Patent ID: 7514001

Claim:
A process for treating a high silica feedwater having a silica concentration at or above 60 ppm silica, prior to being mixed with any recycled retentate produced by the process, using a reverse osmosis treatment apparatus comprising at least a reverse osmosis feed entrance conduit, a reverse osmosis retentate exit conduit, and a reverse osmosis permeate exit conduit, said process operating at high recovery of low-silica liquid while substantially avoiding formation of alkali-soluble silica-containing scale in the treatment apparatus by controlling the pH of a concentrating stream coming from the treatment apparatus, said process comprising at least the following steps: (i) adjusting the pH of a feedwater having a silica concentration at or above 60 ppm silica prior to being mixed with any recycled retentate produced by the process to an acidic pH range to form an acidic pH-adjusted feedwater of sufficient acidity below a pH of 3.9 to produce in step (iii) a reverse osmosis retentate having a pH at or below 3.9 at the reverse osmosis retentate exit of the treatment apparatus; (ii) passing said acidic pH-adjusted feedwater having a pH below 3.9 and a silica concentration at or above 60 ppm silica into the reverse osmosis feed entrance conduit, and operating the treatment apparatus substantially free of potential silica nucleation sites by maintaining the pH of the concentrating stream at or below 3.9; (iii) treating said acidic pH-adjusted feedwater in said reverse osmosis treatment apparatus to produce: (a) a reverse osmosis permeate from the reverse osmosis permeate exit conduit representing about an 85% or greater recovery of liquid having a silica concentration of less than about 2.4 ppm of silica; and (b) a reverse osmosis retentate from the reverse osmosis retentate exit conduit, said reverse osmosis retentate having a pH at or below 3.9 and being stable with a supersaturated silica concentration at or above 386 ppm silica and further being clear and substantially free of precipitating silica; and, (iv) recovering reverse osmosis permeate from the reverse osmosis permeate exit conduit and reverse osmosis retentate from the reverse osmosis retentate exit conduit.