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Ind. Eng. Chem. Res. 1992,31,982-986
A Morphological Study of Bayer Process Desilication Product and Its Application to Laboratory and Plant Digests Desilication product (DSP) formed in the Bayer process is difficult to study using electron microscopy because it is masked by iron oxide particles. Differences in the rate of DSP formation under plant and laboratory conditions, which may be due to particle size and hence surface area, are difficult to resolve. Iron oxides were successfully removed from bauxite refining residue (red mud) without destroying the DSP, enabling the morphology of the DSP to be studied with scanning and transmission electron microscopes. The procedure makes it possible to compare the morphology of DSP formed under plant conditions and using pure components in the laboratory. The DSP formed under plant conditions and removed with the residue has typically a wood-ball-like structure with diameter of around 2 pm. The structure is formed rapidly (within minutea) under plant conditions. Similarities with DSP formed under certain laboratory conditions were observed. The procedure provides a means of confirming the validity of laboratory studies under appropriate conditions for simulating DSP formation under plant operating Conditions. Introduction In the Bayer process for the extraction of alumina from bauxite, a sodium aluminum silicate is formed by the reaction of kaolin and other silica minerals in the bauxite with sodium hydroxide (Flint et al., 1946). The formation of the sodium aluminum silicate, commonly called desilication product (DSP) in the alumina refining industry, has been extensively studied, because ita formation determines the final level of silica in the Bayer process liquor and consequently the purity of the product alumina. The DSP incorporates impurities from the liquor as it crystallizes and acta as an impurity-removalmechanism. The formation of the DSP, however, also means a loss of valuable sodium hydroxide from the liquor to the residue, and has significant implications in the rehabilitation and utilization of fine bauxite refining residue (red mud) (Parekh and Goldberger, 1976;Bell and Meecham, 1978; Barrow, 1982). Properties of DSP have been studied using X-ray diffraction (XRD), infrared spectrometry, and differential thermal analysis (Herrman, 1978; Seimiya, 1963;Breuer et al., 1963;Leiteizen et al., 1975;Kontopoulos et al., 1978). Although DSP is readily detected by XRD, the DSP is difficult to directly observe within the residue. This makes difficult study of the DSP particle size and shape and an estimate of surface area and comparison with laboratory studies of formation of pure DSP. It also makes difficult study of ita subsequent behavior in any treatment of the red mud to reduce the environmental impact. No previous work appears to have been reported on the morphology of DSP in red mud by optical or electron microscopy, either scanning (SEM) or transmission (TEM). Although previous studies on the morphology of DSP produced free of mud have been undertaken (Bell, 1970; Roach and White, 1988),earlier attempts by the authors to observe DSP crystals in red mud using SEM were hindered by interferences from the ubiquitous minute particles of iron oxides coating almost all particles of the mud. The removal of iron oxide particles was considered an essential prerequisite for examination of the DSP using an electron microscope. The application of a method to remove iron oxide particles without affecting the DSP crystals was therefore essential. This paper reports the use of such a method and ita application to the study of the crystal habit and growth of the DSP. Experimental Section Several methods for the removal of iron oxides from soil samples have been developed (Kunze, 1965; Jackson, 1979).
0
M
AFTER 5
11.5
18
24.5
31
37.5
44
50.5
2 8 angle (degrees)
Figure 1. XRD traces of red mud before and after CBD treatment (C = calcite, D = DSP,G = goethite, H = hematite, M = muscovite, Q = Quartz).
These methods rely on the reducing the iron into the more soluble iron(I1) form and complexing it to remain in solution. The methods differ only in the degree of severity of the reduction conditions and the complexing agents used. The citrate bicarbonate dithionate (CBD) method is the mildest in terms of the reducing agent, and the reduction is buffered at a pH of about 8 (Jackson (1979), pp 44-51). This method was considered the most appropriate for removing iron oxide particles from red mud. Because DSP was formed at a very high pH (higher than 13),it was important to ascertain that no alteration of the DSP resulted from the iron-removalprocedure. XRD was used to determine if there was any structural change, and a m888 balance was used to ascertain whether any DSP had been lost by treatment. The DSP is concentrated in the fine fraction of the residue. Particle size separation was therefore carried out by settling and decantation/centrifugation (Jackson (1979), pp 123-40)to obtain fractions with cutoff points of 53,20, 5,and 2 km. Initially particles greater than 53 Fm (sand
0888-5885/92/2631-0982$03.00/00 1992 American Chemical Society
Ind. Eng. Chem. Res., Vol. 31, No. 3, 1992 983 fraction) were separated by wet sieving. The fraction greater than 20 pm (coarse silt) was then collected by repeatedly decanting the suspension which did not settle out 5 cm within 2 min. The suspension was next repeatedly centrifuged to remove the clay fraction (less than 2 pm), each time for 2.9 min at 750 rpm (25OC). Finally the remainder is fractionated into medium (greater than 5 pm) and fine silt by repeatedly centrifuging at 300 rpm for 2.9 min (25 OC), X-ray diffraction analysis was carried out using a Siemens D500 diffractometer with Cu K a radiation. SEM was carried out using an ISI-40 system with a PGT attachment for energy-dispersive X-ray analysis (EDXA), and TEM analysis was carried out using a Philips 430 Analytical EM. The DSP in red mud is quantified by the amount of silica associated with the DSP (DSP silica). It was determined following a standard procedure used in the industry by washing a sample with distilled water and then with excess 0.1 M hydrochloric acid and determining the amount of silica in solution. Experimental Results Recovery of DSP after Undergoing the CBD Procedure. XRD analysis shows that the basic framework structure of the DSP did not change during the iron oxide removal or particle sizii procedure. Figure 1shows XFtD traces of red mud before and after CBD treatment. The only difference between the traces is the attenuation of goethite, hematite, and calcite. The last partly dissolved as calcium bicarbonate (Jackson, 1979). Not all of the iron oxides were removed, but more than sufficient to clearly observe DSP with an electron microscope (see below). A ma88 balance of DSP in red mud before and after the procedures indicated that within experimental errors no DSP was lost (Table I). Experimental errors arose from the analysis of DSP silica,transfer of solids,and weighing. Although difficulty was experienced with the handling of the solids samples during the fractionation process, the analysis of DSP silica involving very small samples contributed significantly to measurement errors, estimated at about 10%. Size fractionation alone (Table I, run 2) did not affect the total amount of DSP in the mud. An increase was indicated, but this was within experimental errors. CBD treatment resulted in over 30% weight loss of iron oxides, but again there did not appear to be any significant loss of DSP. Subsequent experiments treating synthetic DSP with the CBD method also showed that treatment did not affect the DSP. The above conclusion that basic DSP framework is stable at a pH below 12 is consistent with resulb obtained in a related experiment (Wong and Ho, 1988)where red mud was neutralized with hydrochloric acid. DSP could be clearly detected by XRD down to pH 7 where decomposition began to take place. The buffering by bicarbonate resulted in the pH being maintained at about 8 during the CBD procedure, and the buffering capacity of sodium carbonate during size fractionation maintained the pH at about 9. Electron Microscope Studies of DSP. Following treatment of the red mud using the CBD procedure, DSP particles could be readily identified. Initially EDXA was used to determine which particles were DSP based on Na:Al:Si ratio. Subsequently the distinctive morphology of the DSP allowed ready identification, and EDXA was only used occasionally for confirmation.
Table I. Mass Balance of DSP in Mud before and after Iron Oxide Removal o r Fractionation DSP silica weight, g % g 1. Red Mud Treated by CBD Method To Remove Iron Oxides 0.55 before treatment 12.23 4.5 after treatment 8.40 5.0 0.42 2. Red Mud Fractionated
before fractionation after fractionation >53 pm 20-53 2-20 53 pm 20-53 5-20 2-5 53 pm 20-53 5-20 2-5 53 pm 20-53 5-20 2-5 53 pm 3.65 6.0 0.219 20-53 1.48 8.3 0.123 5-20 0.19 13.7 0.026 2-5 0.44 9.6 0.042
