Treatment of industrial wastewaters

Claim 1

An improved method for treating acidic wastewater which contains a quantity of dissolved heavy metals exceeding the level of environmental discharge criteria and having a pH between about 2.5 and 5.0 whereby such quantity of dissolved heavy metals is reduced to a level below environmental discharge criteria and the resolvation of the heavy metals in the acidic wastewater is reduced to a minimum, comprising: (a) mixing particles of at least one carrier agent taken from the group consisting of sludge formed in the method, silica, sand and alumina and an aqueous mixture of at least one neutralizing agent taken from the group consisting of sodium hydroxide, potassium hydroxide, calcium carbonate, sodium carbonate, limestone, calcium oxide and dolomitic limestone in a first reactor for a time sufficient to allow the neutralizing agent to be adsorbed on the particles of the carrier agent and to form an aqueous suspension of the carrier-neutralizing agents, (b) charging the acidic wastewater and a sufficient amount of the aqueous suspension of the carrier-neutralizing agents formed in step (a) into a second reactor wherein the pH of the acidic wastewater is adjusted to between about 8.4 and 10.6, (c) mixing the acidic wastewater and the aqueous suspension of the carrier-neutralizing agents formed in step (a) in the second reactor for a time to precipitate a portion of the heavy metals as oxides and/or hydroxides which are precipitated on the surfaces of the carrier-neutralizing agents and the quantity of heavy metals dissolved in the wastewater is reduced to below environmental discharge criteria, (d) passing the aqueous mixture formed in step (c) to a thickener, (e) allowing the aqueous mixture to remain in the thickener for a time sufficient to allow the carrier-neutralizing agents coated with the precipitated oxides and/or hydroxides to settle to the bottom of the thickener whereby a sludge containing at least 10 weight percent solids is formed, (f) removing the sludge from the bottom of the thickener, and (g) flowing water containing a quantity of dissolved heavy metals which is below environmental discharge criteria over the top of the thickener.