The presence of solid sodium oxalate in the precipitation circuit affects product quality by a number of mechanisms. Laboratory tests aimed at defining and quantifying these are described, and a case study of the effect of humate removal from the Plant liquor stream is given.
It is well known that the presence of solid sodium oxalate dramatically inhibits agglomeration. It also tends to promote nucleation of aluminium hydroxide. The effects of oxalate on hydrate classification are complex, and impact product quality by affecting the particle size distribution. The presence of other organic compounds in the liquor is critical in determining the nature and extent of sodium oxalate effects.
The phenomena so far mentioned impact the particle size distribution of the product.
Sodium oxalate can also affect the soda content of the product. Because sodium oxalate accounts for only a small proportion of the total organic content of the liquor, this is usually only significant if solid sodium oxalate is physically included with the product.
The sodium oxalate in Bayer liquor originates in digestion, primarily by degradation of organic compounds in the bauxite. The remainder arises by degradation of compounds already in the liquor. The magnitudes of these effects are examined for the case of the Worsley Refinery.