Current situation of recycling technology of the h

2022-09-22
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Status quo of cemented carbide recycling process

since the advent of cemented carbide, recycling has been concerned by the industry. As tungsten carbide and rare metal cobalt are the main raw materials of cemented carbide, their economic value and manufacturing cost are relatively high. The recovery of tungsten and cobalt is a very valuable recovery field. Since the 1950s, some recycling processes have been developed and applied to the actual production process. The earliest recycling process has high energy consumption, complex equipment and great impact on the environment. Cemented carbide has very large hardness and high density, which is difficult to be dissolved by some inorganic acids and bases at room temperature. Therefore, it takes a lot of trouble on how to recover cemented carbide. According to the information we have, the existing recycling processes mainly fall into several categories. First, the so-called high-temperature treatment methods, including: nitrate melting method, air oxidation sintering method, oxygen calcination method, etc; The second is mechanical crushing method, including cold crushing method, hot crushing method, zinc melting method, etc; The third is chemical treatment, including metal multivalent salt treatment, chlorination, phosphoric acid leaching, hydrochloric acid treatment, etc; IV. the further optimization of the enterprise innovation environment is the electrochemical method, which has different process routes with alkali as the dielectric, hydrochloric acid or sulfuric acid, nitric acid as the dielectric; There are also methods of high-pressure oxygen and ammonia or amine solution leaching; Carbonyl compound method and decomposition method of tungsten trioxide by steam sublimation, etc

in recent years, in the practice of cemented carbide recycling, due to the increasingly strict requirements for environmental protection, some recycling processes will be discontinued due to pollution. At present, mechanical crushing method, zinc melting method and electrochemical selective electrodissolution method are widely used. The hard phase tungsten carbide and bonding phase cobalt of cemented carbide are sintered at a certain temperature to form the microstructure of powder metallurgy. How to decompose the dense and hard alloy structure and re separate these hard phases from the bonding metal is the first and key step to be solved in the recycling process. For the disintegration of cemented carbide, many researchers have adopted different ideas, and the recycling process routes are also different. For the evaluation of these processes, it is difficult to choose a more reasonable, more economical and more worthy of promotion and application, because the first and basic principle of the selection of process route is that the quality of recycled products should be high, the process flow should be simple, there will be no secondary pollution to the environment, and the working conditions should be clean and safe. Now several commonly used recycling processes are briefly introduced

I. high temperature treatment method

cemented carbide is sintered by protective gas at a certain temperature. If the alloy is heated in a protective atmosphere above the sintering temperature, the volume of the cemented carbide will expand, and cobalt, as a bonding metal, will liquefy and boil. The volume of the alloy will become loose and porous, and the hard alloy will become very easy to be broken and processed. After crushing and grinding, the same mixture of tungsten carbide and bonding metal as the original cemented carbide can be obtained. The principle of high-temperature treatment method is to use a special high-temperature furnace to disassemble the station metal from the alloy structure at a sintering temperature far greater than that of cemented carbide (1800 ℃). Due to the high temperature treatment of the cemented carbide regenerated raw materials obtained by this process, the trace other metal and non-metallic impurities and harmful gases originally contained are removed, and the heating time is longer. Tungsten carbide grains are obviously longer and coarser, intragranular defects are reduced, and the structure and properties of the alloy are also improved, so it has better mechanical properties and longer service life. This kind of recycled mixture is suitable for reproducing cemented carbide with coarser grain and higher cobalt content. For cemented carbide with fine grains and low cobalt content, not only the temperature should be increased during high-temperature treatment, so as to make the cemented carbide waste have enough stress to produce expansion and porosity, but also the preparation and sintering process of mixture should be changed correspondingly when preparing cemented carbide with medium and fine grains. The high temperature treatment method has the characteristics of short process flow, simple equipment matching, relatively clean recycled cemented carbide mixture, low environmental pollution and high recovery rate. However, this process consumes high energy, and a part of cobalt will be lost in the high temperature process. The biggest problem is that the recycled mixture is only suitable for making coarse grain carbide alloy. At present, some manufacturers in some industrialized countries such as Japan and Sweden still use this method to treat waste cemented carbide

II. Crushing method

for some cemented carbides with low cobalt content, due to their relatively low hardness, they can be crushed manually or mechanically to a certain fineness and then loaded into a wet mill for grinding for a period of time, reaching a certain particle size for remanufacturing cemented carbides. This method has the advantages of simple process, short process, low energy consumption and no pollution to the environment. However, when the cemented carbide is broken manually, the metal material debris of the tool will be brought into the broken material to cause pollution. In addition, because the cemented carbide with high cobalt content is not easy to break, the mechanical crushing method is greatly limited; It is also difficult to ensure the quality of recycled products with this method for cemented carbide mixtures with complex components. The technological process of the crushing method is: manual crushing, crushing it into powder (about 200 mesh) or using a ball mill with large cemented carbide as the hitting ball, then adding alcohol into the octagonal ball mill for wet grinding, and then entering the cemented carbide reproduction process. Some enterprises use quenching method to crush: first heat the waste cemented carbide in muffle furnace to above 800 ℃ and immediately put it into water for quenching, causing the cemented carbide to crack, and then enter the mechanical crushing process. This method was popularized in Qinghe and other places in Hebei Province in the 1990s. There are dozens of recycling plants of different sizes in the county that use this method to recycle and remanufacture cemented carbide. The annual output of remanufactured cemented carbide is more than 1000 tons, with a total output value of more than 300 million yuan, making it one of the local pillar industries. At present, there is still a certain development space for the crushing method. The crushing method still needs to be improved by using relatively advanced and clean crushing equipment or by using efficient methods that do not damage the microstructure of cemented carbide

III. zinc melting treatment of cemented carbide

basic principle of zinc melting method

the mechanism of zinc melting treatment of cemented carbide is based on the fact that zinc and bonding phase metals (cobalt, nickel) in cemented carbide can form low melting point alloys, so that bonding metals can be separated from cemented carbide and form zinc with zinc. This project breaks through reverse osmosis membrane, nanofiltration membrane Membrane bioreactor membrane and water purification membrane and other membrane materials have been prepared in a wide range, and a production line - cobalt solid solution alloy solution has been built, which destroys the structure of cemented carbide and turns the dense alloy into a loose hard phase skeleton. Because zinc will not react with carbides of various refractory alloy metals, the vapor pressure of zinc is far greater than that of cobalt at a certain temperature, so that zinc can be evaporated for recycling. Therefore, the carbide powder obtained by zinc melting method retains its original characteristics. After the zinc melting process, cobalt or nickel is extracted into the zinc melt. After zinc distillation, cobalt and carbides are retained. After zinc recovery, it continues to be used in the regeneration process. Zinc melting process flow

waste cemented carbide and zinc block are put into the sintering melting crucible together in the proportion of 1:1 ~ 2, vacuumized, electrified and heated to 900 ~ 1000 ℃, vacuum extracted zinc after holding for a certain time, cooled, and then unloaded the spongy cobalt powder and tungsten carbide block. After ball milling, crushing, adjusting the alloy composition, the cemented carbide is made again

The main characteristics of zinc melting method

zinc melting method was invented by the British in the 1950s. Later, the United States improved the process and equipment. After the 1970s, it was popularized in many countries. In our country, many manufacturers who recycle waste hard alloy have mastered this method. The main advantages of this method are simple process, short process, simple equipment, small investment and low cost. It is especially suitable for processing waste cemented carbide with cobalt content less than 10%, and suitable for small enterprises to use waste cemented carbide to produce cemented carbide. However, this process also has some disadvantages: the high residual zinc content in the mixture is a problem worthy of attention; In recent years, in order to save the amount of cobalt, most of the new cemented carbides are titanium carbide tungsten carbide cobalt series alloys. If the waste cannot be sorted clearly, the recycled mixture will contain a certain amount of titanium, which will limit the selection of recycled products. The increase of titanium will increase the brittleness of the alloy and have a certain impact on the service life of the products; In addition, the power consumption in the whole process is large. The power consumption per ton of cemented carbide is about 12000kwh high and above 6000kwh low; In addition, in the process of zinc melting and zinc collection, whether the equipment is reasonable has a certain impact on the recovery efficiency of zinc. Another is environmental protection. The escape of zinc will have a certain impact on operators

IV. selective electrochemical dissolution method

in the early 1980s. The Institute of material recycling and utilization of the Ministry of domestic trade has launched the selective electrochemical dissolution method (hereinafter referred to as the electro dissolution method), and has successively carried out technical promotion and application in Linqu, Shandong, Qinghe, Hebei and other places, and achieved good economic and social benefits. The promotion of this method has played a great role in the recycling and utilization of cemented carbide. Many cemented carbide recycling and remanufactured products enterprises have taken this process as the starting point of the cycle, forming a complete process of waste cemented carbide recycling → alloy disassembly process → cobalt powder and carbides re batching → remanufactured cemented carbide. For some cemented carbides with low cobalt content, the methods of high temperature treatment or pre crushing and rotating drum anode are adopted, and the application scope of selective electrodissolution method is further expanded

basic principle of selective electrodissolution method

the selective electrodissolution method of waste cemented carbide is a lightweight way of "replacing steel with plastic". The old alloy is used as an anode and placed in the dielectric hydrochloric acid solution. Under a certain selective interelectrode voltage, direct current is applied, and the scrap of waste cemented carbide is under the action of direct current electric field, The bonding phase metal cobalt is oxidized on the anode into divalent cations and enters the solution to combine with nitrogen ions to form cobalt nitride solution. The tungsten carbide in the alloy gradually separates from the alloy body and remains in the anode groove or sinks into the bottom of the dielectric liquid in the form of solid particles or sheets. By recovering cobalt in the dielectric and tungsten carbide in the solid phase, the cemented carbide can be reproduced. The reason why this method is called selective electrolysis rather than electrolysis is that the principle of electrolysis is that the metal cations on the anode are transferred to the cathode, and the electrons obtained on the cathode are reduced and deposited as metal substances. In the electrolysis process, other metals other than the main metal are usually not expected to precipitate on the cathode; The selective electrodissolution method hopes that the main metal will remain in the dielectric, and other impurity metals such as iron, copper, nickel, hydrogen will precipitate on the cathode, so that the cobalt solution can be purified. The process characteristics of selective electrolysis are also here. The technological process has strict requirements on the hydrochloric acid concentration, current density, cell voltage, terminal voltage, solution temperature and dielectric flow state of the dielectric. If the control is improper, chlorine and oxygen will separate out on the anode

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