Acid, thickened with silica

To inject acid to the desired point in an oil well, the acid is thickened with silica. On dilution with water once it is in place the mixture liquefies and the acid becomes. active (629). Silica can also be used to thicken acid in lead storage batteries. 

About 10% of the raw rock, comprising silica and the like, does not dissolve. However, this material is readily removed by settling the dilute slurry in a thickener, sometimes aided by coagulants (Fig. 6.6). The clarified solution from the thickeners is then extracted with a C4, C5 alcohol mixture or trialk-ylphosphate in a series of three or more mixer settlers producing a solvent phase rich in phosphoric acid, and a raffinate of calcium chloride brine freed of phosphate. Presence of the calcium chloride salt in the aqueous phase undoubtedly assists in driving the phosphoric acid transfer to the organic phase by making the aqueous phase more polar. 

It is well Icnown that the solid-liquid separation characteristics of residues become a problem when zinc concentrates with high silica content are treated in a hydrometallurgical process. Once treated improperly, the silica dissolved in the sulfuric acid solution turns to a gel which significantly reduces the solid-liquid separation characteristic of the residue. As a consequence, larger capacity solid-liquid separation facilities such as thickeners, filter presses and others are required. In some cases, the slurry may completely turn into a gelatinous form, which cannot be treated. For this purpose, a technology to settle the dissolved silica in a form easy for solid-liquid separation becomes necessary. 

The process begins with a weak acid leach (pH 1.5-1.8) followed by a pH bump to 3.8-4.0 for iron, silica, arsenic and antimony precipitation. The slurry is thickened, and the solid residues are filtered and washed to recover water-soluble zinc in two stages of American disk filters. This filtration step is being upgraded as described in a following section. These iron-rich residues are then repulped and combed with the residue slurry from the oxide leaching circuit and are pumped to the smelter where the slurry is filtered, combined with other materials and process in the JCIVCET furnace. 

The zinc raw material at the proper particle size is mildly leached at atmospheric pressure with dilute sulfuric acid at a controlled pH at about 50°C and for the required residence time. The reaction mixture is purified by precipitation with limestone and/or lime at controUed pH to remove some impurities, mainly iron, silica and aluminium. The leach residue together with the precipitated gypsum and hydroxides are thickened and filtered. A zinc PLS and a leach residue are produced. 

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