Sulfurous acid and its salts are less effective at inhibiting cyanide than cyanide, but they have the advantages of being non-toxic, insoluble in precious metals, and easily reactivating minerals inhibited by them. They also have weak inhibitory power on copper sulfide minerals and are beneficial for copper-zinc separation, thus they are frequently used in production. However, the following precautions should be taken to ensure the inhibitory effect:
(1) Sulfurous acid and its salts are highly sensitive to pH; therefore, the pH of the slurry must be strictly controlled. A pH of 5–7 provides good inhibition of sphalerite and pyrite. A pH of 4 inhibits copper, lead, and zinc sulfide minerals. A pH of 8 significantly reduces the inhibitory effect on sphalerite. Therefore, the pH range of the slurry must be strictly controlled when using these agents.
(2) Strictly control the dosage. Small dosages result in weak inhibition; large dosages inhibit not only sphalerite and pyrite but also galena and chalcopyrite. The appropriate dosage for a particular ore needs to be determined experimentally.
(3) Strictly control the slurry conditioning time. Sulphite and its salts are easily oxidized; if the conditioning time is too long, they will be oxidized to sulfates and become ineffective. However, the conditioning time should not be too short either, as the inhibitory effect will be insufficient. In the production field, to prevent oxidation, a staged addition method is generally adopted.
(4) To improve their inhibitory effect, they can be used in combination with other agents. Lime can be used to inhibit pyrite, and zinc sulfate or sodium sulfide can be used to inhibit sphalerite.
(5) When using sulfite and sulfur dioxide gas, the slurry is acidic, and xanthate is easily decomposed. Use collectors that are not easily decomposed in acidic slurries whenever possible. If xanthate is used, use high-grade xanthate (which decomposes slowly in acidic slurries).