The almost universally used process for the manufacture of alumina is the Bayer process. In a typical commercial Bayer process, raw bauxite ore is pulverized to a finely divided state. The pulverized ore is then fed to a slurry mixer where a slurry is prepared using water, spent liquor and added caustic. This bauxite slurry is then diluted and sent through a series of digesters where, at about 300.degree.-800.degree. F. and 100-2000 p.s.i., most of the total available alumina is extracted from ore which may contain both trihydrate and monohydrate forms of alumina. The effluent from the digesters passes through a series of flash tanks wherein heat and condensate are recovered as the digested slurry is cooled to about 230.degree. F. and brought to atmospheric pressure. The aluminate liquor leaving the flashing operation (blow-off discharge) contains about 1-20% solids, which consist of the insoluble residues that remain after reaction between the bauxite ore and basic material used to digest the ore and the insoluble products which precipitate during digestion. Herein, all percentages are by weight, based on total weight, unless otherwise stated. The coarser solid particles are generally removed with a "sand trap" cyclone. To separate the finer insoluble solid particles from the liquor, the slurry is normally fed to the center well of a primary mud settler where it is treated with a flocculant such as a polyacrylamide polymer, polyacrylate polymer, hydroxamated polymer, flour and/or starch. As the mud settles, clarified sodium aluminate solution, referred to as "green" or "pregnant" liquor, overflows a weir at the top of the mud settling tank and is passed to the subsequent process steps. The settled solids ("red mud") are withdrawn as underflow from the bottom of the primary mud settler and passed through a countercurrent washing circuit generally comprised of a series of washers, for recovery of sodium aluminate and soda. Aluminate liquor overflowing the primary settler still contains typically 50 to 200 milligrams of suspended solids per liter. This liquor is then generally further clarified by filtration to give a filtrate with less than about 10 milligrams suspended solids per liter of liquor. After passage through the filtration step, the level of suspended solids should be sufficiently low to provide an alumina product from the precipitation step which meets all of the industry standards.
Alumina, in relatively pure form, is then precipitated from the filtrate as alumina trihydrate crystals. The remaining liquid phase or spent liquor is returned to the initial digestion step and employed as a digestant of additional ore after being reconstituted with additional caustic.
The efficient removal of suspended solids from Bayer process streams has been a major problem for many years. The aforementioned insoluble components should be separated at a relatively fast rate to make the overall Bayer process efficient. Ideally, a highly efficient Bayer process would separate the insoluble materials from the aluminate liquor cleanly and completely, to give high solids red mud containing little or no caustic or aluminate liquor, and solubilized alumina liquor with little or no insoluble dispersed residue.
Among the methods of overcoming the above problems and materially speeding up separation of suspended solids from process streams as well as effecting a cleaner separation of the constituents are those disclosed in U.S. Pat. No. 3,390,959 which employs polyacrylates as anionic flocculants and U.S. Pat. No. 3,681,012, which uses combinations of polyacrylates and starch in Bayer alumina recovery circuits. Also of interest in this connection are U.S. Pat. No. 3,975,496 which uses a copolymer of acrylic acid and methylolated acrylamide for the same purpose, and U.K. Patent Specification Nos. 2080272 and 2112366, which use, sequentially, combinations of polyacrylic acid and acrylate-acrylamide copolymers. Other approaches have been proposed: in Japanese Patent Publication No. 56092116 (Jul. 25, 1981) is disclosed starch cationized with a quaternary ammonium salt for use as a coagulant; U.S. Pat. No. 4,083,925 promotes separation from alkali metal aluminate liquor by contacting it with anionic polyacrylamide under special conditions within the mud settler; East German (DE) Pat. No. 2552804 (Aug. 11, 1977) subjects starch to treatment with sodium tetraborate and a magnesium salt to provide improved flocculating properties with lower levels of starch; Russian Pat. No. 507526 (Apr. 6, 1976) reports that cationic flocculants of the formula (R--AR--CH.sub.2 --N--Ph).sup.+ Cl.sup.- are better for solids flocculation than other known flocculants; Japanese Pat. No. J74018558 (Oct. 5, 1974) discloses using an inorganic calcium compound and sodium polyacrylate for sedimentation and filtration. The use of hydroxamated polymers as flocculants for cassiterite is disclosed in Jour. So. African Inst. of Mining and Metallurgy; Vol. 76; pgs. 117-119 (1975) by Appleton et al. Polymers containing hydroxamic acid groups for reduction of suspended solids in Bayer process streams are described in U.S. Pat. No. 4,767,540, which is hereby incorporated herein by reference. Separation processes such as settling in the presence of a synthetic flocculant, filtering, or centrifugation, wherein the separation must be carried out at a pressure above atmospheric pressure, are described in U.S. Pat. No. 4,994,244.
Because of the rheological characteristics of the solids in most Bayer process streams, including flocculated solids, centrifugation is not typically used to separate solids from the process stream. Instead, separation of the solids is generally accomplished by the use of settlers, decanters, thickeners, classifiers, and filters. With the exception of filters, these devices rely on the gravitational settling of the solids to achieve separation. Flocculation of the solids aids in the settling process by tending to agglomerate smaller particles into larger ones, which tend to settle faster. Flocculation also aids in the filtering process because larger agglomerates are easier to filter than smaller ones, and less likely to plug the filtering means. "Red mud" is generally obtained as the underflow from a settler. It consists mainly of extremely fine, difficult-to-filter, insoluble residue that remains after the caustic-soluble components have been extracted. Typically, the red mud underflow from a primary settler passes through a countercurrent washing circuit, generally comprised of a series of washers, for recovery of sodium aluminate and soda. Current practice is to dispose of the last washer underflow stream and other red mud streams by pumping them as a relatively dilute slurry to holding ponds and lakes constructed for that purpose. The practical limit of such a slurry is 25 to 40 percent solids; 40 percent solids content is usual. Those skilled in the art agree that red mud impoundment is not an ideal solution to the disposal problem. The dikes of mud lakes must be maintained, and there is always the risk of a break and spill of the mud into a nearby stream or waterway. In addition, the large amounts of water going to the impoundment along with the suspended mud may contain caustic and solubilized alumina, which are lost from the process and present a large economic penalty.
Experts have studied the problem of red mud utilization, e.g. "An Assessment of Technology for Possible Utilization of Bayer Process Muds" by B. K. Parekh and W. M. Goldberger, U.S. Environmental Protection Agency, Off. Res. Dev., EPA-600/2-76-301 (1976), which is hereby incorporated herein by reference. Virtually all potential commercial uses of the muds require that the muds be dewatered to the extent that the solids can be transported and/or stored in a consolidated dry form without tendency to leach or pulp. Low-cost dewatering of the muds is therefore considered the key to their possible future utilization. It would be very advantageous to increase the solids content of red mud. High-solids mud could be economically transported to other locations and utilized as an ingredient in e.g. ceramics, cement, construction materials, etc. Such uses might also mitigate the impoundment problem. Centrifugation is a well-known process for achieving solid-liquid separations, but has not been typically used to dewater red mud. It is known to those skilled in the art that centrifugation of red mud is only marginally effective for increasing the solids.
Surprisingly, it has now been discovered that greatly improved dewatering of Bayer process streams, particularly settler underflow and digester blow-off, is obtained by a combination of centrifugation and the use of, as aflocculant, a polymer which contains hydroxamic acid groups. The efficiency of solids/liquids separation in the Bayer process is thus improved by increasing the solids content of the separated solids stream.
The processes of the present invention are designed to increase red mud solids and to recover the dissolved components, such as aluminate and soda, contained therein. The improvement forming the basis of the present invention lies in the centrifugation of suspended solids-containing streams, particularly those containing red mud solids, that have been flocculated using polymers that contain hydroxamic acid groups. Centrifugation of solids that were not flocculated with polymers, or that were flocculated with polymers that did not contain hydroxamic acid groups, has been found to be less effective than current commercial procedures.