Systems and methods are provided for processing metal sulfate compounds and sequestering CO.sub.2. These systems and processes involve one or more electrochemical cells for producing an alkali-containing catholyte and involve a CO.sub.2 absorption reactor operatively connected to the electrochemical cell and to a CO.sub.2 source. The CO.sub.2 absorption reactor receives the alkali-containing catholyte and CO.sub.2 gas for forming an alkaline carbonate solution. The alkaline carbonate solution is directed to a vessel where it reacts with an acidic sulfate solution comprising metal ions resulting in precipitation of solid metal carbonate compounds. The acidic sulfate solution may comprise sulfide leachates from acid mine drainage, sulfide mine tailings and/or reacted pyrite concentrate. The acidic sulfate solution may be circulated through an optional SO.sub.2 reduction reactor prior to reaction in the vessel. The SO.sub.2 reduction reactor reduces trivalent metal compounds present in the acidic sulfate solution to divalent metal compounds.

A process for removing sulphur dioxide from the gaseous effluent of a smelter furnace comprising the steps of: providing the gaseous effluent from a smelter; separating the sulphur dioxide from the gaseous effluent to provide concentrated sulphur dioxide and effluent for discharge into the atmosphere; mixing the concentrated sulphur dioxide with a fuel gas; heating the mixture such that the concentrated sulphur dioxide and fuel gas react to form a gaseous product mixture comprising sulphur and hydrogen sulphide; and removing the majority of preferably substantially all of the sulphur and hydrogen sulphide from the gaseous product mixture; wherein the remaining gaseous product mixture is incinerated before being vented into the atmosphere or is recycled into the smelter furnace.

Gaseous perfluorocarbons in a waste gas are adsorbed by an adsorption device. Subsequently a decomposition of the perfluorocarbons takes place with formation of hydrogen fluoride. The hydrogen fluoride is converted with an oxide of a metal to be reduced, to the metal fluoride thereof. The metal fluoride formed is then fed again to the reduction process.

A process for the treatment of a silicate mineral, includes: preparing a first composition including an alkali metal magnesium orthosilicate and optionally either (i) magnesium oxide or (ii) an alkali metal silicate, by reaction, at a temperature from 500 to 1200 C., of an alkali metal carbonate compound, which compound is an alkali metal carbonate, an alkali metal bicarbonate or a mixture thereof, with a magnesium silicate, the molar ratio of alkali metal carbonate compound, expressed as alkali metal oxide of the formula R.sub.2O, in which R represents an alkali metal, to magnesium silicate, expressed as silicon dioxide, of the formula SiO.sub.2, being from 4:1 to 1:4, and contacting the first composition with water to produce a second composition comprising an amorphous magnesium silicate hydrate (M-SH).