PROCESS FOR THE REMOVAL OF SODIUM VALUES FROM SODIUM CONTAMINATED SOLIDS

Abstract: Removal of sodium values from sodium contaminated solids, such as red mud, is achieved by contact of a slurry of such solids with a particulate cation exchanger and occurs at an enhanced rate compared with water washing or washing with an acidic solution. Regeneration of sodium loaded resin occurs more efficiently with a mixed solution of sodium sulphate and an acid, such as sulphuric acid, than with a sulphuric acid solution.

Description: PROCESS FOR THE REMOVAL OF SODIUM VALUES FROM SODIUM CONTAMINATED SOLIDS This invention relates to a process for the removal of sodium values from sodium contaminated solids, such as red mud. Industrial processes which utilise caustic soda and which produce sodium contaminated solid products or byproducts can give rise to significant environmental damage upon exposure of such contaminated solid products or byproducts to rain water or to ground water due to leaching of alkali. As an example of a process of this kind there can be mentioned the Bayer process for the production of alumina from bauxite which yields corresponding quantities of red mud. Another such process is the cleaning of coal with caustic soda in order to remove ash and sulphur therefrom. Red mud is the major waste product from the production of aluminium, being the residue remaining after leaching of bauxite by the Bayer process. In this process bauxite is heated under pressure with sodium hydroxide. The aluminium oxide reacts to form the aluminate ion which remains in solution. The solid impurities are removed by filtration and are known as red mud. Aluminium values in the sodium aluminate solution are reprecipitated upon cooling the filtered sodium aluminate solution by seeding with a crystal of alumina trihydrate: extraction A1 2 0 3 .3H 2 0 + 2NaOH 2NaA10 2 + 4H 2 0 (1). decomposition The resulting alumina trihydrate precipitate can then be calcined to yield alumina: A1 2 0 3 .3H 2 0 > A1 2 0 3 + 3H 2 0 (2). calcination A description of the Bayer process can be found, for example, in "Non Ferrous Extractive Metallurgy in the United Kingdom", edited by . Ryan, published by The Institution of Mining and Metallurgy, London (1968) at pages 1 to 5 under the heading "Aluminium". Aluminium is the non-ferrous metal produced in largest quantity throughout the world. It is estimated that between 40 and 50 million tonnes per year of red mud are produced, essentially all of which is stored as landfill. It is also estimated that this quantity of red mud contains the equivalent of about 2 million tonnes of sodium hydroxide. The red mud is a mixture of oxides, such as those of iron, titanium and calcium, together with desilication products resulting from the Bayer leaching of the bauxite ore. Such desilication products are sodium alumino-silicates, or sodalites as they are alternatively called, which act as ion exchange compounds for sodium. As a consequence sodium slowly leaches from the red mud and produces a highly alkaline residue, typically having a pH of 12 or more. This in turn means that, if the red mud is dumped without treatment, considerable pollution will occur since rain water run off from, and ground water contact with, the red mud will lead to leaching of sodium with accompanying impact on the environment. As a result, soil under and around an exposed dump of red mud will become contaminated with sodium values. There is also difficulty in rehabilitating red mud lagoons because, for example, revegetation is hindered by the alkalinity and red mud is slow to settle and to undergo dewatering. Moreover the presence of sodium in the red mud prevents its finding a use as a raw material for other processes. Much work has been carried out with a view to neutralising red mud. For example, it has been proposed to use mineral acids, such as sulphuric acid, process off-gases containing C0 and S0 , organic acids produced by bio- activity, sea water addition to cause precipitation of magnesium hydroxide, and so forth. Although these techniques lessen the environmental impact of storing red mud as a landfill, there remains a leachable sodium salt in the residue. Moreover any process that involves use of a pH about 3 or lower will result in dissolution of iron from the red mud. - Other disposal methods that employ drained lagoons and dry stacking of the red mud reduce its adverse environmental influence. However they increase disposal costs and the potential problems due to the sodium content of the mud still remain. Previous attempts to treat red mud by processes involving use of ion exchangers include Hungarian Patent Application No. 82/2733 (Publication No. 31570), (Chem. Abs., Vol. 101, 1984, 114640t), which describes a process for removal of alkali and alkaline earth metals from red mud by d.c. electrodialysis or osmosis or without electricity by dialysis using a separating semipermeable diaphragm or membrane. In the cell opposite to the red mud the concentration of ions to be separated is kept near zero to increase the migration rate by increasing the concentration gradient by ion exchangers. The ion exchangers can be regenerated. Thickening of a red mud pulp and use of a mixture of water-insoluble anion and cation-type ion exchangers as a flocculant is described in SU-A-1100232 (Chem. Abs., Vol. 101, 1984, 113240a). An additional consideration to be taken into account when neutralising red mud using mineral acids, process off-gases, organic acids, and the like, is that the treated red mud will be contaminated by the reaction products, such as sodium and calcium salts. In order to reduce contamination of stored material or to enable the mineral values in the red mud to be exploited, extensive washing is necessary. This is a procedure which produces a large volume of liquid effluent. When the neutralised red mud contains reaction products which include calcium or other metal salts of low solubility, the red mud will be contaminated by anions that may be detrimental to the storage of the neutralised red mud or to the use thereof as a raw material for other processes. For example, if sulphuric acid is used to neutralise red mud, the neutralised red mud will contain calcium sulphate which has a solubility of about 2 gpl and which will continue to leach slowly from the neutralised red mud. Alternatively, if the resulting neutralised red mud is used for cement manufacture, then S0 2 will be released during calcination. Alumina produced in the Bayer process may also be contaminated with sodium values. Another potential source of alkaline pollution of the environment due to sodium contamination is steel plant dust from a sinter plant, blast furnace or a steelmaking plant. There is accordingly a need for an improved process for removing sodium values from red mud and from other sodium-contaminated solids economically and efficiently so as to reduce the potential damage to the environment, due to its sodium content, upon storage of this material. There is also a need for an efficient and economical process for removing sodium values from red mud which permits re-use of such sodium values. There is additionally a need to provide a process which will yield viably a red mud of reduced sodium content which can be used with reduced risk of damage to the environment or which can be used as a raw material for other processes, for example as a cement feedstock, as a flux for steelmaking, as a pigment for use in making bricks, or as a source of iron or titanium. The use of ion exchange resins to absorb metal values, such as sodium ions, from solutions thereof is well known. A problem can arise in regeneration of ion exchange resins when the resin has been loaded with metal values using a solution that contains finely divided particulate matter or a solute, such as a calcium salt, which tends to yield a precipitate in finely divided form. This difficulty in regeneration can be attributed at least in part to a blockage of pores in the resin beads. There is further a need to provide a process for regenerating sodium loaded resins more efficiently, particularly when such resins are also loaded with calcium values, which yields an eluate that is more readily disposable or can be treated more easily for recovery of sodium values than existing procedures. The present invention accordingly seeks to provide a viable process for removal of sodium values from red mud and other sodium contaminated solids (such as alumina, alkali washed coal, sodium contaminated soil, for example contaminated soil from the neighbourhood of red mud dumps, and steel plant dusts), which will reduce the sodium content of the red mud or other solids and hence minimise the pollution problems associated with storage of the treated red mud or other solids, as well as potentially yielding useful products from the metal-containing residues and a source of sodium ions suitable for production of, for example, sodium hydroxide for recycle to the Bayer process. The invention further seeks to provide a process for regenerating sodium loaded cation exchange resins, particularly such loaded resins that are also loaded with calcium values, that results in an eluate with less acidity and a higher sodium tenor than conventional regeneration procedures and that is more suitable for disposal or for further treatment for removal of sodium values. According to the present invention there is provided a process for the removal of sodium values from sodium contaminated solids which comprises contacting a slurry of sodium contaminated solids with a particulate cation exchanger, separating sodium loaded cation exchanger, regenerating said sodium loaded cation exchanger to liberate sodium values therefrom, and recycling resulting regenerated cation exchanger for contact with further slurry of sodium contaminated solids. It has thus been surprisingly found that it is possible to effect a more complete and a more rapid removal of sodium values from sodium contaminated solids, such as a red mud, when using a cation exchange resin than by washing with water or with an acidic solution or by displacement with dissolved cations. The process of the invention is applicable to treatment of a wide variety of sodium contaminated solids, including red mud, sodium contaminated soil (e.g. soil from the vicinity of a red mud dump) , alumina produced by the Bayer process, coal that has been washed with a caustic soda or soda ash solution, and steel plant dusts. It is of particular applicability to treatment of red mud. In a red mud the particles are very fine; typically the particles are smaller than about 0.1 mm and 80% or more of the particles may be smaller than about 10 μm. In order to enable separation of the red mud or other solid being treated and the cation exchanger by particle size, the particulate cation exchanger preferably has a significantly larger particle size than that of the red mud or other solid being treated. Hence in the treatment of red mud the particulate cation exchanger preferably has a particle size in the range of from about 0.1 mm to about 5 mm or more, more preferably from about 0.5 mm to about 2 mm. However separation by particle size is not the only method of separating cation exchangers from slurries of red mud or other solids and the use of particulate cation exchangers having the specified range of particle sizes is not an essential feature of the invention. As suitable cation exchangers there can be mentioned cation exchange resins, chelating resins, and zeolites. Cation exchange resins typically comprise polymeric chains, such as polystyrene chains, having pendant functional groups, typically acidic groups such as sulphonic acid groups, carboxylic acid groups, or phenolic groups. Such resins may have a macroreticular structure or may have a gel-like structure. They may be classed as strong cation exchange resins, such as those resins containing pendant sulphonic acid groups, or as weak cation exchange resins, such as those containing pendant carboxylic acid groups. Suitable cation exchange resins include, but are not limited to, Amberlite IR120, Amberlite 200C, Purolite C105, Purolite C106, and Purolite 160TL. (The words "Amberlite" and "Purolite" are trade marks). Such resins typically have an ion exchange capacity ranging from about 1.7 to about 4 equivalents per litre. The red mud or other sodium contaminated solid is supplied to the process in the form of a slurry containing typically from about 50 gpl to about 250 gpl up to about 500 gpl, e.g. about 200 gpl, of red mud solids or other solids. Desirably the solids concentration is as high as is possible so as to enable the equipment to be as compact as possible but not so high that its viscosity renders the slurry insufficiently fluid for handling. The slurrying liquid is conveniently water. In the process of the invention the red mud or other solid being treated can be contacted batchwise as a slurry with the cation exchanger. In this case the solids:cation exchanger mass ratio preferably ranges from about 1:1 up to about 10:1 or more, e.g. 20:1. More preferably, however, it ranges from about 1.5:1 to about 5:1. In such a process the pH of the pulp can be controlled by selection of the solids:cation exchanger mass ratio. In an alternative procedure the red mud or other solid being treated is contacted with the particulate cation exchanger in a contactor of the type used in resin-in-pulp or carbon-in-pulp metallurgical processes and the red mud or other solid is continuously supplied to the contactor in the form of a slurry. The red mud slurry (or other solid slurry) and the cation exchanger may pass in countercurrent through the contactor. In this case the solids:cation exchanger volume mass ratio is preferably from about 1:1 up to about 10:1 or more, e.g. 20:1, even more preferably from about 1.5:1 to about 5:1. Moreover, in this case the pH of the pulp can be controlled by selection of an appropriate solids:cation exchanger mass flow ratio. In a particularly preferred process the sodium contaminated solid comprises red mud. For convenience the following description refers in the main to treatment of red mud. However it will be appreciated by those skilled in the art that other types of sodium contaminated solids, such as sodium contaminated soil, alumina produced by the Bayer process, or steel plant dust, can be treated in place of red mud by the process of the invention Although other mechanisms can be postulated, two of the possible mechanisms for sodium removal from red mud using cation exchange resins will be similar to those involved in water washing and acid treatment. In a slurry of red mud in water an equilibrium is set up between sodium in the form of a solid sodalite and sodium ions in solution according to the equation:- Na sodalite (solid) + H 2 0 * =^ Na + (solution) + OH " + H sodalite (solid) (3). If a cation exchange resin in the H + form is introduced into the slurry, then sodium ions are removed from the solution so that the equilibrium is driven to the right. Removal of sodium ions from the solution occurs by the following equation:- RH + Na + + OH " R Na + H 2 0 (4), where R denotes resin. It would therefore be expected that the pH of the resulting solution would tend towards a neutral pH. However, as will appear from the succeeding Examples, this does not occur in practice. Instead the pH of the solution tends to reduce substantially below 7 when a cation exchange resin in the hydrogen form and a red mud slurry are contacted. The hydrogen ions in solution then act on the sodalites present in the red mud to release further sodium according to the following equation:- Na Sodalite (solid) + H + H sodalite + Na + (5). The sodium ions then exchange with hydrogen ions on the resin according to equation (4) and the controlled leaching reaction continues, releasing more sodium ions into the solution. It is, however, surprisingly found that the overall process represented by the aggregate effect of equations (3) to (5) above occurs more rapidly than removal of sodium valves from red mud by water washing or by leaching with sulphuric acid. In addition it has been observed that, although the sodium content of the solution initially declines, the final sodium content may be significantly higher after contacting the red mud slurry with the resin (in the H + form) than before such contact. These observations suggest that additional unexpected, and as yet unknown, mechanism pathways may be operating. Furthermore, it is also found that, upon removing the sodium loaded resin from the pulp, the pH of the pulp rises only slowly due to equation (3) above. This also implies that the overall process is faster than can be accounted for by a combination of equations (3) to (5) . The removal of calcium, which is also soluble in acid, from red mud can be expected to occur in a similar way when the red mud slurry or pulp is contacted with the cation exchange resin in the hydrogen form. After a time an equilibrium is reached when the cation exchange resin becomes substantially converted to the Na form and requires separation from the slurry and regeneration, for example by treatment with a mineral acid such as sulphuric acid, to reconvert the resin to the H + form. The Na + loaded resin can be regenerated in any convenient manner. However, for recycle for further use in the process, of the invention it needs to be converted to the H form. Thus, for example, it can be regenerated by treatment with a regenerant solution containing an organic or inorganic acid. Alternatively it can be treated with a regenerant solution containing cations other than Na + ions. As examples of organic acids there can be mentioned formic acid and acetic acid. Hydrochloric acid, sulphuric acid, sulphurous acid, phosphoric acid, are examples of inorganic acids. Other regenerant solutions that can be used include solutions containing calcium ions, such as calcium chloride, or a magnesium salt, such as magnesium chloride. The calcium or magnesium loaded cation exchange resin should, however, be converted by treatment with an acid to the H + form before return to the process of the invention. In a preferred procedure the loaded resin is regenerated using a regenerant solution containing an acid, such as sulphuric acid, and also sodium ions, for example, a solution containing sodium sulphate and sulphuric acid. In this case the mole ratio of sodium ions to sulphuric acid is desirably less than about 5:1. In an alternative method of regenerating the Na + - loaded resin it is subjected to electrochemical ion exchange, for example by the so-called Helix process developed by AEA Technology, Harwell, Didcot, Oxfordshire 0X11 ORA. The process can be operated in batch fashion by stirring a given quantity of red mud in water with an appropriate quantity of cation exchange resin in the H form, followed by filtration through a mesh filter of appropriate mesh size which permits passage of red mud particles but retention of the Na + loaded resin, regeneration of the Na + loaded resin, and recycle of regenerated resin in H + form for further use. Alternatively and preferably the process is conducted on a continuous basis using contactors of the type conventionally used in resin-in-pulp or carbon-in-pulp metallurgical processes for recovery of metal values, such as uranium or gold values. Stirred contactors can be used. An example of such apparatus is described, for example, in GB-A-2106806. In one form of plant one or more such contactors connected in series can be used with a continuous flow of red mud slurry (pulp) passing through the contactor or train of contactor? while the resin is transferred countercurrent to the julp flow in order to remove and recover the sodium values from the red mud. The sodium loaded resin is then transferred to another contactor or train of contactors operating in a regeneration mode in order to regenerate the hydrogen form of the resin for recycle and to release the sodium and calcium values therefrom. In the regeneration mode e regenerant solution and sodium-loaded resin can pass countercurrent to one another through the contactor or train of contactors. Although the on line phase may be continued for a particular contactor until the resin is loaded to its equilibrium capacity with sodium and other metal values, it will usually be preferred to stop short of this point when the resin has been loaded with sodium and other metal values to, for example, about 90% up to about 95% or more of its equilibrium ion exchange capacity. A suitable form of resin transfer apparatus is disclosed in GB-A-2130109. In the regeneration of the sodium loaded resin it has further been found surprisingly that sodium elution from a sodium loaded cation exchange resin is enhanced by the presence of a sodium salt, such as sodium sulphate, in an acid eluant solution, for example a sulphuric acid eluant solution. This finding is surprising since it would normally be expected that the presence in the regenerant solution of the metal ion (i.e. sodium) being released in the course of regeneration of the loaded resin would inhibit elution of that metal ion rather than enhancing said elution. Hence in another aspect the present invention provides a process for regenerating a cation exchange resin loaded with metal values selected from sodium values, calcium values, and mixtures thereof, which comprises contacting said loaded resin with a regenerant solution comprising a mixture of sodium ions and an acid. The acid may be for example, sulphuric acid or sulphorous acid. Hence in such a process there may be used, for example, a solution containing a mixture of sodium sulphate and sulphuric acid. Preferably the sodium loaded resin is one which has been used for removal of sodium values from red mud. Such a resin may be, for example, a weak acid cation exchange resin. When using a weak cation exchange resin the acid used may be sulphurous acid but is preferably sulphuric acid. In the regenerant solution the ratio of sodium sulphate to sulphuric acid may range, for example, up to about 5:1 on a molar basis, preferably in the range of from about 0.5:1 to about 2:1. The concentration of sulphuric acid is typically from about 5 gpl to about 500 gpl, preferably from about 10 gpl to about 200 gpl. In the regeneration of the sodium loaded cation exchanger the pH of the eluate resulting from regeneration of the sodium loaded cation exchanger with the regenerant solution can be controlled by control of the volume flow rate of the regenerant solution through the sodium loaded cation exchanger. Alternatively it can be controlled by control of the composition of the regenerant solution applied to the cation exchanger. The selection of appropriate regeneration conditions will be dependent upon a number of factors, such as resin type, sodium loading, required elution time, and downstream processing of the eluate. Thus, for example, in general a higher acid strength will be used with a strong acid resin than with a weak acid resin. In the removal of sodium values by the process of the invention from red mud the cation exchange resin takes up calcium values in addition to sodium values. In the course of regeneration of the resin using a sulphuric acid- containing regenerant calcium sulphate is formed. This tends to form a supersaturated solution in the regenerant eluate and to crystallise out therefrom. It is an advantage of using a mixed sodium sulphate/sulphuric acid regenerant solution that the crystals of calcium sulphate formed are finer than those formed when sulphuric acid is used alone. This observation and the observation that improved elution of cation exchange resins occurs in the presence of bisulphate ions can perhaps be attributed to the initial formation of calcium bisulphate in the solution followed by precipitation of calcium sulphate. Seeding of the eluate with calcium sulphate crystals may help to ensure that any precipitation of calcium sulphate occurs at a location away from the resin. It may be advantageous, when regenerating a bed of sodium loaded resin that has been used to treat red mud, to use an upflow regeneration procedure since the upflowing regenerant solution can be used to carry away any fine crystals of calcium sulphate. The sodium loaded eluate obtained upon regeneration of the loaded resin can be treated in any desired manner for the recovery of sodium hydroxide or sodium salts, e.g. sodium sulphate, therefrom. For example, the sodium loaded eluate can be subjected to crystallisation or to electrolysis in a membrane cell such as that developed by Imperial Chemical Industries p.I.e. or by the Aquatech Systems Division of Allied-Signal Inc. of 7 Powder Horn Drive, P.O. Box 4904, Warren, NJ 07059-5191, U.S.A., in order to recover in a commercially usable form the sodium values removed from the red mud. Such sodium values can be used, for example, for recycle to the Bayer process. When using, for example, a mixture of sodium sulphate and sulphuric acid for regeneration of a sodium loaded resin, it is an important advantage that the eluate from the regeneration stage is less acidic than that obtained when sulphuric acid is used alone. Hence electrolysis of the eluate solution is facilitated. Alternatively, if the eluate is to be discharged from the plant or crystallised, less neutralisation is required. In addition the sodium tenor of the eluate is higher when using a mixed Na 2 SO j /H 2 SO ij eluant in the regeneration step. This makes subsequent recovery of sodium values easier by crystallisation, electrolysis or other treatment steps. An additional advantage of the use of a mixed Na 2 Sθ4/H 2 Sθ4 regenerant solution is that the pH of the eluate changes dramatically once the peak of the elution curve is passed. Thus, in the initial stages of elution, the pH of the eluate is typically in the range from about 4 to about 7 and requires addition of only a minor amount of an alkali, such as lime or sodium carbonate, to produce a neutral solution which can be treated or discharged. In the later stages of elution, however, the pH of the eluate drops below about 4. The resulting eluate can be mixed with further sulphuric acid and/or sodium sulphate to produce an eluant solution for regeneration of a subsequent batch of loaded resin. Hence a split elution process can be advantageously used. When the sodium loaded resin also contains calcium ions, the use of a mixed Na^O^/^SO^ regenerant solution may assist calcium removal by forming calcium bisulphate, which is soluble, in place of insoluble calcium sulphate. Hence, although calcium bisulphate may eventually crystallise as calcium sulphate, possibly as a result of the presence of a seed crystal of calcium sulphate, this will tend to occur away from the resin. Hence blockage of resin pores will be reduced. Treated red mud resulting from the process of the invention may be substantially free from leachable sodium values or may have a markedly reduced leachable sodium content and can be used for addition to cement, as a fluxing agent, as a building material feedstock, or as a source of iron and/or other metal values. In the drawings: Figure 1 is a flow diagram of the process of the invention used for treatment of red mud; Figure 2 is a series of graphs showing the effect, when using a strong cation exchange resin, of changes in the pulp:resin ratio upon the change in sodium concentration in the solids with time in the course of treatment of red mud by the process of the invention; Figure 3 is a series of graphs showing the change of sodium content of the solids with time when using a weak cation exchange resin at various pulp/resin ratios in a red mud treatment process conducted in accordance with the teachings of the invention; Figure 4 is a graph showing how elution of calcium ions in three successive tests varies with the volume of eluate during resin regeneration; Figure 5 is a graph showing acid utilisation in the same three tests; Figure 6 plots two graphs showing elution of sodium ions from a loaded resin using, on the one hand, sulphuric acid and, on the other hand, a mixture of sodium sulphate and sulphuric acid for resin regeneration; Figure 7 provides a comparison of free acid concentration during resin regeneration determined by titration with the corresponding calculated concentrations; Figure 8 is a graph of pH against eluate analysis during resin regeneration; Figure 9 is a plot of sodium distribution as sulphate and bisulphate during the course of elution during resin regeneration; Figure 10 shows a graph of acid utilisation during regeneration of a sodium loaded weak cation resin, assuming (a) sodium sulphate formation and (b) sodium bisulphate formation; and Figure 11 is a flow diagram of a process for regeneration of sodium loaded cation exchange resin produced by treatment of red mud according to the process of the invention. The invention is further illustrated in the following Examples in which all percentages are by weight unless otherwise stated. Example 1. (a) A sample of 250 g of red mud with an analysis of 3.4% Na and 40% moisture was added to 540 ml of water. The resulting pulp was stirred and allowed to settle, after which the supernatant wash water was decanted. The wash water and the solids residue were analysed for sodium content. This washing procedure was then repeated a number of times. After 26 washes the residue contained 1.4% sodium, thus indicating that approximately 40% of the sodium still remained in the mud after 26 washes. The initial wash water had a pH of 12.4; after 26 washes the pH of the wash water was 11.0. The initial wash water contained approximately 3 gpl sodium. After 26 washes the final wash water contained 65 ppm sodium as measured by flame photometry. (b) A solution containing 21.5 g calcium chloride in 50 ml of water was added to a pulp formed by slurrying a further 250 g sample of the same red mud in 540 ml of water. The slurry was periodically sampled over a 12 day period and the solution was analysed for sodium and calcium ions. At the end of the test the pulp was vacuum filtered and analysed. Calcium hydroxide rapidly precipitated leaving sodium chloride in solution and the pH was reduced to 10.5. Over 12 days the calcium level declined and the sodium in solution increased. Sodium release may be due to calcium/sodium exchange or hydrogen/sodium exchange followed by calcium hydroxide precipitation. V 1st the process is effective in lowering the pH, reactio: ^.s too slow for it to deal with sodium retained in sodalites before disposal of the red mud and sodium continues to be released during storage. (c) The procedure of (b) was repeated with a slurry of a further 250 g of red mud in 540 ml of water using, in place of the calcium chloride solution, a solution of 47.85 g of MgSθ4.7H 2 0 dissolved in 30 ml of water was added to give the same molar concentration as the calcium chloride addition. The pH declined to 8.5 due to precipitation of magnesium hydroxide. As in the calcium test (b) the initial pH was reduced but sodium remained in the red mud as sodalites and the long term problem of sodium release remained. (d) Another 250 g sample of the same red mud was treated with 98% sulphuric acid to attain pH5. After a while it was noted that the pH had risen and more acid had to be added to retain the pH at 5. Reaction was relatively slow and it was only after 5 hours that 72% of the total acid requirement had been added. Two overnight periods of standing and 19 hours of stirring were required until the pH was constant. Analysis also showed that the acid had reacted with free lime and had released sodium but that no other ions had been affected. The residue analysed at 0.64% sodium of which 0.53% could be accounted for by the liquor retained with the mud. Analysis of the filtrate indicated, however, only 79% of the feed sodium in the solution. Hence the sodium accountability was poor. The total consumption of acid was 0.125 t/t red mud. Repeated water washing was required to effect further removal of dissolved sodium from the residue which typically retains about 40% moisture. (e) In a further experiment in which the procedure of paragraph (d) was repeated at pH 3, sodium removal occurred at a faster rate but required more acid, i.e. 0.235 t/t red mud. At this pH care must be taken to avoid dissolution of iron. The residue analysed at 0.76% sodium of which 0.46% could be accounted for by retained liquor. The filtrate analysis accounted for 77% of the feed sodium, again indicating poor. sodium accountability. Again a water wash step was required to remove soluble sodium salts. (f) In a further test, using the process of the invention, a slurry of 250 g of wet red mud in 540 ml H 2 0 was stirred with 250 g of Amberlite IR120 resin (a strong acid cation exchange resin) , prepared in the H form. This corresponds to a slurry: resin ratio of 3:1. The hydrogen form resin had been washed extensively until no pH change was detectable in the wash water. After addition to the red mud slurry the pH dropped very rapidly to 5.9 in 1 minute, to 4.05 in 8 minutes, and then rose slowly to 4.85 over 140 minutes. Analysis of the treated red mud solids indicated that over 90% of the sodium content of the red mud and over 50% of the calcium content of the red mud had been removed and that the residue contained 0.22% sodium compared with an initial value of 3.4%. Analysis also indicated that about 30% of the aluminium and silicon content of the red mud had been removed. (g) The procedure of paragraph (f) was repeated and the resin was removed using a 212 μm screen. After 60 minutes 250 ml of fresh resin were added and similar results were observed. Sufficient 280 gpl sulphuric acid solution was then added to attain a pH of 3 following the procedure described above under (d) and (e) . In this case the efficiency of sodium removal from the red mud was about 90% and the efficiency of calcium removal from the red mud was approximately 100%. The residue contained 0.23% sodium. Example 2. 500 g dry weight of red mud in the wet form was slurried in water to form 2.1 litres of pulp. This was stirred with 1400 ml of a strong cation exchange resin, i.e. Purolite C160 TL, in the hydrogen form so as to give a pulp:resin ratio of 1.5:1. The results are summarised in Table I from which it will be noted that 86% sodium removal was effected in 30 minutes. Sodium accountability was close to 100%. Table I The sodium loaded resin was then eluted in a column at a nominal rate of 1 bed volume per hour with successive aliquots each of 1000 ml (0.7 bed volume) of 100 gpl HCl. The results are summarised in Table II. Table II The overall experimental results indicate the sodium balance as set out in Table III . Table III Sodium Balance : In : Liquid 0.65 g Out : Liquid 0.96 g Balance : (OutxlOO ) = 15.24 x 100 Solid 16 .55 g Solid 2.15 g ( In ) 17 .20 - g Eluate 12 . 13 g Total 17 .20 g Total 15.24 g = 88.6% The low mass balance can be attributed to incomplete elution. Example 3 . The procedure of Example 2 was repeated using 595 g dry weight of red mud dispersed in water to form 2.55 litres of pulp. This was stirred with 1000 ml of a weakly cationic exchange resin, i.e. Purolite C105. This resin is described by its manufacturers as being of gel type. The results of this stirring test are summarised in Table IV. Table IV Upon elution with 250 ml aliquots of 100 g/1 HCl solution the results summarised in Table V were obtained. Table V ND means "not detected" . The sodium balance was as set out in Table VI . Table VI In: Liquid 0.86 g Out : Liquid 1.61 g Balance : (OutxlOO ) = 24. 82 x 100 Solid 22 .43 g Solid 6 .01 g ( in ) 23.29 - g Eluate 17 .20 g Total 23.29 g Total 24 .82 g = 106 .6% Example 4. (a) The general procedure using for extraction of sodium from red mud in Example 2 was used in a series of three experiments at pulp:resin ratios of 5:1, 2.5:1 and 1.25:1, using in each case the strong cation exchange resin Purolite C160 TL in the hydrogen form. The results are plotted in Figure 2. It will be seen that extraction of sodium is essentially completed in less than 1 hour. (b) When a weak cation exchange resin, i.e. Purolite C105, was used in place of the strong cation exchange resin then the results of Figure 3 were obtained. Example 5_ A weak cation exchange resin, namely Purolite C105, was loaded, at a 10:1 pulp:resin ratio, to 31.9 g sodium per litre of resin using a slurry of red mud. A portion of the loaded resin was regenerated using (i) 50 gpl sulphuric acid as regenerant and (ii) a regenerant solution containing 35 gpl sulphuric acid plus 22 gpl sodium sulphate, both at 5 bed volumes (BV) per hour. The results obtained upon elution with 50 gpl sulphuric acid solution are set out in Table VII for the first three bed volumes, each subdivided into 5 aliquots. Table VII Some deposition of calcium sulphate crystals was observed. The figures for H 2 S0 4 are calculated figures. The results obtained upon regeneration with an eluant solution containing 35 gpl sulphuric acid and 22 gpl sodium sulphate are shown in Table VIII. Table VIII The sodium sulphate figures given in Table VIII exclude the 22 gpl sodium sulphate already present in the eluant; in other words the total sodium sulphate for aliquot 1A was 50.19 gpl, corresponding to a total sodium content of 16.26 gpl. As in Table VII the figures for H 2 S0 4 are calculated figures. During this experiment it was noted that fine crystals of calcium sulphate precipitated but these were finer than those observed in the preceding experiment (a) in which the eluant did not contain sodium sulphate. However, after 7 aliquots of eluant had been passed through the resin the porous plug support in the elution column blocked. 2 litres of 100 gpl HCl were passed through the bed to dissolve the crystals. The sodium content of the resulting eluate was 1 gpl and the calcium content was 0.89 gpl. The proceeding experiment was repeated but with upflow of eluant (35 gpl H 2 S0 4 plus 22 gpl Na 2 S0 4 ) in order to avoid blockage of the porous plug by CaS0 4 crystals. The results are tabulated in Table IX. Table IX As in Table VIII the Na 2 S0 4 figures exclude the 22 gpl Na 2 S0 4 already present in the eluant solution and the H S0 4 figures are calculated figures. Washing with 2 litres of 100 gpl HCl to dissolve the precipitated crystals of CaS0 gave a liquor with a sodium content of 0.52 gpl and a calcium content of 0.27 gpl. Average peak eluate concentrations averaged over a single bed volume were as set out in Table X. Table X It was found that the percentage of the acid utilised was greater for eluant (ii) than for eluant (i) for the same sulphate content yet the sodium removal was similar. The results from Tables VII to IX are plotted in Figures 4 and 5, which show the concentrations after elution with an increasing number of bed volumes of eluant; Figure 4 shows the calcium concentration of the eluate whilst Figure 5 illustrates the acid utilisation with increasing bed volumes of eluate. From Figure 4 it can be seen that calcium elution is similar in all three experiments and reaches a similar level of supersaturation. Figure 6 illustrates figures for the sodium concentration of the eluate (less the sodium initially present in eluant (ii) ) measured after different numbers of bed volumes of eluate have emerged from the resin bed. Figure 7 shows a comparison of the free acid in the eluate during regeneration of a sodium loaded resin using a mixture of Na 2 S0 4 (22 gpl) and H 2 S0 4 (35 gpl) as determined by titration as compared with concentrations determined by calculation assuming (a) that sodium forms sodium bisulphate and (b) that sodium forms sodium sulphate, while in either case calcium forms calcium sulphate. Figure 7 indicates that sodium in the eluate initially forms sodium bisulphate until all the free acid has been removed, whereupon sodium liberated in the eluate forms sodium sulphate. The titrations at the start and end of the elution indicate the presence of 30 to 35 gpl free acid, showing that the Na S0 4 /H 2 S0 eluant behaves as free acid rather than forming sodium bisulphate. Figure 8 shows graphs of the eluate analysis and of pH plotted against numbers of bed volumes of eluant (regenerant) passing through a bed of sodium loaded weak cation exchange resin, loaded according to the method of Example 2 and eluted according to the procedure of Example 5(c). It is notable how the pH value drops dramatically at about bed volume 2E, although the decline in NaHS0 4 concentration is somewhat less marked. This sudden change in pH may be used in a split elution process to mark the change between a near neutral eluate which can be processed further or discharged and a more acidic eluate suitable for recycle as eluant. Figure 9 shows the measured sodium content of the eluate from elution using a mixed H 2 S0 4 /Na 2 S0 4 eluant together with the calculated proportions of the sodium present as sulphate and as bisulphate. Superimposed are the recorded pH and the quantity of alkali required to neutralise the excess acid and attain pH7. Over the first bed volume virtually no alkali is needed to achieve pH7. Between bed volumes 2 and 3 the peak of the elution is passed and the eluted sodium reports as sodium bisulphate with a corresponding drop in pH and increase in alkali requirement to achieve pH7. In this example elution would be split as pH dropped below 4 and very little alkali would be required to neutralise the first eluate, equivalent to less than 1 gpl acid in the combined eluate. Figure 10 shows the calculated acid utilisation assuming (a) that all the sodium eluted forms sodium bisulphate and (b) that all the sodium eluted forms sodium sulphate. This Figure shows the results of a wide range of elutions of resin loaded under various pulp/resin ratios. Each peak corresponds to an individual elution curve. The horizontal axis represents eluate volume on an arbitrary scale. The peak acid elutions are comparable for 50 gpl acid at 5 bed volumes (BV) per hour and are not dependent on pulp/resin ratio during loading. Similarly at 35 gpl acid, also at 5 bed volumes per hour, the acid utilisation is similar to that at 50 gpl acid, although it is higher at 1 bed volume per hour. Consequently the higher utilisations achieved in the mixed eluant system are attributable to the synergistic effect observed, which is contrary to normal observations when the presence of a cation inhibits elution of that cation. In Figure 11 there is illustrated a possible reaction scheme for regeneration of batches of sodium loaded cation exchange resin which has been used for treatment of red mud in accordance with the invention. In this procedure a batch of sodium loaded resin is regenerated using a mixture of a solution of Na 2 S0 4 /H S0 and eluate (herein called "Second eluate") having a pH, at the flow rate and eluant concentration selected, typically less than about 3 to about 4 obtained from the later stages of regeneration of a previous batch of sodium loaded resin. The initial eluate from the regeneration vessel has a pH typically greater than about 4. That initial eluate requires minimal neutralisation to achieve pH 7. For example, if the pH of the collected initial eluate is 4.8, then addition of sodium carbonate equivalent to 0.05 gpl of cone. H 2 S0 4 suffices to achieve a pH of 7. In the later stages of regeneration, however, the pH drops below 4. The eluate collected in these later stages forms the second eluate mentioned above. The initial eluate, possibly after neutralisation, can be discharged or can be subjected to further processing, e.g. crystallisation to produce sodium sulphate or electrohydrolysis to produce sodium hydroxide and sulphuric acid. The raffinate from such a crystallisation procedure or the acid stream from the electrohydrolysis step can be recycled to provide eluant for elution of further sodium loaded resin. Using a mixed Na 2 S0 /H 2 S0 4 regenerant solution the species present in the initial eluate can be described as an NaHS0 4 /Na 2 S0 4 solution. In the second stages the species present will be NaHS0 4 , Na 2 S0 4 and H 2 S0 4 and at this point the pH drops below 4.

Claims: CLAIMS 1. A process for the removal of sodium values from sodium contaminated solids which comprises contacting a slurry of sodium contaminated solids with a particulate cation exchanger, separating sodium loaded cation exchanger, regenerating said sodium loaded cation exchanger to liberate sodium values therefrom, and recycling resulting regenerated cation exchanger for contact with further slurry of sodium contaminated solids. 2. A process according to claim 1, in which the sodium contaminated solids are selected from the group consisting of red mud, sodium contaminated soil, alumina produced by the Bayer process, coal that has been washed with caustic soda, and steel plant dusts. 3. A process according to claim 1 or claim 2, in which the particulate cation exchanger has a particle size in the range of from about 0.5 mm to about 2 mm. 4. A process according to any one of claims 1 to 3, in which the particulate cation exchanger is an ion exchange resin containing pendant acidic groups selected from sulphonic acid groups, carboxylic acid groups, and phenolic groups. 5. A process according to any one of claims 1 to 4, in which the sodium contaminated solids comprise red mud. 6. A process according to any one of claims 1 to 5, in which the solids are supplied in the form of a slurry containing from about 10 gpl to about 500 gpl of solids. 7. A process according to claim 5 or claim 6, in which the solids are supplied as a slurry in water. 8. A process according to any one of claims 1 to 7, in which the solids are contacted batchwise as a slurry with the cation exchanger. 9. A process according to claim 8, in which the solids:cation exchanger mass ratio is from about 1:1 up to about 10:1. 10. A process according to any one of claims 1 to 9, in which the pH of the pulp is controlled by selection of the solids:cation exchanger mass ratio. 11. A process according to any one of claims 1 to 10, in which the solids are contacted with the particulate cation exchanger in a contactor of the type used in resin-in-pulp or carbon-in-pulp metallurgical processes and in which the solids are continuously supplied to the contactor as a slurry. 12. A process according to claim 11, in which the solids and the cation exchanger pass in countercurrent through the contactor. 13. A process according to claim 12, in which the solids:cation exchanger mass flow ratio is from about 1:1 up to about 10:1. 14. A process according to any one of claims 11 to 13, in which the pH of the pulp is controlled by selection of an appropriate solids:cation exchanger mass flow ratio. 15. A process according to any one of claims 1 to 14, in which the pulp has a pH such that substantial solubilisation of iron or iron oxides does not occur. 16. A process according to any one of claims 1 to 15, in which sodium loaded cation exchanger is regenerated for further use by contact with a regenerant solution containing an acid. 17. A process according to claim 16, in which the regenerant solution also contains sodium ions. 18. A process according claim 17, in which the sodium ions are present as sodium sulphate. 19. A process according to any one of claims 16 to 18, in which the regenerant solution comprises a mixture of sodium ions and sulphuric acid. 20. A process according to claim 19, in which the acid is sulphuric acid and in which the mole ratio of sodium ions to sulphuric acid is less than about 5:1. 21. A process according to any one of claims 16 to 20, in which the regenerant solution contains sulphuric acid and in which the concentration of sulphuric acid is in the range of from about 10 gpl to about 500 gpl. 22. A process according to any one of claims 16 to 21, in which the cation exchanger is a weak acid cation exchanger, and in which the eluate resulting from contact of the regenerant solution with the sodium loaded cation exchanger is divided into an initial eluate having a pH of about 4 or higher and a second eluate having a pH of about 4 or less. 23. A process according to claim 22, in which the second eluate is used to provide at least a part of the regenerant solution. 24. A process according to claim 22 or claim 23, in which the regenerant solution contains sulphurous acid. 25. A process according to any one of claims 16 to 24, in which the pH of the eluate resulting from regeneration of the sodium loaded cation exchanger with the regenerant solution is controlled by control of the volume flow rate of the regenerant solution through the sodium loaded cation exchanger. 26. A process according to any one of claims 16 to 25, in which the pH of the eluate resulting from regeneration of the sodium loaded cation exchanger with the regenerant solution is controlled by control of the composition of the regenerant solution applied to the cation exchanger. 27. Solids treated by a process according to any one of claims 1 to 26. 28. Use of a red mud treated by a process according to any one of claims 1 to 26 for manufacture of cement, as a fluxing agent, as a construction material feedstock, or as a source of iron or other metal values. 29. A process for regenerating a cation exchange resin loaded with metal values selected from sodium values, calcium values, and mixtures thereof, which comprises contacting said loaded resin with a regenerant solution comprising a mixture of sodium ions and an acid. 30. A process according to claim 29, in which the acid is selected from sulphuric acid and sulphurous acid. 31. A process according to claim 29 or claim 30, in which the resin is one which has been used for removal of sodium values from red mud. 32. A process according to any one of claims 29 to 31, in which the acid is sulphuric acid and in which the ratio of sodium ions to sulphuric acid is less than about 5:1. 33. A process according to any one of claims 29 to 32, in which the acid is sulphuric acid and in which the concentration of sulphuric acid ranges from about 10 gpl up to about 500 gpl. 34. A process according to any one of claims 29 to 33, in which the resin is a weak acid cation exchange resin. 35. A process according to claim 34, in which the regenerant solution contains sulphurous acid. 36. A process according to claim 35, in which the eluate resulting from contact of the regenerant solution with the sodium loaded cation exchanger is divided into an initial eluate having a pH of about 4 or higher and a second eluate having a pH of about 4 or less. 37. A process according to claim 36, in which the second eluate is used to provide at least a part of the regenerant solution.

Inventors: WIGHTMAN GRAHAM, WIGHTMAN, GRAHAM
Applicants: DAVY MCKEE STOCKTON, WIGHTMAN GRAHAM, DAVY MCKEE (STOCKTON) LIMITED
Application year: 1993
Main classification: C02F
IPC codes: B01J39, B01J47, B01J49, B09C1, C01F7, C22B26