Effect of extended grinding on the dissolution of a Ta/Nb concentrate

A tantalum/niobium concentrate was ball milled for up to 100 hours in a laboratory mill without any evidence of a phase change other than a decrease in unit cell symmetry. After 2 hours of milling, the dry milled powders had passed through their maximum BET surface areas and milling for longer times resulted in a decreased surface area. The surface area of wet milled powders continued to increase with milling time. These differences were attributed to the presence of water hindering, impact induced rewelding of the particles. The extent of dissolution in a mixture of hydrochloric acid and sodium fluoride increased with milling time until over 86% of the concentrate was solubilised in 24 hours. It was found that a powder milled for 50 hours dissolved 4500 times more rapidly than a powder milled for 2 hours. Both strong acid and high fluoride were required for the maximum rate. The dissolution was shown to be two stages: an initial, rapid solution diffusion controlled reaction followed by a slower chemically controlled step. The dissolution associated with the fast step increased with milling time.