Clarification of bayer process liquors

Details Clarification of bayer process liquors Clarification of bayer process liquors

1987-03-25

1988-12-06

Hruskoci, Peter

Liquid purification or separation

Processes

Making an insoluble substance or accreting suspended...

210734, 423121, 423122, B01D 2101

Patent

active

047894857

DESCRIPTION:

BRIEF SUMMARY
The separation of inorganic suspended solids from an aqueous liquor is often promoted by the addition to the liquor of a polymeric flocculant that may be non-ionic, anionic or cationic.
When the liquor is alkaline, the flocculant is usually an anionic or non-ionic polymer but there have been some proposals to use a cationic polymer in alkaline liquors.
In U.S. Pat. No. 3,023,162, partially or wholly quaternised dialkylaminoalkyl (meth) acrylates are used for the flocculation of aqueous suspensions having pH from 9 to 14 and in which the suspended solids are organic (e.g., sewage) or inorganic (e.g., mineral dressing liquors).
In U.S. Pat. No. 4,160,731, a copolymer of acrylamide and 3(methacrylamido)-propyl-trimethyl ammonium chloride (MAPTAC) is used to promote dewatering of sewage sludge to which sufficient lime has been added to raise the pH to at least 12.
Particular problems arise in the clarification of Bayer Process liquors. The Bayer process is a widely used process for obtaining aluminium from bauxite by digesting the bauxite with sodium hydroxide to form a slurry of red mud, separating the red mud solids from the liquor, and then recovering the aluminium from the separated liquor. It is important that the liquor should be of high purity since impurities are carried into later stages of the aluminium recovery process and reduce the purity of the resultant aluminium and/or make the later recovery stages more difficult to operate. For instance the presence of inorganic suspended solids in the liquor is liable to lead to the contamination of the aluminium by, for instance, iron whilst the presence of dissolved organic materials such as humates is liable to interfere with the crystallisation during the recovery processes.
The separation of inorganic suspended solids is therefore usually conducted in two stages, namely a primary settlement stage to remove most of the suspended solids and a pressure filtration stage to clarify or polish the resultant liquor. Flocculants such as starch or sodium polyacrylate are generally used to promote the settlement stage but additional steps usually have to be taken to improve the effectiveness of the pressure filtration stage, for instance because the suspended particles are very fine, and conventionally a filter aid such as a calcium compound is added. Merely increasing the amount of anionic or non-ionic flocculant to the settler stage does not improve results, and generally makes them worse, both because of overdosing in this stage and because the extra amount of dissolved polymer is liable to increase the blinding of the filters.
In JP No. 5009646A, it is proposed to promote settlement of the red mud slurry in the primary settlement stage by addition of sodium polyacrylate followed by quaternised dialkylaminoalkyl(meth)acrylate.
In U.S. Pat. No. 4,578,255 (not published at the priority date of this application), it is proposed to remove humates from Bayer Process liquors by a particular process involving the addition of a water soluble, vinyl, polymeric quaternary ammonium salt. The only vinyl polymers which are shown to work satisfactorily are polymers of diallyl dimethyl ammonium chloride (DADMAC) and although some polymers having intrinsic viscosity (IV) up to 4.8 are mentioned, it is clear that best results are obtained with DADMAC polymers having IV up to about 1. No vinyl polymers other than DADMAC are mentioned in U.S. Pat. No. 4,578,255.
There is therefore still an urgent need to provide better clarification of sodium aluminate liquors, and in particular to promote better filtration since the present methods result in frequent blinding of the filter cloths thereby reducing the rate of flow through the filters and necessitating more frequent cleaning off of the cloths.
In the invention, inorganic suspended solids in a sodium aluminate liquor obtained in the Bayer Process are flocculated by addition of a flocculating agent and are separated from the liquor, and the flocculant comprises a quaternised polymer having intrinsic viscosity above 1 dl/g and which

REFERENCES:
patent: 3023162 (1962-02-01), Fordyce et al.
patent: 4137164 (1979-01-01), Coscia
patent: 4160731 (1979-07-01), Doyle
patent: 4275042 (1981-06-01), Lever
patent: 4396752 (1983-08-01), Cabestany et al.
patent: 4545902 (1985-10-01), Connelly et al.
patent: 4578255 (1986-03-01), Roe et al.
patent: 4678584 (1987-07-01), Brownrigg
"Aluminum Production: Principles and Practice", by A. N. Adamson, The Chemical Engineer, Jun. 1970.

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