Recovery of fine cassiterite, down to 5 µm from gravity plant tailings, by flotation is now practiced successfully at a number of operations. Typically, the tailings from gravity concentration, after removal of the plus 45 µm material, are cycloned at high pressure in clusters of small diameter cyclones for removal of minus 5-7 µm slimes in preparation for flotation. If economically sufficient additional cassiterite can be liberated, the plus 45 µm portion of the gravity plant tailing is reground and combined with the minus 45 µm portion for cyclone treatment.
If sulfides are present, the deslimed fines are treated first in a flotation step with a strong collector, a xanthate or xanthate replacement such as AERO MX-3730 promoter, a frother and copper sulfate if required. The sulfide flotation tailing goes to the cassiterite flotation circuit for rougher flotation and usually several steps of recleaning with cleaner tails returning to the head of the rougher circuit. Concentrates produced assay in the range of 10% to 30% Sn with recoveries of 50% to 70% of the tin in this circuit's flotation feed. The first successful commercial operation, utilizing the process patented by Prof. N. Arbiter, used AERO 845 promoter ( 200 g/t of flotation feed), AEROFROTH 65 frother and sulfuric acid to pH 2-3. This process with some modifications is still in use.
In more recent years the arsonic and phosphonic acids have been tested successfully on more difficult ores to improve selectivity. Of these the styrene phosphonic acid is now commercial. Likewise, in recent technical literature references have been made to the use of alkyl hydroxamates in the flotation of cassiterite. Cytec offers AERO 6493 promoter and other alkyl hydroxamate formulations, which have shown in laboratory test work their effectiveness in floating cassiterite.
Modifiers and selective depressants have been evaluated and successfully introduced. Flotation is always carried out in acid circuit from pH 2 to 5 preadjusted with sulfuric acid. Where necessary, frothers such as AEROFROTH 65, OREPREP F-507, OREPREP F-549, OREPREP F-579, or OREPREP F-587 frothers can be used.
Selectivity is improved by the use of sodium silicate (500-1000 g/t) and sodium fluoride (20-500 g/t). Modifying and depressing agents are usually added to a five-minute conditioning step, followed by collector to the second conditioning step, where acid and frother are also added. Automatic pH control in rougher and cleaner circuits is highly desirable in this very sensitive operation.