Sodium tungstate preparation process

Description

The invention relates to a process for producing sodium tungstate by oxidative reaction of hard metal and / or heavy metal waste in molten salt. Processes developed for the processing of hard metal (WC-Co or WC-Co-TaC-TiC) and heavy metal waste (W-Cu-Ni-Fe) are described in numerous patents. Known processes include the oxidation of hard metal waste at higher temperatures (US-A 3,887,680), treatment with liquid zinc (US-A 3,595,484) or anodic dissolution of the binder (AT-A 380,495). These processes either require large equipment or can only be used for one type of waste. On an industrial scale, the reaction of carbide waste in molten salt is more typical. U.S. Patent No. 4,603,043 describes the processing of W-containing materials in a melt of a mixture of NaNO3 and NaOH at 500-700 °C. In order to better control the reaction, the sodium hydroxide and the W-containing component are first heated to 560-600 °C, and then the sodium nitrate required as an oxidizing agent is added over 2-3 hours. According to DE-A 314 495, the hard metal waste is also processed in an alkali metal hydroxide / alkali metal nitrate melt. The melt contains 40-80% by weight of alkali metal nitrate and 20-60% by weight of alkali metal hydroxide. In order to lower the melting point, 5% by weight of sodium chloride is also added. The melting point is 550 °C. The melt is poured into water. According to DD-A 207 932, the carbide waste is processed in a pure sodium nitrate or sodium nitrite melt. The stoichiometric excess of the alkali metal component is 10-15% by weight, the melt temperature is 900 °C. The reacted melt is cooled to room temperature and then dissolved in water. According to IN-A 157 146, a molten salt containing an alkali metal hydroxide, preferably potassium hydroxide or sodium hydroxide, and an alkali metal nitrate, preferably potassium nitrate or sodium nitrate as an oxidizing agent, is used for the processing of hard metal waste. The temperature of the melt is 350-460 °C. It has been found that at 440-460 °C the yield of tungstate can be increased to 99%. At this temperature, the reaction is difficult to control and nitrogen oxides are formed. The disadvantage of the reaction in nitrates or nitrites is that the highly exothermic reaction is difficult to control, and the entire reaction poses a safety risk. In addition, nitrogen oxides are formed in an uncontrollable manner. The sodium tungstate solution contains nitrate and nitrite residues, which makes processing - especially if the next step is solvent extraction - very difficult. The task of the present invention was to develop a method for processing hard metal and heavy metal waste, which method is free from the disadvantages of known methods. The above task was solved by processing tungsten-containing waste of various compositions by oxidizing it in a molten salt solution consisting of sodium hydroxide and sodium sulfate. The invention therefore relates to a process for producing sodium tungstate by oxidizing hard metal and / or heavy metal waste in molten salt. The process is characterized in that a molten salt containing 60-90% by weight of sodium hydroxide and 10-40% by weight of sodium sulfate is used. The reaction is preferably carried out in a stirred melt, particularly preferably in a directly heated rotary kiln operated in a batch mode. The inner lining of the rotary kiln against wear is preferably a refractory mass. The oxidation is preferably carried out by blowing air into the melt. The reaction temperature is preferably between 800 °C and 1100 °C. Particularly good digestion is achieved when the alkali metal components are present in an excess of 5 to 20% by weight. The method according to the invention is described in more detail below without any intention of limitation. After the melt has become liquid, the energy supply is stopped and the reaction is continued for 4-8 hours by rotating the furnace and blowing air in. The tungsten content of the W-containing waste is directly converted into sodium tungstate. No further energy input is required, the reaction can be controlled solely by the air supply. Once the reaction is complete, the Na2O4 melt is directly fed into water through special funnels. The sodium tungstate dissolves immediately and can be processed into tungsten carbide powder using standard processes. Other components of the starting material, such as Co, TiC, TaC, and Fe, Ni, Cu, remain in the residue and can also be recovered by known methods. One advantage of the process according to the invention is that the reaction of the NaOH / Na2SO4 melt is weakly exothermic. By introducing air, the entire reaction can be well controlled, i.e. it is problem-free from a safety point of view. An additional advantage is that the alkali metal components used do not have a negative impact on the further processing of the valuable components to be extracted. Finally, no toxic gases are released during the reaction according to the invention. The process according to the invention is illustrated below by examples, which are not to be considered as limiting. Example 1 1000 kg of lump carbide scrap, the composition of which is 84% ​​WC, 9% Co, 4% TaC, 3% TiC, together with 400 kg of sodium hydroxide and 150 kg of sodium sulfate, is fed into a rotary kiln lined with refractory mass, operated in batches, and the charge is heated to approximately 900 °C. After a six-hour reaction, the melt is poured into approximately 5 m3 of water and leached there. After filtration, the Co, Ti, Ta components are in the residue, while the tungsten has passed into the filtrate in the form of sodium tungstate. The filtrate is processed into ammonium paratungstate in a known manner. The exploration yield of tungsten is 86.3%. HU 220 255 B Example 2 The procedure is as in Example 1, except that 60 m3 of air per hour is passed through the melt during the six-hour melt reaction. The processing is carried out as in Example 1. The yield of the exploration is 89.5%. Example 3 1000 kg of lumpy hard metal scrap is mixed with 400 kg of sodium hydroxide and 250 kg of sodium sulfate in the Melt and process as described in Example 1. The recovery yield of tungsten is 91.5%. Example 4 100 kg of lump carbide scrap together with 400 kg of sodium hydroxide and 150 kg of sodium sulfate are processed according to Example 1, except that the melt is not agitated. The exploration yield of tungsten is 75.1%. The selected melt parameters and the extraction yield as a function of the Na2SO4 content are illustrated in Figure 1.

Claims

1. Process for the production of sodium tungstate by the oxidizing reaction of hard metal and / or heavy metal waste in a salt solution, characterized by the fact that a salt solution consisting of 60-90% by weight of sodium hydroxide and 10-40% by weight of sodium sulfate is used.

2. The method according to claim 1, characterized in that the reaction is carried out in a stirred melt.

3. The method according to claim 1 or 2, characterized in that the reaction is carried out in a batch-operated, directly heated rotary kiln.

4. The 1-3. Method according to any one of claims, characterized in that air is blown into the melt.

5. The 1-4. Method according to any one of claims, characterized in that the reaction is carried out at a temperature between 800 °C and 1100 °C.

6. The 1-5. Process according to any one of the claims, characterized in that the alkali metal components are used in an excess of 5-20% by weight.