Method of preventing or reducing aluminosilicate scale in a bayer process

Abstract

Abstract of the DisclosureMaterials and a process are provided whereby polymers with the pendant group or end group containing --Si(OR")3 (where R" is H, an alkyl group, Na, K, or NH4) are used to control aluminosilicate scaling in a Bayer process. When materials of the present invention are added to the Bayer liquor before the heat exchangers, they reduce and even completely prevent formation of aluminosilicate scale on heat exchanger walls. The present materials are effective at treatment concentrations that make them economically practical.

Description

Detailed Description of the InventionBACKGROUND OF THE INVENTION[0001] The Bayer process is almost universally used to manufacture alumina. In this process raw bauxite ore is first heated with caustic soda solution at temperatures in the range of 140 to 250º. This results in the dissolution (digestion) of most of the aluminum-bearing minerals, especially the alumina trihydrate gibbsite and alumina monohydrate boehmite, to give a supersaturated solution of sodium aluminate (pregnant liquor). Resulting concentrations of dissolved materials are very high, with sodium hydroxide concentrations being greater than 150 grams / liter and dissolved alumina being greater than 120 g / l. Any undissolved solids are then physically separated from the aluminate solution, and a polymeric flocculant is used to speed the removal of the fine solid particles. Residual suspended solids are removed by a filtration step. The filtered clear solution or liquor is cooled and seeded with alumina trihydrate...

AI Technical Summary

Benefits of technology

[0011] The present invention solves the aforementioned problems and others by providing materials and a process whereby polymers with the pendant group or end group containing --Si(OR)3 (where R is H, an alkyl group, Na, K, or NH4) are used to reduce or eliminate aluminosilicate scaling in a Bayer process. When materials of the present invention are added to the Bayer liquor before the heat exchangers, they reduce and even completely prevent formation of aluminosilicate scale on heat exchanger walls. Moreover, the present materials are effective at treatment concentrations that make them economically practical.

Problems solved by technology

The rate at which the desilication products precipitate out, however, is slow and even when a lengthy "predesilication" step is used, concentrations of dissolved silica remain well above equilibrium values.

The scale has low thermal conductivity compared to the steel of the walls and heat transfer is severely reduced as scale builds up.

This reduced heat transfer caused by aluminosilicate scaling is sufficiently severe that the heat exchange units have to be taken out of service and cleaned frequently, as often as every one to two weeks.

The situation is however complicated by the fact that silica in the solution or liquor is not necessarily proportional to the silica in the starting bauxite.

Cleaning of the heat exchangers with acid is itself a high maintenance cost.

The acid cleaning also reduces the life of the heat exchangers, therefore adding cost due to frequent replacement of the heat exchangers.

Moreover, the reduced heat exchanger efficiency caused by scaling leads to higher demand and cost for energy in the form of steam.

The scaled pipes also result in decreased flow of liquor and potentially lost production.

Altogether the costs directly due to scaling constitute a significant portion of the cost of producing alumina.

Scale build up has also been known to be a problem in boiler water systems and a number of treatments for reducing scale in boiler water systems have been proposed.

In contrast, the supersaturated solutions at high temperatures and high pH of essentially 14, make scaling problems much more serious and difficult to contend with in plants that carry out the Bayer process than in boilers.

Thus, although there have been treatments available for boiler scales, there has been limited success in obtaining methods and / or chemical additives that reduce or eliminate aluminosilicate scaling in the Bayer processing of alumina.

Hence, thus far no economically practical materials or process has been offered to solve the problem of aluminosilicate scaling in the Bayer process industry.

There is, in fact, currently no way at all to eliminate aluminosilicate scaling in the Bayer process.

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