Method of extracting Te and bismuth oxide and recovering byproducts

This application is the U.S. national stage of International Patent Application No. PCT/CN2009/072096, filed on Jun. 2, 2009 and entitled “Method of Extracting Te and Bismuth Oxide and Recovering Byproducts,” which claims the benefit of priority from Chinese Patent Application No. 200810044496.7, filed Jun. 2, 2008. The disclosures of the foregoing applications are incorporated herein by reference in their entirety.

The present invention relates to smelting processes for scattered elements, rare metals and noble metals, and more specifically, to processes of extracting highly pure tellurium directly from various raw materials containing tellurium by hydrometallurgical methods, extracting highly pure bismuth trioxide when the raw material comprises bismuth and collectively recovering other byproducts, which falls within the field of hydrometallurgy; while it substantially achieves no Three Wastes (waste gas, waste water and industrial residue) and zero emission, which falls within the field of environmental protection.

Tellurium is an essential elemental in modern science and belongs to scattered elements. At present, the extraction and recovery of tellurium is primarily achieved by collectively recovering from byproducts during nonferrous metal smelting processes, for example, recovering from anode slimes of copper and lead etc., or extracting from primary tellurium ores.

Tellurium may be incorporated into steels to increase ductility, and serve as brightener in electroplating solution, catalyst in petroleum cracking, coloring material for glass, and also may be added into lead to increase its strength and corrosion resistance. Highly pure tellurium may be used as an alloy component for thermoelectric materials; bismuth telluride is a good refrigeration material; semi-conducting compound As 32 Te 48 Si 2 O is a material for manufacturing storage devices of computer; ultrapure single crystal tellurium is a novel infrared material. Highly pure tellurium plays important roles although little is used. Tellurium is also one of the best primary materials for solar cell, which has the largest conversion coefficient, relatively low cost and best benefit. Cost is essential for popularization of solar energy, the eternal energy source, and it is thus critical to obtain advancement in the process technology of tellurium extraction, minimize the cost, and achieve substantially no Three Wastes, zero emission and best product quality in practical production.

There have been various reports and patents concerning extracting tellurium from tetradymites or other Te-containing raw materials with wet process, and lots of works have been done by the experts. For example, Chinese patent application No. 99111440.X entitled “Process of Extracting Fine Tellurium from Tellurium Polymetallic Ores” discloses a process of extracting tellurium with wet process, in which hydrochloric acid is used for leaching, the leach solution is reduced by sulfur dioxide gas and then precipitated to give crude tellurium powders which subsequently react with oxidants in the hydrochloric acid solution to yield an intermediate product, TeO 2 , and fine tellurium product is finally collected upon electrolysis. Although fine tellurium with 99.99% purity can be obtained after electrolysis employing the hydrometallurgical method disclosed by this application, the whole process has relatively high production cost, high energy consumption, low recovery rate and low yields, wherein serious environmental problems such as large amount of waste gases and waste water occurs and other byproducts are not comprehensively utilized.

In another case, Chinese patent application No. 93115389.1 entitled “A Production Process of Extracting Fine Tellurium from Primary Tellurium Ores by Wet Process” discloses a method of producing fine tellurium from primary tellurium ores, which comprises leaching tellurium and bismuth by high temperature oxidation in hydrochloric acid medium, reducing by sulfur dioxide to give crude tellurium powder, and obtaining fine tellurium through chemical purification and electrolytic purification, and further obtaining an intermediate bismuth-containing product upon hydrolysis of the neutralized bismuth-containing reduction solution. This method requires external heating to 80° C. or more, which doubtlessly increases cost, and employs an electrolysis process leading to issues such as high energy consumption. In addition, the waste solution and waste residues are not comprehensively utilized during the production process whereby serious environmental problems and resource wasting issues emerges.

In bismuth recovery from tetradymites by hydrometallurgical methods, in general, iron is added into the acidic solution after recovering tellurium for reduction, yielding primary spongy bismuth product of low purity. The spongy bismuth produced by this method has low values, high cost and poor economical benefits. It has not been reported to directly produce 99.99% Bi 2 O 3 from tetradymites.

With regards to the Three Wastes concerning environmental protection, less waste gases and residues are produced in hydrometallurgical methods than in pyrometallurgy methods, whilst much more waste water is produced, which is a serious environmental issue for hydrometallurgy.

The present invention is directed to certain drawbacks and problems present in hydrometallurgy of the existing technologies, such as low leach rate, low recovery rate, relatively high production cost, high energy consumption, low product quality; serious environmental problems such as large amount of waste gases and waste water, as well as disadvantages as inability to achieve comprehensive exploitation of waste residues; and the electrolysis employed during isolation and purification which causes issues like high energy consumption, low yield and high cost etc. The present invention proposes a new method with high recovery rate, low cost, low energy consumption, high quality, collective recovery and substantially no Three Wastes and zero emission. The method employed in the present invention to recycle the waste water and recover in closed circuit is the first successful practice, hence a milestone, in the history of environment protection in hydrometallurgy.

Referring to the illustration in FIG. 1 , a method for direct extraction of highly pure tellurium from raw materials by wet process is proposed in the present invention. More particularly, the hydrometallurgical method of extracting tellurium according to the present invention comprises steps of material selecting, leaching the selected material, and reducing and purifying the leach solution. In this method, the material selecting step includes: selecting raw material, usually mineral raw material, with tellurium content ≧1.8%, e.g., fine mineral powder with tellurium content ≧1.8% and 80 mesh or above, or anode slime with tellurium content ≧1.8%, and/or other raw materials with tellurium content ≧1.8%, and placing the selected raw material into a leaching trough; the leaching system comprises H 2 SO 4 , Cl − , Br − , NH 4 + and NaClO 3 . The detailed leaching step includes to the leaching trough in this order: (1) adding a solution and/or solid containing Cl − , Br − and NH 4 + at a ratio of Cl − ≧115 kg, Br≧16 kg and NH 4 + ≧62 kg per ton of the raw material, with the liquid-solid ratio between the liquid and raw material in the leaching trough ≧1.5, and leaching at room temperature for 1 to 3 hours, (2) then adding concentrated H 2 SO 4 in an amount of 500 to 1000 kg per ton of raw material and leaching for 1 to 3 hours, wherein the leaching temperature may rise spontaneously to 45-65° C. due to the heat released by concentrated sulfuric acid, and the residual acidity of the solution after leaching upon addition of concentrated H 2 SO 4 is required to be 0.5 to 1.5 N, (3) further adding NaClO 3 in an amount of 25 to 50 kg of NaClO 3 per ton of raw material, leaching under stirring for 2 hours or more, wherein oxidation reaction occurs upon addition of NaClO 3 while heat is released, which leads to an spontaneous increase of temperature up to 75-95° C.; filtering and washing, and then collecting leach solution. Using the method of the present invention, the washing residue (leaching residue) has a content of Te≦0.035% and can be used as raw material for cement or cement bricks; the collected leach solution is reduced and purified with existing techniques so as to give tellurium with purity of 99.99% or more.

The collected leach solution mentioned above may also be used to produce highly pure product with the following method:

impurity separation by precipitation process: the leach solution is adjusted to pH 2.7 to 3.1 with bases, filtered and washed to obtain waste solution A and a precipitate, wherein the waste solution A comprises H + , SO 4 2− , NH 4 + , Cl − , Br − and other ions leached out from the raw material, and Te remains in the precipitate; the precipitate with hydrochloric acid, filter and collect filtrate which is reduced and purified with existing technologies to yield tellurium with purity of 99.99% or more.

In the present invention, in addition to employ the existing reduction and purification method for the filtrate, the following method may be used for reduction and purification to obtain tellurium with purity of 99.99% or more:

reduction: sulfite or SO 2 gas is introduced into the collected filtrate for reduction until Te≦0.03 g/l, tellurium is precipitated, filtered and washed to obtain a tellurium-containing precipitate and filtrate B;

purification: concentrated HCl is added at a liquid-solid ratio ≧1:1 into the above tellurium-containing precipitate for washing, wherein other impurities in the washed tellurium precipitate may be leached by concentrated HCl since tellurium is not soluble in HCl, a tellurium precipitate and waste solution C are obtained by filtering and washing, the tellurium precipitate is washed with pure water until pH>3, filtered to obtain tellurium precipitate and waste solution D, and the tellurium precipitate is baked to yield fine tellurium powder with purity of 99.99% or more which is sintered at 400 to 500° C. to give block tellurium product with purity of 99.99% or more.

According to the preferred specific embodiment of the present invention, in the leaching step, the solution and/or solid containing Cl − , Br − and NH 4 + is a solution and/or solid of NH 4 Br and NH 4 Cl, and Cl − , Br − and NH 4 + are complexing and catalytic leaching agents; the leach solution in the impurity separation step of the precipitation process is adjusted to pH 2.9 with bases; said bases are NaOH and/or NH 3 ; the sulfite in the reduction step is Na 2 SO 3 .

The waste solution A, waste solution C and/or waste solution D produced in the above steps are returned to the step (1) of the leaching step for leaching, and it will not be necessary to add further solution and/or solid containing Cl − , Br − and NH 4 + which can be completely replaced by the waste solutions for leaching when the amount of Cl − , Br − and NH 4 + in the waste solutions satisfies that Cl − ≧115 kg, Br − ≧16 kg and NH 4 + ≧62 kg for each ton of raw material. In addition, the concentration of Cl − , Br − and NH 4 + in the waste solutions increases as they are returned for leaching for more and more times, that is, the complexing and catalytic leaching effect is getting better, leading to a continuous decrease of leaching time and continuous increase in leach rate, and the tellurium content in the leached residue may be less than 0.02% after leaching with the waste solution for multiple times.

When the bismuth content is ≧2% in the raw material, filtrate B is collected in the above reduction step, and existing method can be used to extract Bi 2 O 3 , for example: the filtrate B is adjusted with base added to pH 8-9 and heated to 80-90° C. so as to convert the precipitate to a yellow Bi 2 O 3 precipitate, which is filtered and washed to obtain Bi 2 O 3 precipitate, said precipitate is then baked to yield Bi 2 O 3 with purity of 99.99% or more. In addition, a new method is employed in the present invention for extraction of Bi 2 O 3 : firstly the filtrate B is adjusted with bases to pH 2.7-3.1, preferably pH 2.9, while bismuth is precipitated in the forms of BiO(OH), Bi(OH) 3 and BiOCl, filtered and washed to obtain bismuth-containing precipitate and waste solution E which contains impurities; the bismuth-containing precipitate is transferred into an agitator with a pure water:solid ratio ≧1:1 and blended with bases added therein under stirring to adjust pH to 8-9, heated to 80-90° C. so as to convert the precipitate to a yellow Bi 2 O 3 precipitate, which is filtered and washed to obtain waste solution F and Bi 2 O 3 precipitate, the precipitate is then baked to yield Bi 2 O 3 with purity of 99.99% or more; other conventional methods may also be used to convert bismuth-containing precipitate for Bi 2 O 3 purification.

Waste solution E may be returned to the step (1) of the leaching step of tellurium extraction for use. When the bismuth content is <2% in the raw material, filtrate B can be used the same as waste solution A, waste solution C, waste solution D and waste solution E to return to the step (1) of the leaching step for leaching. Said bases are NaOH and/or NH 3 ; waste solution F is basic, which may be used as a base in the process for Bi 2 O 3 purification to adjust pH to 2.7-3.1, or may be returned to the process of tellurium extraction to adjust pH to 2.7-3.1, preferably pH 2.9.

The present invention also provides a method for collective recovery of byproducts, wherein, as the waste solution being returned and used for leaching for multiple cycles, the concentration of the metals with trivial amount in the leach solution keeps increasing until reaching the recovery condition, and then the metals are recovered. More specifically, the method of collective recovery of byproducts according to the present invention includes treatment of the waste solutions from the hydrometallurgical process for tellurium extraction and/or from the hydrometallurgical process for bismuth trioxide extraction. In the present invention, as the returned waste solutions, partially or fully replacing the solution and/or solid containing Cl − , Br − and NH 4 + , are used for leaching for multiple cycles, the concentration of other scarce noble metals such as Au, Ag, Pt, Rh, Pd, Co, Ni, Sn, Cu, Se included in the raw material continuously increases, and recovery may be carried out by conventional methods when the collective recovery condition is arrived; further, when the bismuth content in the raw material is <2%, bismuth is collectively recovered together with these scarce noble metals after leaching for multiple cycles. In addition, Na 2 SO 4 .10H 2 O or (NH 4 ) 2 SO 4 crystals will precipitate from the waste water (waste solution) during recycling and may be recovered as byproducts. Namely, the present invention also provides a method for collective recovery of byproducts, wherein the Na 2 SO 4 .10H 2 O or (NH 4 ) 2 SO 4 crystals precipitated from waste solution during recycling are recovered. Specifically, the waste solution is the waste solution produced during the hydrometallurgical process for tellurium extraction and/or during the hydrometallurgical process for bismuth trioxide extraction of the present invention.

The present method may be used for production and recovery for all tellurium-containing raw materials to give products in the form of fine tellurium powders or elementary block products with purity of 99.99% or more; when Bi in the raw material is ≧2%, it may be used for production and recovery of Bi 2 O 3 products with purity of 99.99% or more; and it may also be used for collective recovery of other products or crude products.

Since elementary substances or compounds of scattered, rare elements like tellurium and bismuth are all substantially soluble in the H 2 SO 4 +Cl − +Br − +NH 4 + +NaClO 3 system, the present invention employs a leaching system containing H 2 SO 4 , Cl − , Br − , NH 4 + and NaClO 3 , and integrates acidic oxidation leaching, complexation leaching and catalytic leaching into a comprehensive leaching method. In the first leaching process, after adding the solution and/or solid containing Cl − , Br − and NH 4 + , waste solution can be recycled in the subsequent production to partially or fully replacing the solution and/or solid containing Cl − , Br − and NH 4 + for leaching, which saves the cost and is beneficial for environment protection as almost no waste solution is discharged; furthermore, the concentration of Cl − , Br − and NH 4 + in the waste solution increases as the times of leaching increase, and the complexing leaching effect and catalytic leaching effect of these ions causes the leaching time with waste solution to decreases and leach rate to increases, while the leach rate of other metals increases as well. It has been demonstrated in experiments that the leach rate of tellurium and bismuth may be up to 99.5% and above and the leach rate of other scarce noble metals may be up to 99% after multiple recycling of waste solution, such high leach rate has never been reported previously. Concentrated sulfuric acid is added into the leaching system and spontaneously releases heat, thus allowing the temperature during the leaching process to rise to the desired temperature without heating; further, sodium chlorate is used for oxidative leaching after concentrated sulfuric acid leaching, and leaching temperature further increases and can achieve the desired temperature without heating due to the participation of sodium chlorate in the exothermic oxidation reaction, thereby the higher leaching rate can be achieved without too higher temperature, which saves the cost and shortens the process. Fine tellurium powder with purity of 99.99% or more can be obtained after baking, or block-shaped tellurium may be produced after sintering at 400-500° C. Abandoning expensive electrolysis method and simplifying the hydrometallurgical procedure for tellurium, the present method is more convenient, effective and energy-saving while improving the quality of the product.

In the present invention, the leach solution is adjusted during the process of tellurium purification to pH 2.7-3.1, which step is sufficiently utilized so that many impurities, such as Fe, Cu, Se, Mg, Al, Si, Co, Ni, Cl − , SO 4 2− , NH 4 + , may be separated and precipitated as BiO(OH), Bi(OH) 3 , BiOCl and H 2 TeO 3 , Te(OH) 4 , the precipitates are then dissolved by concentrated HCl and the isolated tellurium is reduced by sulfite or SO 2 , the resulting acidic solution, as a bismuth-containing solution, is subjected to pH adjustment, and then filtered and washed followed by baking to give Bi 2 O 3 with purity of 99.99% or more. Such method produces high quality Bi 2 O 3 product with high quality, and such method has low cost, good operability and short procedure, and is an economic and efficient hydrometallurgical method for Bi 2 O 3 recovery.

Some scarce noble metals, such as Au, Ag, Pt, Rh, Pd, Co, Ni, Sn, Cu, have low leach rate in HCl+NaClO 3 medium, generally at about 60%. However, it has been demonstrated in experiments that up to 99% of leaching may be achieved during the waste water recycling employed in the present process. Although the content of these metals is low, the concentration thereof continue to increase as the waste solution in which they present are recycling for multiple times, and the metals can be recovered using existing recovery method when the condition for collective recovery is achieved; washed residue may be used as raw material for cement or cement bricks etc. after it is washed and passes the test; Na 2 SO 4 .10H 2 O or (NH 4 ) 2 SO 4 crystals that may precipitate during the recycling of waste water may be recovered and used as other chemical raw materials. As seen, the process method provided by the present invention has simple procedure, low cost, high quality, strong adaptability, recover in closed circuit, and also achieves substantially no Three Wastes and zero emission in the hydrometallurgical process, and is beneficial for environment protection.

Based on the idea and practice of hydrometallurgy with no Three Wastes and zero emission proposed in the present invention, no emission of exhaust gases is essentially achieved after the small amount of exhaust gas produced during the production procedure and that from the factory building are absorbed by bases and acids; waste residues may be used as raw material for cement bricks or cement production after multiple washes and reaching the standard; waste water is recycled in closed circuit, the increased amount of water during the production process is entrained in waste residues and crystallizing water for the byproduct Na 2 SO 4 .10H 2 O, while the entrained water in the product may also be recycled during the baking process. Hence, it is practically proved that material balance can be essentially achieved.

With regards to Three Wastes that are concerned in environmental protection, hydrometallurgy process generates less waste gas and waste residues than pyrometallurgy, but large amount of waste water is produced, which is the major environmental problem present in hydrometallurgy. One of the successes of the present invention is in that waste water is recycled and recovered in closed circuit, which is the first successful practice, hence a milestone, in the history of environment protection in hydrometallurgy. It has been discovered in the present invention that the concentrations of complex ions like Cl − , Br − and NH 4 + in the waste water keep increasing as recycling times increase, which results into a more prominent effect of complexing leaching and catalytic leaching. A theory for waste water recycling is proposed in the invention as follows: many ions and compounds with functions of complexation, catalysis and oxidation-reduction exist in the waste water in the hydrometallurgical process, and they can be repeatedly used for leaching, while reinforcing the leaching effect, improving leach rate, shortening leach time, and saving the amount of leaching agents used. This is the theory for waster water returning and recycling, such theory needs to be further testified in more hydrometallurgical projects, so as to determine whether it is adaptable for all hydrometallurgical systems and whether it can become a principle in hydrometallurgy.

FIG. 1 is a flowchart of the hydrometallurgical process for extracting tellurium and bismuth trioxide.

The present invention is described below in more details in connection with Examples. In the following six examples, although the leaching conditions are different from each other, a leach rate of 99.5% Te and 99.5% Bi is reached after leaching completely with waste water in all examples, which clearly demonstrate the advantage of leaching integrating acidic oxidation leaching, complexing leaching and catalytic leaching in the present invention; the recycled waste water is substantially balanced. The concentration of Cl − in the waste water increase with the increase of recycling times, and excellent leaching effect has been demonstrated by data of multiple leaching examples. When the concentration of Cl − increases to the saturated concentrations of its compounds such as NaCl and FeCl 2 , they will precipitate and may subsequently enter the collective recovery procedure; whereas, the amount of NH 4 + and Br − will be somehow consumed, and needs to be supplemented when the data of NH 4 +and Br analyzed and controlled in production drops. The content of individual components in the raw material is in percent by weight. In the present invention, the liquid-solid ratio is based on the convention in industrial production in the art, wherein the dimension for liquid is volume in m 3 , and the dimension for solid is weight in T.