Research on Key Technologies of Recycling and Utilization of Coated WC-Co Hardmetal Scrap

Author:Zuo Hai

Supervisor:tan dun qiang


Degree Year:2019





Coated cemented carbide has been widely used for its good wear resistance,oxidation resistance and corrosion resistance.However,in the recycling process of coated cemented carbide,the interaction of coatings with the matrix and added substances affects the recovery process.Different from the hydrometallurgical recovery of waste cemented carbide to prepare ammonium paratungstate(APT)raw materials,coated WC-Co cemented carbides were directly recycled and utilized by coating stripping,the zinc process,electrolysis and the oxidation and reduction-carbon process in this paper.The influence of coatings on recycling process parameters,reclaimed powders and alloy products was analyzed.The mechanism of coated WC-Co cemented carbide was studied.The main results are as follows:1.TiAlN single coating and TiCN/Al2O3/TiN multi-coating were successfully stripped by chemical method in the solution with hydrogen peroxide and potassium pyrophosphate as the main components.The TiAlN single coating can be completely removed within 6 hours in the solution with a pH of 8 and a temperature of 55℃.The alumina coating can be removed within 76 hours in the solution with a pH of 9 and a temperature of 40℃,and then the loss of matrix is relatively small.This provides technical support for the reuse of the re-deposition new coating on it after stripping the waste blade coating,whose matrix remains intact but has defects on coating during the preparation of coated cemented carbide.2.Reclaimed WC powders are obtained by electrolytically recycling of waste coated cemented carbide using hydrochloric acid as an electrolyte.In the recycling process,the coating can isolate the solution from the substrate and delay the electrolysis reaction.The passivation phenomenon is more likely to occur,leading to the reduction of electrolytic efficiency and resulting in low production efficiency.This may be due to the fact that the cobalt content in the coated cemented carbide is generally less than 10%and the electrolysis recycling method is not suitable for the waste coated cemented carbide.3.The waste TiAlN single-coated and TiCN/Al2O3/TiN multi-coated cemented carbides are melted at 880°C by zinc melting method,and then reclaimed WC powders are successfully obtained by processes of dissolving with hydrochloric acid,separating and washing.During the zinc process,the coating isolates the molten zinc from the matrix,making it difficult for zinc to penetrate the coating and the matrix,which results in slow recycling progress.Thermal stress and the impact effect of molten zinc are the main causes of formation and propagation of defects,which provide fast channels for molten zinc to enter the matrix.The performance of regenerated cemented carbide can be improved by physical cleaning,adding rare earth elements and adding native WC powder in proportion.The bending strength of YG6 cemented carbide with adding 0.4%rare earth can be improved by 14.33%.The bending strength of regenerated YG10 cemented carbide,prepared by mixing and matching the reclaimed WC with the original WC powder in a ratio of 3:7,is close to that of the alloy prepared with the original WC powders and its hardness was slightly improved.4.The coated cemented carbide scrap is recycled by the oxidation and reduction-carbon process,and the effect of the coating on the regenerated alloy is reduced by the high temperature reaction.During the oxidation process,the coating impedes the inward diffusion of oxygen,resulting in a lower oxidation rate and requirement of a higher oxidation temperature.More WC-Co interfaces can directly contact with oxygen after waste coated cemented carbides are broken into powders below 0.15 mm,which can be completely oxidized to a mixture of WO3 and CoWO4under the condition of 900°C for 3 hours.Defects provide a fast channel for the element diffusion.The main causes of defects generation and propagation are thermal stress,porous oxides,and gas volatilization.After complete oxidation,the TiCN/Al2O3/TiN multi-coating is transformed into TiO2/Al2O3/TiO2,and the oxide film is broken down or even exfoliates under thermal stress.The TiAlN single coating is transformed into a three-layer structure with the WO3-concentrating outmost layer rich,the Al2O3-concentrating intermediate layer and the TiO2-concentrating innermost layer.The containing cobalt tungsten powder is obtained by the reduction process at850°C for 3 hours under hydrogen atmosphere,which is carbonized at 1450°C into a WC-Co composite powder.The performance of the regenerated cemented carbide can be improved by adding the original WC powder in proportion.The YG10 cemented carbide prepared by mixing the recycled material with the original WC powder at 3:7has a slightly higher bending strength than that of the alloy prepared with the original WC powder.