Abstract:
The invention relates to a method for separating nickel and other valuable metalsparticularly from material having low nickel content, which contains iron and magnesium in addition to nickel and other valuable metals. The material havinglow nickel content is subjected to pulpingand atmospheric leaching in acidic and oxidising conditions, in which the majority of the metals in themate-rialdissolve and the iron is partially precipitated. The precipitated iron is sepa-rated from the solution, after which nickel and the other dissolved valuable metalsare precipitated as sulphides.

Description:
FIELD OF THE INVENTION 
       [0001]    The invention relates to a method for separating nickel and other valuable metals particularly from material having low nickel content, containing iron and magnesium in addition to nickel and other valuable metals. The nickel-poor material is subjected to pulping and atmospheric leaching in acidic and oxidising conditions, in which the majority of the metals of the material dissolve and the iron partially precipitates. The precipitated iron is separated from the solution, after which nickel and the other dissolved valuable metals are precipitated as sulphides. 
       BACKGROUND OF THE INVENTION 
       [0002]    The separation of a valuable metal mineral from gangue generally takes place by concentration, which includes for example, crushing, grinding and flotation. The purpose of concentration is to achieve a concentrate, in which as large a portion of the valuable minerals as possible is present and as little worthless gangue as possible. Concentration methods are being developed all the time and the recovery of valuable minerals into concentrate is improving. However, there are still always some small amounts of valuable metals ending up in the fraction that is discarded to the waste heap and in particular, as the metal prices have risen, the need to recover the valuable metals in waste also has become apparent. The recovery of nickel in particular from the fraction that has been discarded has become a target of interest, since nickel is an important component of stainless steel. 
         [0003]    Nickel-poor material is formed for instance in the concentration process of a nickel-bearing ore, in which the fraction with higher nickel content is separated from the ore by flotation and the tailings are left, which nevertheless still contain some nickel and other valuable metals such as cobalt, copper and zinc. When the nickel ore is mostly sulphidic, the majority of the nickel remaining in the flotation tailings is also in the form of sulphides and only a small amount as silicates. The tailings formed in a multi-stage flotation process are usually routed to a tailings area, even though there is still a small amount of valuable metals in them. 
       PURPOSE OF THE INVENTION 
       [0004]    The purpose of the invention presented here is to disclose a method by means of which the majority of the nickel and other valuable metals, which have remained in the nickel-poor waste material in the context of the concentration of nickel sulphide ore, can be recovered. 
       SUMMARY OF THE INVENTION 
       [0005]    This invention relates to a method for separating nickel and other valuable metals from nickel-poor iron-containing material generated in the nickel ore concentration process, which method comprises 
         [0006]    a) a pulping stage, wherein the nickel-poor material is pulped into an acid-containing leaching solution, and the pH of the leaching solution is adjusted to a desired range, 
         [0007]    b) an atmospheric leaching stage, wherein the nickel-poor material obtained from the pulping stage is contacted with an acid-containing solution and an oxygen-containing gas. 
         [0008]    More specifically the invention relates to a method for separating nickel and other valuable metals from the nickel-poor material containing iron that is generated in a nickel ore concentration process, whereby the nickel-poor material is pulped in an acid-containing leaching solution, the pH value of which is adjusted to the desired range in order to leach a first part of the nickel contained in the nickel-poor material. 
         [0009]    According to one embodiment of the invention, the pH of the leaching solution is adjusted in pulping to the range of 1.5-3.0. 
         [0010]    According to one embodiment of the invention, after pulping of the nickel-poor material, it is routed to atmospheric leaching, in which a second part of the nickel, the other valuable metals and the iron are leached by means of an acid-containing solution and an oxygen-containing gas. The first part of the iron is precipitated during the atmospheric leaching stage, after which the precipitated iron is separated from the solution containing a valuable metal and the solution containing a valuable metal is routed to the precipitation of nickel and other valuable metals, which is carried out by means of sulphidation. 
         [0011]    Typically the nickel-poor iron-containing material is generated in the nickel ore concentration process. According to one embodiment of the invention, the nickel-poor iron-containing material is tailings of a nickel ore flotation process. 
         [0012]    According to one embodiment of the invention, nickel and the other valuable metals are in the nickel-poor material mostly as sulphides. 
         [0013]    According to one embodiment of the invention, the pulping and leaching solution of the nickel-poor material is sulphuric acid based. 
         [0014]    According to an embodiment of the invention the oxygen containing gas can be any suitable gas mixture which contains oxygen. According to an embodiment of the invention, the oxygen-containing gas is oxygen. According to another embodiment of the invention, the oxygen-containing gas is air. According to a further embodiment of the present invention the oxygen-containing gas is oxygen-enriched air. According to one embodiment of the invention the oxygen-containing gas contains approximately 21-100% oxygen. 
         [0015]    In one embodiment of the invention, the first part of the iron is precipitated as jarosite, whereby atmospheric leaching is carried out in a pH range of 1.3-2. The second part of the iron dissolved in the leaching solution is precipitated as goethite or hydroxide by neutralising and oxidising the solution. 
         [0016]    In another embodiment of the invention, atmospheric leaching is carried out in a pH range of 2.1-3.0, whereby the first precipitated part of the iron corresponds to the total amount of iron. Iron is precipitated as goethite and hydroxide. 
         [0017]    According to one embodiment of the invention, pulping and atmospheric leaching are carried out at a temperature between 60° C. and the boiling point of the solution. 
         [0018]    According to one embodiment of the invention, the nickel-poor material is pre-treated with concentrated sulphuric acid before pulping. 
         [0019]    According to one embodiment of the invention, the leaching of valuable metal sulphides to sulphates is performed at elevated oxygen pressure. According to one embodiment of the invention the oxygen partial pressure is 2-4 bar at the temperature of 115-135° C. 
         [0020]    According to one embodiment of the invention, the leaching solution formed in sulphide precipitation, from which the valuable metals have been removed, contains magnesium and manganese, which are precipitated from the solution by neutralising it. 
         [0021]    According to one embodiment of the invention, the valuable metals in the nickel-poor material in addition to nickel are cobalt, copper and zinc. 
     
    
     
       LIST OF DRAWINGS 
         [0022]      FIG. 1  is a diagram of one embodiment of the invention, 
           [0023]      FIG. 2  is a diagram of another embodiment of the invention, 
           [0024]      FIG. 3  presents a graph of the dissolution of various metals as a function of the amount of acid consumed, and 
           [0025]      FIG. 4  presents a graph of the nickel recovery in a test as a function of time. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0026]    The invention relates to a method for separating nickel and other valuable metals from a material with low nickel content generated in nickel ore flotation, or flotation tailings, which are usually routed to a tailings area. In addition to nickel, the material also includes iron; it has been found that the nickel in the material is mainly bound to sulphidic minerals containing iron, such as pyrrhotite and pentlandite, and only a small amount to silicaceous minerals. The amount of nickel in flotation tailings is typically in the region of half of one per cent. Flotation tailings also include other valuable minerals in addition to nickel, such as cobalt, copper and zinc, but in amounts that are a fraction of the amount of nickel. In addition to the above, flotation tailings also contain a considerable amount of magnesium. 
         [0027]    Since the amount of valuable metals in nickel-poor material such as flotation waste is small, the valuable metal recovery process has to be designed taking process costs in particular into account. If for example the flotation tailings have to be subjected to another flotation treatment with all its scavenging and cleaning flotation stages, the method will be expensive. Likewise leaching the whole of the flotation tailings in an autoclave incurs too high a cost in relation to the price for the valuable metals obtained in the treatment. It has now been found in accordance with the invention that nickel and other valuable metals can be leached from nickel-poor material with atmospheric leaching by means of a leach solution containing sulphuric acid and an oxygen-containing gas. A small amount of copper may also be fed into the leach solution in order to optimise leaching, should there be no copper at all in the flotation tailings. 
         [0028]    The simplest and most cost-effective method of all accordant with the invention is when only partial recovery of nickel is sufficient. In that case the method consists only of pulping of the nickel-poor material in an acidic aqueous solution, because it has been found in tests that by adjusting the pH to the desired range during pulping, for example 1.5-3.0, the first part of nickel, i.e. about 40% of the nickel, dissolves. The reactor volume required for pulping alone remains fairly small and at the same time the consumption of acid required for pH control of the aqueous solution remains low. 
         [0029]    It is also possible to pre-treat the flotation tailings with concentrated sulphuric acid before pulping, so that the waste material is mixed into concentrated acid without water. In this way the metals in the nickel-poor material react with the acid and form sulphates. In pre-treatment silicates are partially dehydrated and their structure changes, facilitating post-leaching liquid-solids separation. Treatment reduces the formation of silicaceous gel during leaching and precipitation and thereby improves filterability. 
         [0030]    When it is desired to recover as much of the nickel and other valuable metals from the nickel-poor material as possible, treatment of the material should be continued by leaching it after pulping. The second part of the nickel can be made to dissolve by means of leaching. In the tests carried out it was found that in the conditions in which nickel and other valuable metals dissolve, i.e. in the range of 1.5-3.0, the majority of the iron and magnesium also dissolve, but the iron is simultaneously re-precipitated. The pH range of the leaching solution is controlled depending on, for example, whether it is desired to dissolve the valuable metals bound to silicates also and in what form it is wished to precipitate the iron from the solution. In the alternative accordant with  FIG. 1  the pH of the solution is adjusted to the range of 1.3-2.0, whereby the valuable metals bound to silicates also mostly dissolve. In the pH range in question, iron is precipitated as jarosite. In order for iron to be precipitated as jarosite, some suitable alkali compound is also fed into the leaching stage, for example sodium sulphate (not shown in detail in the drawing). A lower pH range demands a greater amount of sulphuric acid and for that reason the selected pH range may also depend on cost factors. Again, it was found in tests that the precipitation of iron as jarosite or another compound during leaching further improves the settling of solids in solid-liquid separation and reduces acid consumption. The recycling of the precipitated iron compound to the leaching stage promotes the dissolving of nickel. 
         [0031]    When valuable metals are leached from flotation tailings in a pH range of below 2, as presented in the flowsheet of  FIG. 1 , part of the iron remains in the solution and is not precipitated in these conditions. In order to precipitate this second part of the iron a suitable neutralising agent such as lime is routed to the solution, by means of which the second part of the iron is precipitated as goethite and/or hydroxide. In this case too, recycling to pulping or leaching of the iron precipitate, particularly of the first jarositic part, is advantageous, even though it is not shown in the flowsheet. Preferably the precipitation of the second part of the iron is performed in a pH range of 2.5-2.9, whereby precipitation of the copper in solution along with the iron is prevented. Trivalent iron should be precipitated out of solution before the sulphide precipitation of valuable metals; divalent does not harm precipitation. 
         [0032]    The embodiment accordant with  FIG. 2  operates in a higher pH range, 2.0-3.0, whereby the valuable metals bound to silicates remain mostly undissolved and only the valuable metals bound to sulphides are recovered in leaching. Iron is precipitated in this case mainly as goethite and hydroxides in the leaching stage and therefore precipitation of the first part of the iron during leaching is the precipitation of the entire amount of iron. 
         [0033]    Leaching of flotation tailings is preferably carried out at a solids content of 250-700 g/I. Since the solids content is fairly high, the reactors used for leaching should be equipped with effective mixing. It is advantageous to feed an oxygen-containing gas to the vicinity of the mixer. According to one embodiment, the oxygen-containing gas is fed into the solution outside the reactor, whereby an oxidation unit equipped with a static mixer is located outside the reactor, via which part of the leaching solution is circulated. 
         [0034]    When the valuable metals have dissolved from the nickel flotation tailings, the dissolved and precipitated iron and the undissolved material are separated from the solution by solids-liquid separation. Separation takes place by some appropriate method, for example by countercurrent decantation (CCD), as presented in  FIGS. 1 and 2 , or by filtering or by thickening and filtering. 
         [0035]    The nickel and other valuable metals dissolved in the leaching solution are subjected to sulphide precipitation. Precipitation can be done by means of any appropriate sulphide, such as hydrogen sulphide or sodium sulphide. If required, the solution is neutralised so as to achieve the right pH range. The advantage of sulphide precipitation is that it is selective, so that the impurities in the solution such as sodium, magnesium, calcium, manganese and divalent iron are not precipitated along with the valuable metals. 
         [0036]    The precipitated valuable metal sulphide residue can be further subjected to sulphide leaching, so that the sulphides are leached in acidic conditions into a sulphuric acid solution and a sulphate solution containing valuable metals is obtained. Leaching is carried out either in an autoclave, whereupon the leaching time is around 2-3 h, or in atmospheric conditions, whereupon the leaching time is correspondingly longer. The sulphate solution containing valuable metals can be routed on to solvent extraction, precipitation and electrolysis for fabrication of each metal product. 
         [0037]    It is advantageous to remove the magnesium and manganese from the leaching solution from which the valuable metals have been removed. This can be performed for example by hydroxide precipitation, whereby the pH value of the solution is raised for instance by means of slaked lime (Ca(OH) 2 ). The alkaline solution from which impurities have been removed can be used, for example, as the solution fed into the final stage of solids-liquid separation which takes place as counter-current decantation. Additionally, the alkaline solution can be used as the neutralising agent in the precipitation of the second part of the iron instead of or in addition to the above-mentioned limestone. 
       EXAMPLES 
     Example 1 
       [0038]    A test was carried out with flotation tailings, of which the Ni content was 0.35%, the Cu content 0.07%, the Co content 0.007%, the Fe content 10.7% and the Mg content 15.6%. All the percentages are percentages by weight. About four fifths of the nickel is in sulphide minerals, such as pentlandite, violarite and pyrrhotite and the rest in silicaceous minerals. The test conditions are shown in the tables below, and the recovery of nickel and other metals is also presented as a graph in  FIG. 3 . 
         [0039]    The leaching test was performed in a 5-litre reactor, in which there are baffles, temperature control, mixing with a gls mixer as well as pH and redox measurement. The pH was held constant at a value of 2.6 throughout the test by titrating the solution with concentrated sulphuric acid. The leaching temperature was 95° C., the mixing speed 1000 rpm, slurry density 400 g/L and oxygen feed 0.3 L/min. The oxygen feed was oxygen. 
         [0040]    Samples were taken regularly during the test. Approx. 30 mL of slurry was taken as a sample from the central stages of the reactor, and was filtered by vacuum filtration. A solids sample was taken from the precipitate after the precipitate was washed and a solution sample was taken of the filtrate. The samples of Co, Cu, Fe, Mg, Mn and Ni were analysed using the ICP technique (ICP=Inductively Coupled Plasma). The samples of precipitate were analysed after total leaching. The precipitate samples were also analysed for sulphidic metals after bromine-methanol leaching. 
         [0041]    00 h means the starting point when the slurry had reached the test temperature and 0 h the point when the target pH had been reached and oxygen feed started, in other words, pulping occurs between 00 and 0 h. 
       Solution Analyses: 
       [0042]      
         [0000]    
       
         
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
             
           
               
                   
               
               
                 Time 
                 Co 
                 Cu 
                 Fe 
                 Mg 
                 Mn 
                 Ni 
               
               
                 h 
                 mg/l 
                 mg/l 
                 mg/l 
                 mg/l 
                 mg/l 
                 mg/l 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 00 
                 0 
                 0 
                 &lt;1 
                 1790 
                 2 
                 0.7 
               
               
                 0 
                 7.3 
                 31.8 
                 5340 
                 11600 
                 165 
                 616 
               
               
                 1 
                 7.7 
                 9.4 
                 6540 
                 13600 
                 180 
                 665 
               
               
                 2 
                 8.3 
                 2.7 
                 9490 
                 14500 
                 195 
                 724 
               
               
                 4 
                 7.9 
                 1.7 
                 5760 
                 14700 
                 189 
                 701 
               
               
                 6 
                 9.2 
                 12.8 
                 2630 
                 18600 
                 223 
                 816 
               
               
                 10 
                 11 
                 17.8 
                 2600 
                 24800 
                 250 
                 896 
               
               
                 14 
                 10.2 
                 33.6 
                 1050 
                 25400 
                 228 
                 814 
               
               
                 18 
                 11.8 
                 88.6 
                 671 
                 31000 
                 239 
                 899 
               
               
                 24 
                 14.4 
                 131 
                 481 
                 39000 
                 256 
                 1030 
               
               
                 30 
                 16.7 
                 150 
                 321 
                 44400 
                 271 
                 1140 
               
               
                   
               
             
          
         
       
     
       Solids Analyses: Total Leaching and (S)=Sulphidic Minerals 
       [0043]      
         [0000]    
       
         
               
               
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
               
             
           
               
                   
               
               
                 Time h 
                 Co % 
                 Cu % 
                 Fe % 
                 Mg % 
                 Mn % 
                 Ni % 
                 Co(S) % 
                 Cu(S) % 
                 Fe(S) % 
                 Ni(S) % 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 00 
                 0.007 
                 0.07 
                 10.8 
                 15.6 
                 0.1 
                 0.35 
                 0.005 
                 0.07 
                 5.4 
                 0.28 
               
               
                 0 
                 0.006 
                 0.07 
                 10.5 
                 14.9 
                 0.06 
                 0.23 
                 0.004 
                 0.07 
                 4.2 
                 0.18 
               
               
                 1 
                 0.005 
                 0.07 
                 10.4 
                 14.9 
                 0.06 
                 0.22 
                 0.004 
                 0.07 
                 3.6 
                 0.16 
               
               
                 2 
                 0.005 
                 0.06 
                 9.8 
                 14.8 
                 0.06 
                 0.21 
                 0.004 
                 0.07 
                 2.2 
                 0.15 
               
               
                 4 
                 0.005 
                 0.06 
                 10.8 
                 14.3 
                 0.06 
                 0.19 
                 0.003 
                 0.06 
                 1.0 
                 0.14 
               
               
                 6 
                 0.005 
                 0.06 
                 11.6 
                 13.5 
                 0.05 
                 0.18 
                 0.003 
                 0.06 
                 0.6 
                 0.13 
               
               
                 10 
                 0.004 
                 0.05 
                 12.1 
                 12.3 
                 0.05 
                 0.16 
                 0.003 
                 0.05 
                 0.4 
                 0.12 
               
               
                 14 
                 0.004 
                 0.05 
                 12.6 
                 11.6 
                 0.05 
                 0.16 
                 0.003 
                 0.05 
                 0.3 
                 0.12 
               
               
                 18 
                 0.004 
                 0.04 
                 13.5 
                 10.2 
                 0.05 
                 0.15 
                 0.003 
                 0.03 
                 0.2 
                 0.1 
               
               
                 24 
                 0.004 
                 0.03 
                 13.3 
                 9.02 
                 0.04 
                 0.13 
                 0.003 
                 0.02 
                 0.3 
                 0.08 
               
               
                 30 
                 0.004 
                 0.03 
                 13.8 
                 7.62 
                 0.04 
                 0.11 
                 0.003 
                 0.01 
                 0.2 
                 0.06 
               
               
                   
               
             
          
         
       
     
       Recoveries: 
       [0044]      
         [0000]    
       
         
               
               
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
               
             
           
               
                   
               
               
                 Time h 
                 Co % 
                 Cu % 
                 Fe % 
                 Mg % 
                 Mn % 
                 Ni % 
                 Co(S) % 
                 Cu(S) % 
                 Fe(S) % 
                 Ni(S) % 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 00 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
               
               
                 0 
                 23.5 
                 10.4 
                 11.1 
                 16.9 
                 42.0 
                 42.8 
                 30.4 
                 15.5 
                 32.3 
                 44.1 
               
               
                 1 
                 24.8 
                 3.1 
                 13.6 
                 17.7 
                 46.2 
                 45.9 
                 31.1 
                 20.0 
                 42.6 
                 50.8 
               
               
                 2 
                 26.7 
                 0.9 
                 19.8 
                 20.3 
                 48.4 
                 49.6 
                 32.8 
                 22.0 
                 65.8 
                 55.0 
               
               
                 4 
                 25.4 
                 0.6 
                 12.0 
                 19.5 
                 48.8 
                 52.3 
                 47.3 
                 23.5 
                 84.4 
                 56.1 
               
               
                 6 
                 29.6 
                 4.2 
                 5.5 
                 25.3 
                 52.3 
                 55.4 
                 48.2 
                 32.2 
                 90.6 
                 60.2 
               
               
                 10 
                 35.4 
                 5.8 
                 5.4 
                 33.7 
                 58.9 
                 60.6 
                 49.6 
                 37.6 
                 94.5 
                 63.7 
               
               
                 14 
                 32.8 
                 11.0 
                 2.2 
                 39.0 
                 59.8 
                 61.6 
                 50.8 
                 43.8 
                 95.2 
                 64.8 
               
               
                 18 
                 37.9 
                 28.9 
                 1.4 
                 51.1 
                 64.1 
                 68.8 
                 55.1 
                 70.1 
                 96.8 
                 73.3 
               
               
                 24 
                 46.3 
                 42.7 
                 1.0 
                 57.9 
                 66.6 
                 73.6 
                 56.3 
                 82.3 
                 96.2 
                 78.4 
               
               
                 30 
                 53.7 
                 48.9 
                 0.7 
                 65.8 
                 67.9 
                 78.6 
                 58.0 
                 88.0 
                 97.9 
                 84.2 
               
               
                   
               
             
          
         
       
     
         [0045]    The recovery table shows that 44% of the sulphidic nickel dissolved during pulping (00-0), so in some cases actual leaching can be omitted or performed only for a period of a few hours. The same thing is also shown in graph form in  FIG. 3 , in which the recovery of various metals is shown as a function of sulphuric acid consumption. 
       Example 2 
       [0046]    The test accordant with the example was made in conditions in accordance with Example 1 other than for the fact that, at the start of the test, the pH was adjusted to be 1.5. In these conditions iron was precipitated as jarosite. The waste to be treated neutralised the solution during leaching so that the pH rose to the range of 2.3 and remained there until the end of leaching. The recovery of nickel was over 60% at the stage when pH 1.5 was reached and the recovery rose to a value of over 75% during the test. The solution recoveries are depicted in  FIG. 4 , in which the yields of the different metals are shown as a function of time. 
         [0047]    The yields of the different metals are given below: 
         [0000]    
       
         
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
             
           
               
                   
               
               
                 Time h 
                 Al % 
                 Co % 
                 Cr % 
                 Cu % 
                 Fe % 
                 Mg % 
                 Mn % 
                 Ni % 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 0 
                 — 
                 — 
                 — 
                 — 
                 0.0 
                 2.7 
                 0.0 
                 0.0 
               
               
                 1 
                 5.9 
                 40.4 
                 5.9 
                 86.3 
                 28.7 
                 35.2 
                 54.6 
                 63.9 
               
               
                 2 
                 4.3 
                 41.1 
                 5.4 
                 89.3 
                 27.9 
                 40.6 
                 61.3 
                 72.6 
               
               
                 3 
                 4.4 
                 45.9 
                 4.1 
                 84.8 
                 24.9 
                 45.7 
                 58.1 
                 70.7 
               
               
                 4 
                 4.6 
                 51.8 
                 3.0 
                 84.1 
                 21.9 
                 52.9 
                 62.8 
                 74.9 
               
               
                 5 
                 3.1 
                 46.7 
                 1.7 
                 81.7 
                 15.5 
                 49.2 
                 64.4 
                 75.6