Tungsten Ores

Scheeelite (CaWO4) is the principal tungsten mineral in domestic ores, but other important tungsten minerals include wolframite (Fe, Mn) WO4, ferberite (FeWO4), and hubnerite (Mn WO4). All of these minerals may be floated in a soda ash alkaline pulp with fatty acid type collectors.

Because of the high specific gravity of the tungsten minerals, they are amenable to gravity concentration methods. However, the fine dissemination of the ore minerals in the gangue usually makes fine grinding necessary, and requires at least some flotation concentration for the recovery of the finer sizes. The trend has been toward all-flotation recovery with gravity processes being used only as a means of upgrading a part of the concentrates.

If the ore contains pyrite or other sulfides, a preliminary sulfide flotation stage is usually employed ahead of both tabling and flotation of the tungsten. The low cost and high collecting power of xanthates makes them the favored collector for this sulfide float. Soda ash or sulfuric acid is usually added for pH control. Acid is frequently preferred for those ores containing pyrrhotite. However, its use with calcite gangue ores will result in the formation of calcium sulfate, which may float with and dilute the subsequent tungsten flotation concentrate.

The flotation of tungsten is conducted at a pulp pH of 9.0-10.5. Soda ash, caustic soda, or sodium silicate may be used for alkalinity control. 

The other modifying agents required will depend on the gangue minerals to be depressed. Quebracho or tannin is used to depress the calcite, but an excess will also depress tungsten minerals. Sodium dichromate acidified with sulfuric acid has also been reported as a calcite depressant and copper silicate (sodium silicate and copper sulfate) has been reported as a depressant for fluorite. Figure 8 shows a tungsten flotation flowsheet. The most commonly used tungsten collectors are crude oleid acids or tall oils, or the more refined fatty acids or soaps.

Most tungsten flotation concentrates and some table concentrates are not of sufficiently high grade to meet the exacting specifications of the market. The principal diluents are phosphates and carbonates, which are difficult to depress sufficiently in flotation. The removal of these minerals is most frequently accomplished by leaching the concentrates with hydrochloric acid. Because of the relatively simple equipment required, the trend has been toward leaching at the flotation plant. However, a number of custom leaching plants are equipped to treat flotation concentrates from the smaller concentrators.

Figure 8 – Tungsten Ore Flowsheet