Removal of humates from bayer process liquors

#This invention relates to a process for reducing the concentration of humate type organic impurities in Bayer Process liquors derived from the digestion of bauxites with a caustic solution.

#Organic contamination of Bayer liquor is a problem of particular importance to Australian alumina producers since Australian bauxites, particularly those in the Darling Range of Western Australia, contain significantly higher levels of organic matter than their counterparts elsewhere in the world. A major portion of the organic matter associated with the bauxite is in the form of complex cellulose and lignin substances which are extremely difficult to characterise. These organics are extracted from the ore during digestion and undergo oxidative attack in the caustic liquor to form "humates" and the sodium salts of a variety of lower molecular weight organic acids. Since the process liquors are continuously recycled to digestion where additional organic extraction from the bauxite occurs, the organic impurity levels build in the liquor stream and become quite significant. Organics are suspected of seriously interfering with hydrate production through a variety of detrimental effects such as increased liquor foaming and precipitator scaling, reduced yields in digestion and precipitation, increased liquor viscosity, poor settling rates in the thickeners and classifiers, poor agglomeration of fines, increased soda contamination and uncontrollable precipitation of sodium oxalate. Therefore, organics must be continually removed from the process liquor to avoid excessive levels of contamination. In most alumina refineries, the only control of organic levels in the liquor is through the residue disposal system where organic impurities absorb onto the mud solids and the loss of liquor to the mud lakes functions as a purge stream. Depending upon the operating conditions of the refinery and the type of bauxite being processed, steady-state organic levels can range between 3-40 g/L as total organic carbon.

#Considering the problems that organics can cause in a refinery, the nature of the organic species in Bayer liquors has received very little serious attention in the literature. Where work has been done, most of it has focused on the effects of high molecular weight humic acid salts since humates are suspected of being responsible for the most severe "organic" problems in alumina refineries. The exact composition of these humates is uncertain but they appear to consist of a mixture of complex species, some of which are intensely colored due to extensive conjugation. These particular species impart to the Bayer liquor a characteristic dark, red/orange color and affect the quality of the product hydrate, especially with respect to "whiteness". Humates also stabilize low molecular weight acid salts, such as sodium oxalate, making oxalate removal control difficult.

#FIG. 1 is an graphical view showing the electronic absorption of Alcoa-Kwinana Green Liquor.

#FIG. 2 is a graphical analysis showing the infrared spectra of poly-DADMAC and poly-DADMAC/Organic Coascervate.

#FIG. 3 is a diagrammatic view showing the electronic absorption spectra of Alcoa-Kwinana Process Liquor after extraction with poly-DADMAC.

#From the experimental data presented in the patent related to the efficiency of the poly-DADMAC for removing humates, reagent dosages as low as 56 mg/L (e.g. approximately 11 mg/L of cationic polymer since the original reagent was only 20% active) are claimed to remove as much as 27% of the humates from the liquor. Humate removal was determined solely by the reduction in colour of the liquor, as measured by visible spectrophotometric methods at 691 nm, before and after treatment with poly-DADMAC. Considering that humates can represent a significant portion of the organics present in Bayer liquors (i.e. approximately 5% of the 30-40 g/L TOC present in Alcoa-Kwinana liquor), it is doubtful that small amounts of poly-DADMAC are capable of removing large percentages of the total organic carbon from high TOC Bayer process liquors. Unfortunately, no TOC information or humate levels for the Kaiser/Gramercy liquor are given in the Nalco/Kaiser patent. Recent studies indicate that the actual humate precipitation capacity of poly-DADMAC for Alcoa of Australia Worsley & Queensland Alumina process liquors is far less than that reported in the Nalco/Kaiser patent for Kaiser/Gramercy liquors.

#The electronic absorption spectrum of green liquor from the Alcoa-Kwinana refinery is shown in FIG. 1 of the drawings.

#A single low intensity absorbance with a shoulder at .lambda..sub.max 430 nm is observed. However, in the ultraviolet region, liquor absorbance increases dramatically and there is a strong absorption band, with a shoulder at 310 nm, attributable to electronic transitions characteristic of the organic contaminants in the liquor. The absorbance value at 310 nm is more useful for determining organic removal from Bayer liquors than the absorbance value at 690 nm since not all organics in the process liquor are coloured species. Although the absorbance value at 310 nm is due, in large part, to complexes other than humates (e.g. Alcoa Research and Development has evidence that the highest molecular weight "humates" contribute only 10-15% of the absorbance value at 310 nm), it is the change in absorbance at 310 nm upon treatment of the liquor with poly-DADMAC that is significant for organic removal determination. Since poly-DADMAC is capable of forming precipitates only with high molecular weight "anionic" species, any reduction in liquor absorbance at 310 nm upon poly-DADMAC addition must be due to "humate" removal from the liquor.