide addition was doubled to 8 g/L in Test 32, the residual chloride concentration was low enough so that the resultant solution could be recycled in the process. The complete removal of 811 mg/L Cl −  theoretically requires 4.1 g/L of the wet cuprous oxide. Therefore, approximately 200% of the theoretically required quantity of cuprous oxide was needed to achieve a satisfactory removal of chloride ions. The copper ion addition to the starting solution was increased to 2 g/L in Test 33 from 0.25 g/L in the previous two tests. There was only a marginal decrease in the final chloride concentration in comparison with the results of Test 32.  
         [0073]    Using the solution containing 1600 mg/L Cl − , a wet cuprous oxide addition rate of 16 g/L, approximately 200% of the theoretically required quantity, was needed to lower the chloride ion concentration in Test 34. In Test 35, pH of the starting solution was raised to 2.5 from 2.0 in the previous test, while other test conditions were unchanged. Similar test results as those in Test 34 were obtained.  
         [0074]    Test 36 was performed at a temperature of 13° C. lower than ambient temperature of 18-20° C., which was used for Test 34 otherwise under similar test conditions. The lower reaction temperature marginally improved the chloride ion removal rate. On the other hand, the reaction temperature was increased to 38° C. in Test 37, while other test conditions were maintained to be the same as in Tests 34 and 36. The chloride removal was incomplete at this temperature, indicating that lower temperatures favored the chloride ion removal.  
       EXAMPLE 4  
     Chloride Recovery II  
       [0075]    A synthetic solution representing the composition of copper leach pregnant solution was prepared by dissolving copper sulphate crystals in distilled water. This synthetic copper solution contained 70 g/L cupric ions. To this solution, various amount of impurities such as chloride and ferric ions were added as hydrochloric acid and ferric sulphate, respectively. One liter of this solution was placed in a reactor with a lid. While wet cuprous oxide produced in Test 52 in Example 6, which consisted of 24% copper as cuprous ion, was added to the solution. pH of the mixture was controlled in an appropriate range by adding sulphuric acid solution. Samples of the solution mixture were taken at 5, 15 and 30 minutes after the start of test. Samples were immediately filtered and their pH and chloride concentration were determined. Table 5 summarizes the results of these tests.  
                                                                                                                                                                                                   TABLE 5                           Summary of Chloride Recovery II Tests                    *Initial Solution   **Wet Cu 2 O   Samples            Test           Fe 3+     Fe 2+     Addition   0 min   5 min   15 min   30 min            Number   Temp ° C.   pH   (g/L)   (g/L)   (g/L)   Cl − (mg/L)   pH   Cl − (mg/L)   pH   Cl − (mg/L)   pH   Cl − (mg/L)   pH                    Test 41   Ambient   2.0   0.00   0.00   2.0   150   2.0   38   2.4   55   2.6   28   2.2       Test 42   Ambient   2.0   0.00   0.00   2.0   93   2.0   44   2.5   59   2.8   33   1.9       Test 43   Ambient   2.0   0.00   0.00   5.0   93   2.0   100   3.5   146   3.7   26   2.1       Test 44   Ambient   1.5   0.50   0.00   3.0   248   1.5   265   2.7   169   2.5   163   2.2       Test 45   Ambient   2.0   1.00   0.00   5.0   157   2.0   199   3.5   197   3.9   201   2.1       Test 46   Ambient   2.0   0.09   0.41   5.0   92   2.0   32   3.7   60   3.3   41   2.5       Test 47   Ambient   2.0   0.14   0.36   5.0   71   2.0   62   2.1   61   2.2   40   2.2                                  
 
         [0076]    In Tests 41-43, the chloride concentration in the pregnant copper solution was varied at 93 and 150 mg/L. No ferric ions were added in these tests as impurity. Whereas the wet cuprous oxide addition rate was varied between 2 and 5 g/L. Though the pH of the solution started at 2, it increased above 2.5 during the tests. While pH of the solution was above 2.5, the chloride ion concentration was higher than 50 mg/L. When the pH value was brought down to below 2.5, the chloride ion concentration notably came down below 50 mg/L. To obtain a final chloride concentration below 50 mg/L, it is important to keep the solution pH below 2.5. The cuprous oxide addition rates between 2 and 5 g/L to the solution containing chloride ions at 93-150 mg/L had no significant impact on the final chloride concentration.  
         [0077]    The previous example (Example 2) illustrates that although the iron dissolution from the pressure oxidation residues may be maintained at low levels, small amounts of iron will dissolve as ferric ions. These ferric ions even at low concentrations may have a negative impact on the efficiency of chloride removal. In Tests 44 and 45, the test solution was spiked with ferric ions at 0.5 and 1 g/L, respectively. Cuprous oxide was added at 3 and 5 g/L in Tests 44 and 45, respectively. In spite of the apparently appropriate pH range maintained in these tests, the final chloride concentrations were well above 100 mg/L. Especially noteworthy is the fact that the final chloride concentration in Test 45 after 30 minutes was higher than the starting concentration of 157 mg/L, which was putatively due to chloride contamination of the wet cuprous oxide used in this test. The ferric ions consumed all the cuprous oxide without precipitation of insoluble cuprous chloride.  
         [0078]    In Tests 46 and 47, part of the ferric ions added to the test solution were reduced to ferrous ions by contacting the solution with copper scrap in a tumbling mill for 5 minutes. The solution was separated from the scrap copper pieces and was reacted with wet cuprous oxide for chloride removal. The final chloride concentration range of 40-41 mg/L was achieved in these tests, in which cuprous oxide was added at 5 g/L.  
       EXAMPLE 5  
     Cuprous Chloride Conversion  
       [0079]    Table-6 lists the test results for the Cuprous Chloride Conversion. 150 g of reagent grade CuCl and distilled water were placed in a 2-L beaker. In Test 51, a mechanical mixer provided the necessary mixing action, 64 g of reagent grade CaO were added to the slurry mixture over one hour. The addition of CaO was performed at ambient temperature and no heating was provided. At the end of the one hour reaction time, the slurry pH was 11.4. The slurry was filtered and the cake was washed with 300 mL of distilled water to remove soluble chloride ions in the moist cake. 278 g of a wet cake consisting of cuprous oxide, unreacted cuprous chloride and unreacted calcium hydroxide were obtained and stored in a freezer to prevent oxidation. Chemical analysis of the wet cake indicated that the wet cake contained 35% copper and 3.2% chlorine. A conversion of cuprous chloride to cuprous oxide was estimated to be 83%.  
                                                                                                                                                                 TABLE 6                           CuCl Conversion test Summary                    CuCl                       Solid Product (in 1 L)                    Dry           Reagent           Wet   % Cu   % Cl   Cu/Cl   Estimated       Test   Temp.   Weight   Time       Added       Cl −     Weight   in wet   in wet   Weight   Conversion       #   ° C.   (g/L)   (min)   Reagent   (g/L)   pH   (g/L)   (g)   solids   solids   Ratio   to Cu 2 O (%)                    Test   Ambient   150   0   CaO   50   5.4   32   278   35   302   10.9   83       51           10           5.5   33                   30   CaO   14   5.7   41                   40           11.4   33                   60           11.4   42       Test   Ambient   150   0   CaO   50   5.1   22   288   24   1.8   13.3   90       52           10           5.5   26                   30           5.7   25                   40   CaO   30   12.5   33                   60           12.7   44                  
 
         [0080]    The above procedure was repeated but with an increased CaO amount of 80 g in Test 52. The final slurry pH after one hour of reaction time was 12.7. 288 g of a wet reaction product was obtained. This wet cake contained 24% copper and 1.8% chlorine. The estimated rate of conversion of cuprous chloride to cuprous oxide was 90%. As being indicated in Example 5, this second cuprous oxide product was used to carry out the tests for chloride ion removal from the copper leach pregnant solution.  
       Conclusion  
       [0081]    Although various embodiments of the invention are disclosed herein, many adaptations and modifications may be made within the scope of the invention in accordance with the common general knowledge of those skilled in this art. Such modifications include the substitution of known equivalents for any aspect of the invention in order to achieve the same result in substantially the same way. Numeric ranges are inclusive of the numbers defining the range. In the specification, the word “comprising” is used as an open-ended term, substantially equivalent to the phrase “including, but not limited to”, and the word “comprises” has a corresponding meaning. Citation of references herein shall not be construed as an admission that such references are prior art to the present invention. All publications, including but not limited to patents and patent applications, cited in this specification are incorporated herein by reference as if each individual publication were specifically and individually indicated to be incorporated by reference herein and as though fully set forth herein. The invention includes all embodiments and variations substantially as hereinbefore described and with reference to the examples and drawings.