Recycling of WC-Co scraps by sub-molten salt electrolysis

In this thesis, the aim is to develop an efficient, economical and environment friendly process for the recovery of WC-Co scraps. To achive this goal, the most effective methods tried to be determined after the literature studies and preliminary experiments were carried out. The most suitable method for WC-Co recovery was determined after preliminary experiments including aqueous system electrolysis, oxidation process, nitrate process and molten salt electrolysis. In the experiments using molten salt electrolysis with NaOH, KOH electrolytes, it has been found that the hydroxide compounds make the WC-Co brittle without dissolving the materials. The biggest problem in recycling of WC-Co scraps is that they cannot be grinded because the material is very hard. The dissolution of the hydroxyl compounds in the crystal water at a temperature below the melting temperature enables the scrap to be brittle by anodic oxidation. For this reason, the system has been called "sub-molten salt electrolysis" and the development of this system has been targeted. Sub-molten salt electrolysis is the anodic oxidation process of the scraps at a temperature below the melting temperature of the soluble compounds in the crystal water, such as NaOH, KOH. As a result of this process, scrap can be crushed and grinded. Sub-molten salt electrolysis experiments were carried out at 250, 325, 400 ºC, at current densities from 500 to 2500 A / m2 for 30-90 minutes using NaOH and KOH electrolytes. As a result of the parameter scan, milling in a ring mill pulverizer for 15 seconds and treating at 250 ºC for 60 minutes with current density of 1400A / m2 were determined as the optimum experimental conditions to obtain fully grindable samples and WC-Co powders with grain size below 125 μm were obtained. Experiments were carried out for acidic leaching of WC-Co powders at temperatures of 25-70 ºC, stirring speed of 360 rpm, for 1-120 minutes and solvent concentrations of 0.5-2 M H2SO4. Experiments showed that Co dissolution yield was 99.64% at the experimental conditions of 120 min, 0.5 M H2SO4, 360 rpm, 70 ºC and that the change in the final acid concentration did not affect the Co dissolution efficiency. By the acidic leaching process, all of the cobalt was separated from the solution and pure WC powders were obtained. Co (OH)2 precipitation was carried out at pH values of 10 and 25 ºC using NaOH. No further description could be made but the obtained Co (OH)2 powder is amorphous. With the sub-molten salt electrolysis system, the WC-Co scraps achieved to be made brittle and a new process has been developed which is the first in the world to overcome the biggest problem in scrap recovery. Experimental works have resulted in the recovery of pure WC powders and Co(OH)2 succesfully.