Abstract:
A silicate ore containing gold and silver is melted with the slag from a copper converter and a reducing agent. A source of CaO may also be added to the slag so that the treated slag may contain at least 3.0% by weight of CaO. Copper is recovered from the slag and absorbs gold and silver from the ore. This method drastically improves the capacity for the treatment of a silicate ore containing gold and silver during the smelting of copper. It enables the complete melting of the ore without leaving any ore undissolved.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a method of efficiently recovering gold and silver from a silicate ore. 
     2. Description of the Prior Art 
     There are two main methods known for smelting a silicate ore containing at least 0.1 g of each of gold and silver per ton. One of them is a hydrometallurgical method which is intended only for the recovery of gold and silver. The other is a pyrometallurgical method in which the ore per se is used as a flux for the pyrometallurgical smelting of copper or lead, while gold and silver are recovered as by-products. 
     Slurry cyaniding is a widely used hydrometallurgical method, but the disposal of the waste solution and tailings which contain cyanogen is a big problem. The thiourea method and flotation are hydrometallurgical methods which do not use any cyanogen solution. The former is, however, still under study and not yet employed in practice (Studies and Prospects of Gold Extraction from Carbon Bearing Clayey Gold Ore by the Thiourea Process, Chen Deng Wen: Preprints--The Canadian Institute of Mining and Metallurgy, XIV International Mineral Processing Congress, Toronto, Canada, Oct. 17-23, 1982, II--8.1 to II--8.11). The latter can only recover gold and silver at a low yield (about 93% in the case of gold--The Japanese Association of the Mining Industry: Report on the Principal Costs of Flotation and Cyaniding, 1981). 
     The method in which the ore is used as a flux in, for example, a flash smelting furnace or converter for copper has the disadvantage that the resulting slag has an Fe (%)/SiO 2  (%) ratio which is limited by its physical properties. The ratio is about 1.0 in the case of a slag from a flash smelting furnace and about 1.8 to 2.0 in the case of a converter slag. Any attempt to melt the silicate ore until a lower FeO/SiO 2  ratio is reached gives rise to a great increase in the viscosity and melting point of the slag (The Japan Institute of Metals: Nonferrous Smelting, p. 63), resulting in an increased loss of copper to the slag. It is necessary to raise the molten bath temperature in order to cope with any such increase in the viscosity and melting point of the slag. It is, however, practically difficult to employ a higher molten bath temperature, since it promotes the erosion of the refractories. The converter slag usually contains 25 to 35% of Fe 3  O 4 . Even a small increase of SiO 2  greatly raises the melting point of the slag. A part of the silicate ore remains undissolved with a resultant reduction in the primary yield of recovered gold and silver. 
     SUMMARY OF THE INVENTION 
     It is a primary object of this invention to provide a novel method of melting a silicate ore containing gold and silver to recover the gold and silver effectively, and which can drastically increase the capacity of treating any such ore in the field of copper smelting. 
     According to a first aspect of this invention, its object is attained by a method which comprises melting a silicate ore containing gold and silver with the slag from a copper converter and a reducing agent, so that the copper extracted from the slag may absorb the gold and silver. 
     This method can drastically increase the capacity of treating the ore, but the slag which is composed of FeO, Fe 2  O 3  and SiO 2  finally contains a saturated amount of SiO 2 . If there is a slight drop in temperature or a slight change in composition, it is likely that some of the ore may remain undissolved, with a resultant reduction in the yield of gold and silver recovery. The ore melts rather slowly and requires a high temperature and a long time for its complete dissolution. The use of a still higher temperature for shortening the melting time is likely to cause the erosion of the refractories. When a reducing agent is blown into the converter through its tuyeres or top lancing tube, the slag foams heavily as the silicate ore is dissolved therein. This increases the slopping of the slag out of the furnace and the material which is recycled. Moreover, it is possible that the hot molten material may blow out of the furnace. 
     It is, therefore, another object of this invention to provide a method which can completely melt a silicate ore container gold and silver in a short time, while reducting the slag from a copper converter, to decrease the consumption of the fuel maintaining the temperature of the molten bath, prolong the life of the refractories and prevent the foaming and slopping of the slag. 
     According to a second aspect of this invention, its object is attained by a method which comprises melting a silicate ore containing gold and silver with the slag from a copper converter and a reducing agent so that the copper extracted from the slag may absorb the gold and silver, characterized by adding a source of CaO to the slag so that the treated slag may contain at least 3.0% by weight of CaO. 
     The method of this invention enables the efficient recovery of gold and silver from a silicate ore, while simultaneously accomplishing the cleaning of the slag from a copper converter. Thus, it can drastically increase the capacity of treating such silicate ore during the smelting of copper. 
     The addition of a reducing agent enables the treatment of the silicate ore until the SiO 2  content of the slag is comparable to that of the slag from a matte smelting furnace of, for example, the flash or reverberatory type. As a low Fe/SiO 2  ratio is maintained, this invention is advantageous for the reduction of Fe 3  O 4  and effective for preventing the overreduction which would otherwise be likely to occur to even Fe at a high temperature. 
     The addition of a source of CaO accelerates the complete melting of the ore and thereby makes it possible to decrease the consumption of fuel, to prolong the life of the furnace refractories and to prevent the slopping of the slag. Therefore, it is possible to decrease the amounts of the material which blows out and the material which is recycled, and the handling loss of gold and silver, and thereby improve productivity. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a graph showing the copper content of the slag in relation to its Fe 3  O 4  content; 
     FIG. 2 is a graph showing the gold content of the copper recovered from the slag in relation to the gold content of the slag; 
     FIG. 3 is a graph showing the thickness of a foam layer formed on the slag of a copper converter when the slag to which a silicate ore containing gold and silver, limestone powder and pulverized coal had been added was cooled, in relation to the amount of the limestone; and 
     FIG. 4 is a graph showing the results of silicate ore melting as obtained when it was melted with the slag from a copper converter and pulverized coal, and when limestone was further added, in relation to the slag temperature and the melting time. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     According to this invention, a reducing agent is added to the molten slag from a copper converter to reduce Fe 3  O 4  therein. The viscosity of the slag is lowered, the copper suspended in the slag settles down, and a part of the copper oxide is reduced. If a silicate ore containing gold and silver is added during the progress of those reducing reactions, gold and silver are absorbed into the copper which settles down and SiO 2  forms a fayalite slag with the FeO which is produced by the reduction of Fe 3  O 4 . As the reducing agent is added to the slag from a copper converter while the silicate ore is also added, the reduction takes place at a lower Fe/SiO 2  ratio than what is achieved during the mere cleaning of the slag by reduction. The resulting slag has a smaller activity of FeO and Fe 3  O 4  is, therefore, easier to reduce. It is also possible to prevent any over-reduction to Fe that is likely to occur at a high temperature. 
     In order to add the reducing agent, it is possible to use a metallurgical furnace having tuyeres, for example, a nonferrous metal smelting converter, and blow pulverized coal and oxygen-enriched air into the slag through the tuyeres. Alternatively, it is possible to use a top lance. It is also possible to use an electric furnace or a furnace having a burner for heat compensation and introduce a reducing agent, such as coal, into the slag. A reducing gas, such as propane, can also be employed. 
     The slag temperature drops with the progress of reduction or the addition of the silicate ore. It is necessary to maintain a temperature of 1200° C. to 1300° C. to melt the ore completely. If the slag temperature is lower than 1200° C., heavy slopping occurs and the ore remains undissolved. A temperature exceeding 1300° C. should also be avoided, since it causes the heavy erosion of the refractories and also heavy slopping. Therefore, it is necessary to control the rate at which the silicate ore is added, using an auxiliary burner for heat compensation, to control the load of an electric furnace if one is used, and to control the oxygen content and temperature of the air if pulverized coal and oxygen-enriched air are blown through the tuyeres. 
     The loss of gold and silver to the slag depends on the final copper content of the slag and the gold and silver contents of the copper recovered from the slag, since it is in the majority of the cases due to the mechanical suspension in the slag of the copper-containing particles which have absorbed gold and silver. 
     The copper content of the slag is affected by its Fe 3  O 4  content. The inventors of this invention melted a silicate ore in a PS converter having a slag treating capacity of three to four tons per charge by blowing pulverized coal and air through its tuyeres until the slag finally had an Fe/SiO 2  ratio of about 1.0. The results are shown in FIG. 1. It has been found that in order to lower the final copper content of the slag to about 0.5% by weight, it is advisable to lower its final Fe 3  O 4  content to a level not exceeding 2% by weight. 
     The inventors examined the relationship between the gold content of the copper recovered from the slag and the final gold content of the slag by employing silicate ores having different gold contents and different amounts of recovered copper. The results are shown in FIG. 2, in which the black circles indicate that the slag had a final copper content not exceeding 0.5% by weight, while the white circles indicate that the slag had a final copper content of 0.7 to 1% by weight. As is obvious therefrom, it is advisable to lower the final copper content of the slag to a level not exceeding 0.5% by weight and the gold content of the recovered copper to a level not exceeding 60 g/ton in order to lower the final gold content of the slag to a level not exceeding 0.1 g/ton. In order to lower the final gold content of the slag to a level not exceeding 0.2 g/ton, it is advisable to lower the final copper content of the slag to a level not exceeding 0.5% by weight and the gold content of the recovered copper to a level not exceeding 170 g/ton. The same is presumably the case with silver. Therefore, if the gold and silver contents of a silicate ore are too high for the copper recovered from a converter slag, it is advisable to add a substance containing copper, such as an anode furnace slag or copper residue, in order to lower the gold and silver contents of the recovered copper. 
     The silicate ore can be treated until the slag has a final Fe/SiO 2  ratio of about 0.8. The finer the particle size, the higher the melting speed is. Lump ore having a size of, say, 20 mm can, however, be melted if a temperature of 1200° C. to 1300° C. is maintained for, say, 100 to 150 minutes. Lump ore can be fed through the working mouth of the furnace and pulverized ore can be blown through the tuyeres or lance. 
     The inventors tested the addition of a source of CaO as will hereunder be described. The slag from a converter, pulverized silicate ore containing gold and silver, limestone powder and 2% by weight of pulverized coal were melted at 1250° C. for an hour in the presence of nitrogen gas in an alumina tammann tube on a laboratory scale. The slag had an Fe/SiO 2  ratio of 0.8. After the molten material had been cooled, the thickness of a foam layer (a mixture of foams and slag on the dense slag layer) was measured. The results are shown in FIG. 3. The foam has a large thickness if less than 2.5% by weight of CaO is added (or if the slag contains less than 3.0% by weight of CaO). The foaming and slopping of the slag are very likely to occur. It is necessary for the slag to contain at least 3.0% by weight of CaO. No correspondingly improved results can, however, be expected even if the slag contains more than, say, 13% by weight of CaO. The source of CaO may be limestone or quicklime. While various shapes of limestone or quicklime can be added by various methods, it is usually sufficient to throw lumps into the furnace. 
     The slag finally contains only small amounts of copper, gold and silver and can be thrown away. The recovered copper, however, still contains a large amount of impurities, such as iron, and must be treated in an ordinary copper smelting converter. 
     The invention will now be described with reference to several examples thereof. 
     EXAMPLE 1 
     A PS converter lined with a brick wall having an inside diameter of 1.5 m and an inside length of 1.7 m was charged with three tons of a copper converter slag of the composition shown in Table 1. Pulverized coal was blown into the converter through four tuyeres having an inside diameter of 21 mm at a rate of 4.6 kg/min. by oxygen-enriched air having an average flow rate of 15.1 Nm 3  /min. and an average oxygen content of 25.6% by volume. At the same time, 1.0 ton of 15 to 20 mm lump silicate ore of the composition shown in Table 1 was charged into the converter by a chute which had been inserted through its working mouth. The blowing time was 150 minutes. The silicate ore was continuously charged during the first 90 minutes. 
     The final product had a weight of 3.5 tons of slag and 0.15 ton of copper was recovered. Their compositions are also shown in Table 1. The recovered copper contained 96% by weight each of gold and silver. 
     
                       TABLE 1______________________________________Cu           Fe                  Au    Ag(wt. %)      (wt. %)  SiO.sub.2 (wt. %)                            (g/ton)                                  (g/ton)______________________________________Slag    4.5      45.0     20.0     0.2   10Silicate ore   --        1.0     84.0     20    10Final slag   0.4      38.0     39.5     0.1   &lt;1.0Recovered   80.3      3.0     --       132   256copper______________________________________ 
    
     EXAMPLE 2 
     The procedure of Example 1 was substantially repeated for treating a silicate ore containing gold and silver in the converter used in Example 1. Three tons of a copper converter slag, one ton of silicate ore and 0.6 ton of an anode furnace slag were treated. Their compositions are shown in Table 2. There were obtained 3.7 tons of a final slag and 0.54 ton of recovered copper, of which the compositions are shown in Table 2. The copper recovered 98% by weight of gold and silver from the silicate ore. 
     Oxygen-enriched air having an average oxygen content of 26.9% by volume was blown through the tuyeres at an average rate of 15.9 Nm 3  /min. Pulverized coal was fed at a rate of 4.8 kg/min. Example 1 was repeated for the blowing time and the manner in which the silicate ore was charged. 
     
                       TABLE 2______________________________________   Cu     Fe       SiO.sub.2                            Au    Ag   (wt. %)          (wt. %)  (wt. %)  (g/ton)                                  (g/ton)______________________________________Copper con-      4.5     45.0     20.0   0.2   10verter slagAnode furnace     60.0     9.0      10.4   0.2   42slagSilicate ore     --       1.0      84.0   20    10Final slag      0.5     36.2     37.4   &lt;0.1  &lt;0.1Recovered 87.0     1.0      --     37.4  118copper______________________________________ 
    
     EXAMPLE 3 
     A PS converter used in Example 1 was charged with two to four tons of a copper converter slag of the composition shown in Table 3. Pulverized coal was blown in the quantity of 10 to 20% by weight of the slag by oxygen-enriched air having an oxygen content of 25 to 33%; by volume and a flow rate of 15 to 20 Nm 3  /min. through four tuyeres. A silicate ore of the composition shown in Table 3 was continuously supplied until the slag had an Fe/SiO 2  ratio of 0.8 to 1.0. Limestone was also continuously supplied until the slag had a CaO content of 7% by weight. The material in lump form was supplied by a chute inserted through the working mouth of the converter. The material in powder form was supplied through the tuyeres or lances. 
     After blowing had been continued for a certain period of time, the converter was tilted to discharge the slag and the copper recovered therefrom. The speed at which the silicate ore was supplied and the oxygen content of the oxygen-enriched air which was blown through the tuyeres were controlled to maintain the slag at or near a temperature of 1200° C. to 1300° C. 
     The silicate ore contained 85.0% by weight of SiO 2 . It was either in the form of lumps having a size of 15 to 30 mm, or a powder containing 70% by weight of particles having a particle size not exceeding 200 mesh. The limestone was in the form of lumps having a size of 10 to 20 mm. 
     
                       TABLE 3______________________________________Cu        Fe       SiO.sub.2                       CaO    Au    Ag(wt. %)   (wt. %)  (wt. %)  (wt. %)                              (g/ton)                                    (g/ton)______________________________________Slag 4.5      45.0     20.0   1.0    0.2   10Sili---        1.0     85.0   --     20    10cateore______________________________________ 
    
     About 96 to 98% by weight of gold and silver were recovered from the ore. 
     FIG. 4 shows the results of silicate ore melting in relation to the melting time and the slag temperature. The black squares in FIG. 4 indicate that some lump silicate ore remained undissolved when the slag had an Fe/SiO 2  ratio of 1.0 and a CaO content of 7% by weight. The white squares indicate that no ore remained undissolved. The black triangles indicate that some lump silicate ore remained undissolved when the slag had an Fe/SiO 2  ratio of 0.8 and a CaO content of 7% by weight, while the white triangles indicate that no ore remained undissolved. The inverted white triangles indicate that no powdery silicate ore remained undissolved when the slag had an Fe/SiO 2  ratio of 1.0 and a CaO content of 7% by weight. As is obvious from FIG. 4, the lump ore can be melted to an improved degree if the slag has a temperature of 1200° C. to 1300° C. and an Fe/SiO 2  ratio of 1.0 when 7% by weight of CaO is added. The use of powdery ore produces still better results. 
     Table 4 shows how the addition of CaO and the control of the slag temperature are effective for preventing the slopping of the slag. In Examples 3-1 and 3-2, limestone was used and the speed at which the silicate ore was supplied and the oxygen content of the oxygen-enriched air were accurately controlled to maintain a slag temperature of 1200° C. to 1300° C. The amount of the material which was blown out through the working mouth of the converter was substantially reduced. This means that the slopping of the slag could be substantially prevented. 
     
                                           TABLE 4__________________________________________________________________________     Number           Converter                            Blown out     of   Lime-              Temperature                      slag (kg/                            material (wt.     charges          stone              control charge)                            % of slag)__________________________________________________________________________EXAMPLE3-1       17   Added              The slag temp.                      3124   7.0              was accurately              controlled to              1200° C. to              1300° C.3-2        6   Added              The slag temp.                      3900   5.1              was accurately              controlled to              1200° C. to              1300° C.COMPARATIVEEXAMPLE1         52   Not The slag temp.                      4195  41.0          added              was allowed to              vary until a              final level of              about 1100° C.2         27   Not The slag temp.                      2180  24.5          added              was allowed to              vary until a              final level of              1350° C. to              1400° C.3         21   Not The slag temp.                      3139  10.5          added              was accurately              controlled to              1250° C. to              1300° C.__________________________________________________________________________ 
    
     Table 5 shows how the addition of CaO and the control of the slag temperature are effective for preventing any serious erosion of the refractories for the converter. 
     
                                           TABLE 5__________________________________________________________________________                      Erosion of refractories     Number           (mm/charge)     of   Lime-              Temperature                      Gas                         Molten     charges          stone              control zone                         bath zone                              Tuyeres__________________________________________________________________________EXAMPLE3         129  Added              The slag temp.                      1.5                         1.0  1.5              was accurately                      to to   to              controlled to                      2.5                         1.6  1.6              1200° C. to              1300° C.COMPARATIVE      52  Not The slag temp.                      3.0                         1.7  1.7EXAMPLE        added              was allowed to                      to to   to              vary until a                      5.0                         3.0  2.3              final level              of 1300° C. or              above.__________________________________________________________________________ 
    
     In Example 3, limestone was added and the speed at which the silicate ore was supplied and the oxygen content of the oxygen-enriched air were precisely controlled to maintain a slag temperature of 1200° C. to 1300° C. The appropriate control of the slag temperature and the prevention of slopping enable a reduction in the erosion of the refractories in any portion of the converter. 
     COMPARATIVE EXAMPLE 
     The procedure of Example 3 was repeated unless otherwise stated. The slag temperature was controlled when required, or allowed to vary. Referring to FIG. 4, the black circles indicate that some lump silicate ore remained undissolved when the slag had an Fe/SiO 2  ratio of 1.0, while the white circles indicate that no ore remained undissolved. In either event, no limestone was added. The results shown in FIG. 4 teach that if no limestone is added, some ore is likely to remain undissolved unless a high slag temperature exceeding about 1300° C. and a long melting time of at least about 100 minutes are employed. 
     Referring again on Table 4, no limestone was added in Comparative Example 1, nor was any oxygen-enriched air used. The slag temperature was allowed to vary and finally dropped to about 1100° C. As a result, heavy slopping occurred. In Comparative Example 2, no limestone was added, but the oxygen content of oxygen-enriched air was held at a constant level. The silicate ore was supplied at a constant rate. The slag temperature was allowed to vary and reached as high as 1350° C. to 1400° C. When it exceeded 1300° C., heavy slopping occurred, and a large amount of molten material was blown out despite a reduction in the amount of the slag per charge. In Comparative Example 3, no limestone was added, but the oxygen content of the oxygen-enriched air and the speed at which the ore was supplied were controlled to maintain a slag temperature of 1250° C. to 1300° C. 
     Referring to the Comparative Example in Table 5, no limestone was added, and the slag temperature was allowed to vary until it finally reached a level exceeding 1300° C.