A "design-of-experiment" (DOE) approach was used to study the purification of a scheelite ore concentrate in HCl. The ore concentrate contained about 65% CaWO4, 18% CaCO3, 14% Ca-10(PO4)(6)(OH)(2,) and 3% SiO2. In addition to these major phases, the ore concentrate also contained trace amounts of toxic elements such as arsenic, lead, and uranium. These elements were associated with the apatitic part of the ore. The results of DOE experiments indicated that calcite can be selectively removed with a low HCl concentration, and the resulting calcium-containing solution may be used for other applications. Apatite can then be removed with a high concentration of HCl. This solution contains toxic elements such as arsenic, lead, and uranium because of the association of these elements with apatite. High temperature seemed to enhance the dissolution of unwanted tungsten (scheelite part of the ore concentrate), perhaps due to the formation of P- and As-containing heteropoly anions of W, such as PW12O403- and AsW12O403- (formation of heteropoly anions may be favored at high temperature).