The Bayer Process is well-known and commonly employed for producing aluminum hydroxide from alumina-containing ore such as bauxite. This process usually comprises the steps of:
(i) treating bauxite with an alkaline solution such as an aqueous sodium hydroxide solution or an aqueous mixture solution of a sodium hydroxide solution and a sodium carbonate solution to obtain a slurry (bauxite-dissolving step),
(ii) extracting alumina contained in bauxite as sodium aluminate (extraction step),
(iii) separating insoluble residues (dissolution residue) such as iron oxides, silicates and titanium oxides from the slurry to obtain a sodium aluminate solution (red mud-separation step),
(iv) adding seeds of aluminum hydroxide into the sodium aluminate solution so that aluminum hydroxide is precipitated at a temperature from 50.degree. C. to 70.degree. C. and the precipitated aluminum hydroxide is separated from the sodium aluminate solution (precipitation step), and
(v) recycling a portion of the separated aluminum hydroxide as seeds in the precipitation step and recovering the rest of the separated aluminum hydroxide as a product, while the sodium aluminate solution remaining after the recovery of the aluminum hydroxide is recycled as it is or after condensation for the use in the bauxite-dissolving step.
Bauxite usually contains an alkali-soluble silica (hereinafter, referred to as a reactive silica), a content thereof being dependent on a place where it is mined. Therefore, in the above extraction step, the reactive silica contained in bauxite is dissolved as well as the alumina component.
When an extract solution (sodium aluminate solution) containing the reactive silica dissolved therein is used for the precipitation step to obtain aluminum hydroxide, the silica in the solution is also decomposed and precipitated together with aluminum hydroxide, which causes deterioration of the quality of aluminum hydroxide obtained. To avoid the deterioration, the reactive silica in the extract solution is allowed, prior to the precipitation step, to react with a portion of the alumina component and a portion of the alkaline component in the extract solution, so as to precipitate as sodalite or zeolite which is alkali-insoluble (desilication step), and be separated and discarded together with the other insoluble substances such as the iron oxides and the titanium oxides in the red mud-separation step.
In this process, the reactive silica is precipitated by consuming a large amount of the alumina component and the alkaline component in the extract solution and, therefore, a large amount of losses of these components can not be avoided and the process is not economical.
For suppressing the losses of alumina and the alkaline components, Japanese Patent Laid-Open Publication No. 5-193931-A (corresponding to U.S. Pat. No. 5,545,384) discloses a process for producing aluminum hydroxide from an alumina-containing ore described wherein:
(i) the alumina-containing ore is mixed with an alkaline solution to obtain an alumina-containing slurry,
(ii) the alumina-containing slurry obtained in step (i) is mixed with a preheated alkaline aqueous solution, to obtain an alkaline aqueous mixture,
(iii) alumina is extracted as sodium aluminate from the alkaline aqueous mixture obtained in step (ii) at a temperature of 120.degree. C. to 160.degree. C. within 10 minutes, to obtain a mixture of an extract solution of sodium aluminate and a dissolution residue of a reactive silica,
(iv) the dissolution residue is separated from the mixture obtained in step (iii), to obtain the extract solution of sodium aluminate,
(v) the extract solution of sodium aluminate obtained in step (iv) is desilicated, to obtain a mixture of a desilication product and a desilicated-extract solution,
(vi) the desilication product is separated from the mixture obtained in step (v), to obtain the desilicated-extract solution and
(vii) seeds of aluminum hydroxide are added to the desilicated-extract solution obtained in step (vi), to precipitate aluminum hydroxide.
With the process in the JP-5-193931-A, the losses of alumina and alkaline components is quite suppressed and aluminum hydroxide is produced from an alumina-containing ore at a high yield and with little contamination by silica. However, the suppression of the losses of alumina and alkaline components is not sufficient, and the yield of aluminum hydroxide is not sufficient, either and, therefore, a more improved process has been desired.