Patent Publication Number: US-2004052706-A1

Title: Process for removing contaminants from bayer liquors

Description:
TECHNICAL FIELD  
       [0001] The present invention relates to a process for removing contaminants from a Bayer process stream and, more particularly, to a nano-filtration process for removing organic contaminants from a Bayer process stream.  
       BACKGROUND  
       [0002] The Bayer process was first developed in 1888 and presently accounts for over 90% of the world&#39;s alumina production. The process utilises a digestion solution at elevated temperature to digest aluminium hydroxides or oxy-hydroxides (termed “alumina” in this context) from bauxite or other aluminium containing ores. The digestion solution is typically caustic soda but other caustic solutions in which alumina can be dissolved may be used. For example, the digestion solution may be potassium hydroxide or ammonium hydroxide.  
       [0003] To digest the alumina ore, the caustic solution is typically at a temperature in the range of 150-280° C., with the temperature used being largely dependent upon the nature of the alumina ore. Typically, an alumina rich liquor phase is separated from undissolved impurities by settling and alumina is recovered from the liquor phase by precipitation of aluminium hydroxide (typically gibbsite) crystals. The aluminium hydroxide crystals are calcined to produce aluminium oxide.  
       [0004] The undissolved impurities resulting from digestion of the bauxite with caustic soda at clevated temperature is commonly referred to as red mud and typically comprises inert iron oxides, titanium oxides and silica compounds Prior to discharge from the process, the red mud is typically washed with water to recover entrained caustic soda in solution.  
       [0005] Organic material enters the Bayer process from a variety of sources with the bauxite ore typically accounting for the majority of the total organic carbon of Bayer liquors. Organic contaminants may also be introduced into Bayer processes from other raw materials used in Bayer processes, such as defoaming agents, flocculants and caustic soda. On digestion in the Bayer process typically over 50% of the organic carbon and sometimes to over 90% is extracted into the liquor and its concentration gradually builds up to an equilibrium level, (dependant on organic losses from the process) with recycling of the liquor.  
       [0006] The presence of organic contaminants typically causes problems in the precipitation stage resulting in lower yields of acceptable quality alumina, colouring of the is liquor, lowering of red mud settling rates, and consumption and lower availability of caustic.  
       [0007] Various techniques have been proposed for removing organic contaminants from Bayer process liquors but very few techniques have found commercial acceptance.  
       [0008] U.S. Pat. No. 4,280,987 relates to the so-called “liquor burning” technique. Other techniques for removing organic contaminants include a Wet Oxidation process with pressurised oxygen (JP 30548/1970), a Wet Oxidation process with manganese or copper based catalysts (WO 85/05095, AU 200017606, AU 200171796), an ultraviolet irradiation method (JP 20097/1974), an adsorption method (U.S. Pat. No. 5,093,092/AU86730/91), a bio-degradation method for oxalate and associated organics (U.S. Pat. No. 5,271,844), a cooling method (U.S. Pat. No. 3,508,884), a magnesium hydroxide co-precipitation method (UK 1520352, JP 130692/1976), a causticising method (U.S. Pat. No. 3,120,996 and U.S. Pat. No. 3,341,286), a sodium oxalate removing method (U.S. Pat. No. 3,649,185, U.S. Pat. No. 3,372,985, U.S. Pat. No. 3,899,571. JP 11480/1973, JP 398/1978 and JP 400/1978), and a bauxite burning method (JP 21395/1972).  
       [0009] The liquor burning technique of U.S. Pat. No. 4,280,987 has found limited commercial acceptance due to the significant cost of energy required to evaporate liquor and calcine the resulting residue. Its use has also been dogged by environmental emission issues particularly with odour and alkaline dusts to the surrounding populations.  
       [0010] A recent development has combined the processes of liquor burning and wet oxidation (WO 00/10918) which teaches that a weakness of each process is overcome by utilising a strength of the other process.  
       [0011] A number of membrane based techniques have also been proposed for removal of organic contaminants in Bayer processes.  
       [0012] U.S. Pat. Nos. 4,676,959 and 4,786,482 teach removal of colour producing humate material from a Bayer solution by passing the solution through a semi-permeable membrane. U.S. Pat. No. 4,678,477 teaches purification of a Bayer solution by contacting the solution under pressure with the outside of a plurality of porous hollow fibres coated with a semi-permeable membrane, preferably polysulfone fibers coated with a sulfonated polysulfone membrane, with removal of organic sodium salts, humates and sodium oxalate. The membranes taught by U.S. Pat. Nos. 4,676,959, 4,678,477, and 4,786,482 are relatively loose, high molecular weight cut-off, polysulfone membranes.  
       [0013] WO 88/07975, AU 15433/88, EP 310553 and EP 310554 teach removal of Bayer organics via ion exchange and dialysis using at least one anion perm-selective membrane, at least one cation perm-selective membrane and at least one bi-polar water-dissociating membrane. This is claimed to purify Bayer process streams by recovery of sodium hydroxide from sodium carbonate, oxalate, chloride, sulfate, fluoride, etc thereby removing impurities and organics for disposal and returning purified permeate to the process. Seed wash liquor and washer streams are mentioned as suitable feed streams.  
       [0014] AU 16258/95, WO 95/27681 relates to the use of nano-filtration membranes in reclaiming caustic from dilute alkaline cleaning effluents containing 2-5% caustic (for example from the food, biotechnological and pharmaceutical industries) with recovery of chelating agents (eg EDTA, NTA, etc).  
       [0015] To date, no membrane based technique has found commercial acceptance in a Bayer process to remove liquor organics or other impurities and to thereby improve liquor purity and productivity.  
       [0016] Bayer process streams have complex compositions and caustic strengths ranging from low (eg. 5 g/L free caustic) to high (eg. in excess of 150 g/L free caustic). It is well accepted in the industry literature that the types and molecular weight or size of organics present in a Bayer plant liquor will depend mainly on the type of bauxite feed and the digestion conditions, particularly whether it is a low temperature plant (eg 140-170° C.) or high temperature Bayer digestion plant (eg. 230-280° C.).  
       [0017] Recently, organic characterisation work has been performed on both a low temperature Bayer plant liquor [M. A. Wilson, A. V. Ellis, G. S. H. Lee, H. R. Rose. X. Lu &amp; B. R. Young,  Industrial  &amp;  Engineering Chemistry Research,  38 (12) 1999, pp 4663-4674] and a high temperature plant liquor [D. E. Smeulders. M. A. Wilson, H. Patney and L. Armstrong,  Industrial  &amp;  Engineering Chemistry research,  39 (10) 2000, pp 3631-3639]. These studies separated plant organics generally according to molecular size in solution into several fractions ranging from &lt;1200 molecular weight (Daltons) to &gt;300,000 Daltons. Most of the Bayer plant organic material was &lt;1200 Daltons in molecular weight, 75% in the low digest temperature case, and &gt;87% in the high temperature case, notwithstanding that different bauxites are processed in each case.  
       [0018] Further unpublished studies [L. Armstrong, V. Mitchell. K. Adams, D. E. Smeulders and M. A. Wilson “Impact of organic fractions from Weipa high temperature digest on gibbsite and oxalate precipitation”, Sixth International Alumina Quality Workshop, submitted for publication, 2001] indicate that a major amount of the organic compounds in a high temperature Bayer plant processing bauxite from Weipa in Australia is &lt;250 molecular weight  
       [0019] Throughout this specific the below listed nomenclature will be utilised.  
       [0020] “Free caustic” (fc) denotes caustic present as sodium hydroxide in solution (expressed in g/L as Na 2 CO 3  equivalent).  
       [0021] “Caustic” (C) denotes caustic present as sodium hydroxide (ie “free caustic”) plus sodium aluminate (mostly NaAl(OH) 4 ) also expressed in g/L as Na 2 CO 3  equivalent.  
       [0022] “Pure caustic solutions” contain just sodium hydroxide, ie just “free caustic”.  
       [0023] “Caustic aluminate liquors” contain sodium hydroxide and sodium aluminate and possibly other impurities such as carbonate. Aluminium concentration for these liquors is expressed as g/L Al 2 O 3  equivalent.  
       [0024] “Soda” as S is the sum of caustic and sodium carbonate, expressed as g/L Na 2 CO 3 .  
       [0025] “Total Organic Carbon” (TOC) of process streams is expressed as carbon, in reality present as sodium salts of a range of molecular weight organic compounds.  
       [0026] “Non Oxalate Organic Carbon” (NOOC) is organic carbon other than sodium oxalate.  
       [0027] “Nano-filtration membrane” is defined as a membrane having a molecular weight cut-off (MWCO) range of 100-1000 Daltons for uncharged organic molecules, as is the usual membrane industry convention.  
       [0028] “Permeate” is defined as the solution which is forced under flow pressure through the nano-filtration membrane and has a resulting composition according to the membrane&#39;s MWCO, the size of ions in solution, and other operating factors and membrane characteristics.  
       [0029] “Retentate” or “Concentrate” is defined as the solution which is not forced through the nano-filtration membrane and has a resulting composition according to the membrane&#39;s MWCO, the size of the ions in solution and other operating factors and membrane characteristics. The terms “Retentate” and “Concentrate” are used interchangeably.  
       DISCLOSURE OF THE INVENTION  
       [0030] In a first aspect, the present invention provides a process for removing organic contaminants from a Bayer process stream of a Bayer process, the process including passing the process stream through a nano-filtration membrane to form a retentate containing organic contaminants and a purified permeate.  
       [0031] Preferably, a portion of the retentate is purged from the Bayer process and/or subjected to further processing for recovery of chemical values therefrom, for example, in a wet oxidation or liquor burning unit. The whole of the retentate may be purged and/or further processed. However, a further portion of the retentate is preferably recycled as feed to the nano-filtration membrane. Preferably, the further portion is the balance of the retentate.  
       [0032] Preferably, the permeate is returned to the Bayer process.  
       [0033] In a second aspect, the present invention provides an apparatus for purifying a Bayer process stream containing organic impurities, the system including:  
       [0034] a filtration module housing a nano-filtration membrane having a feed side and a permeate side,  
       [0035] means for introducing the Bayer process stream as a feed stream to the feed side of the membrane at super atmospheric pressure,  
       [0036] means for removing a purified stream from the permeate side of the membrane, and  
       [0037] means for removing an impurity bearing retentate stream from the feed side of the membrane.  
       [0038] The process of the present invention is applicable to a variety of Bayer process streams, including seed wash liquor, lime-treated seed wash liquor, washer overflow liquor, spent plant liquor (including diluted with water or other low caustic liquor), liquor to digestion (including diluted with water or other low caustic liquor), pregnant liquor (including diluted with water or other low caustic liquor), or any other caustic aluminate liquors including a product stream of a liquor purification process. Any convenient arrangement or configuration of membranes may be employed including banks of membrane elements in series or parallel. However, for increased throughput at minimal capital costs, the available surface area of membrane within a membrane element is preferably maximised by spiral winding the membrane or the like.  
       [0039] The present invention offers improved performance of Bayer plants. For example, Bayer plants which purge organic containing streams such as spent liquor or seed wash solution or lime treated seed wash solution or washer liquor to balance organic contaminant input could utilise the present invention to recover alkali rich permeate for re-use in the process and purge a concentrated retentate stream rich in organic contaminants, thereby reducing alkali losses, reducing the volume of purged material, and improving the environmental aspects of the purge operations.  
       [0040] Advantageously, the present invention may be used as a precursor to other contaminant removal processes, such as liquor burning or wet oxidation or some other such organics removal process, with improved performance resulting from the concentration of contaminants achievable by the present invention.  
       EXAMPLES  
       Example 1  
       [0041] Introductory Membrane Analysis  
       [0042] An indication of the caustic tolerance in a pure caustic solution of membranes (A)-(G) specified below in Table 1 was obtained by comparing the average water flux rate (L/m 2 /hr) of the membranes in flat sheet configurations before and after a caustic immersion treatment. The caustic immersion treatment consisted of immersing each membrane in a (non-Bayer process stream) solution of free caustic (230 g/L sodium hydroxide expressed as Na 2 CO 3 ) for 24 hours at 60° C. The average water flux rate (L/m 2 /hr) was measured at constant pressure and temperature for a period of at least 30 minutes. A change of less than 25% after the caustic immersion treatment as compared with the water flux rate prior to the caustic immersion treatment was considered to be indicative of the membrane having caustic tolerance. The results of the indicative testing are summarised in Table 1, together with a summary of whether each membrane met the aforementioned definition of a nano-filtration membrane based on the molecular weight cut-off (MWCO) range designated by the supplier of the membrane.  
                               TABLE 1                                   Indicative                       Caustic   Nano-       Membrane   Trade Name   Supplier   Tolerance   Filtration                  (A)   GR90PP   Dow DDS   Yes   No       (B)   7410   Nitto Denko   No   Yes       (C)   7450   Nitto Denko   No   Yes       (D)   TFC   Fluid Systems   No   Yes       (E)   MPF 34   Koch Membrane   Yes   Yes               Systems       (F)   MPF 36   Koch Membrane   Yes   Yes               Systems       (C)   N30F   Hoescht   Yes   Yes                  
 
       Example 2  
       [0043] Short Term Membrane Performance  
       [0044] Membranes (A) and (E)-(G) having indicative caustic tolerance from Example 1 were analysed to assess their ability to remove organic contaminants from a Bayer process stream. Membranes (B)-(D) were not similarly analysed because of the indicative caustic tolerance results of Example 1.  
       [0045] Membranes were tested as flat sheets in a stirred cell set-up and/or in a cross-flow configuration. Under stirred conditions Bayer liquor to digestion (“L-D”) was placed in a reservoir and allowed to equilibrate prior to sampling. The L-D had a nominal composition of 260 g/L caustic, 0.39 A/C. 0.80 C/S and 153 g/L free caustic.  
       [0046] About 50 ml of the L-D liquor was removed from the reservoir and placed in the stirred cell to ensure coverage of the membrane. This was performed to ensure that when pressure was applied no disturbance of the membrane occurred. Sampling of permeate was performed periodically. At the completion of the test the concentrate was also sampled.  
       [0047] Under crossflow conditions L-D liquor was placed in a feed tank. The feed tank was completely sealed to ensure that no evaporation took place. Sampling of the permeate and concentrate was performed periodically. The L-D liquor was maintained at 60° C. by a water bath and recirculation was sufficient to maintain this temperature throughout the system.  
       [0048] Organic rejections (% TOC Rejection) were calculated in two ways. Under stirred conditions the rejections were calculated with respect to the organic concentration in the initial feed since sampling of the concentrate could not be performed. Equation 1 was used co perform the calculations.  
       %  TOC   (food) Rejection=(1 −TOC   permeate)   /TOC   (feed) )*100   (Eq 1)  
       [0049] where TOC (permeate) =Organic concentration in the permeate  
       [0050] TOC (food) =Organic concentration in the initial feed  
       [0051] However, when testwork was undertaken under crossflow conditions the concentrate was also sampled and as a consequence organic rejections were also given with respect to the concentrate as shown in equation 2 below.  
       %  TOC   (conc) Rejection=(1 −TOC   (permeate)   /TOC   (Conc) )* 100   (Eq 2)  
       [0052] where TOC (conc) =Organic concentration in the concentrate at the time of sampling.  
       [0053] TOC rejections with respect to the concentrate were undertaken due to the fact that the concentration of organics in the initial feed solution was constantly changing. This was due to a number of reasons. Firstly, because the organics in the initial feed were being concentrated and since the concentrate was recycled back to the feed tank the organic concentration in the initial feed was thus increasing. Secondly, the concentration in the initial feed during diafiltration studies was constantly changing due to the addition of water, caustic or spent plant liquor to the feed tank.  
       [0054] Due to the above, the % TOC rejection in most of the crossflow studies were based on the concentration of organics in the concentrate at the time of sampling and not the initial feed concentration. However, for some tests the % TOC rejection was calculated with respect to both the initial feed and the concentrate.  
       [0055] The results of analysis of membrane (A) are set out below in Table 2.  
                                   TABLE 2                                      Flux   Pressure   % Removol                                             Membrane   (L/m 2  · hr)   (kPa)   TOC   Al2O3   Caustic   Conditions               (A)   60-12   900   0.2   0.1   0.1   Stirred cell                  
 
       [0056] The results of analysis of membranes (E) and (F) are set out below in Table 3  
                                       TABLE 3                                      Flux   Pressure   Time   % Removol                                                 Membrane   (L/m 2  · hr)   (kPa)   (hrs)   TOC   Al2O3   Caustic   Conditions                                                     E     2-0.5   600   25   7.5(f)   0   0   Stirred       E     1-0.4   1000   30    15(c)   2   0   Caustic                                   diafiltration                                   crossflow       E   2.5-1.5   1000   9     5(f)   2   0   Crossflow                                   with PTGC                                   pretreatment       E   1.7-0.9   2500   5    10(f)   5   0   Crossflow       E   3.5-3.0   2500   5    10(f)   5   0   Crossflow                                   with PTGC                                   pretreatment       F     5-3   600   25   7.5(f)   0   0   Stirred       F     8-5   1250   25    16(f)   0   0   Stirred       F     7-2   1000   24    14(c)   0   0   Crossflow       F       1000   24    14(c)   0   0   Crossflow and                                   spacers       F    25-20   2500   5    27(c)   7   0   Crossflow       F     8-5   900   20    17(c)   0   0   Water                                   diafiltration                                   crossflow       F    10-4   900   50    25(c)   2   0   Caustic                                   diafiltration                                   crossflow       F    10-3   1000   50    20(c)   2(c)   0   Caustic                                   diafiltration                                   crossflow       F     7-3   900   60   20(c)   2   0   Spent plant                                   liquor                                   diafiltration                                   crossflow       F    10-2   1000   270   26(c)   4.5(c)   0   Spent plant                                   liquor                                   diafiltration                                   crossflow                  
 
       [0057] (f)=Calculated with respect to organics concentration in the initial feed, (c)=Calculated with respect to organic concentration in the concentrate at the time of sampling.  
       [0058] Membrane (G) was tested under crossflow conditions with Bayer plant liquor to digestion at a pressure of 1000 kPa. Organic rejections of 15% were achieved within 40 hours of operation and aluminium trihydrate rejections of 3-4% were achieved within 7 hours of operation and thereafter remained constant. No caustic was rejected during this test. The flux ranged from 4-1 L/m 2 hr.  
       [0059] Examples 1 and 2 indicated that:  
       [0060] (a) based on the indicative caustic Tolerance testing, nano-filtration membranes (B), (C) and (D) were unlikely to be capable of removing organic contaminants from Bayer process streams;  
       [0061] (b) nano-filtration membranes (E), (F) and (G) successfully removed organic contaminants from Bayer process streams; and  
       [0062] (c) notwithstanding the indicative caustic tolerance testing, ultra-filtration membrane (A), which had a MWCO of 2000 Daltons as specified by the supplier, was incapable of removing organic contaminants from Bayer process streams.  
       [0063] Without wishing to be bound by theory, ultra-filtration membranes are believed to be too “loose” with respect to pore size to have any appreciable affect in removing organic contaminants from Bayer process streams.  
       Example 3  
       [0064] Relative Caustic Durability and Performance of Nano-Filtration Membranes for Low Free Caustic (5-50 g/L) Streams  
       [0065] The commercially available or developmental nano-filtration (NF) membranes (as defined by supplier companies) listed in Table 4 below were subjected to more detailed investigation of relative caustic durability and performance over longer time periods.  
                       TABLE 4                       Membrane   Trade Name   Supplier                  (E)   MPF 34   Koch Membrane Systems       (F)   MPF 36   Koch Membrane Systems       (G)   N30F   Nadir (formerly Hoescht)       (H)   P005F   Nadir (formerly Hoescht)       (I)   NF PES 10   Nadir (formerly Hoescht)       (J)   NF-70   Filmtec (Dow Chemical Group)       (K)   DK   Desal Inc / Osmonics       (L)   7470   Nitto Denko                  
 
       [0066] To assess the caustic durability of the membranes, each membrane was immersed in a pure caustic solution or caustic aluminate solution for various lengths of time at 60° C. After immersion, each of the membranes was then tested in a flat sheet membrane rig using equivalent free caustic strength liquor, with TOC rejection, flux rate and aluminate and soda rejections being plotted against time. The degree of change in flux rate and/or decrease in TOC, aluminate and soda rejections indicates the degree of degradation of the membrane during the period of immersion.  
       [0067] The testwork was undertaken under crossflow conditions with the retentate (concentrate) recycled back to the feed using the equipment schematically illustrated in FIG. 1.  
       [0068] Organic rejections were calculated with respect to the concentrate in accordance with equation 2 set out in Example 2.  
       [0069] The initial test work involved immersion of membranes in 20 g/L free caustic solution for over 2 months. The caustic durability and separation performance of the membranes were tested on an analogous low caustic plant stream, lime-treated seed wash liquor (“LSOF”), with the results set out in FIG. 2. The test work demonstrated that six out of eight membranes were stable in 20 g/L free caustic liquor for at least 9 weeks, although one of the six (membrane (F)) gave inconsistent results. Maximum TOC removal was 70-75% for membranes (E) and (L), combined with a high caustic recovery (ie nearly all caustic reporting to the permeate, as desired).  
       [0070] Membranes (J) and (K) were removed from further testing as they had demonstrated degradation and membranes (B) and (c) from Example 1 were included for further testing based on promise shown by membrane (L) from the same supplier (Nitto Denko).  
       [0071] Follow-up test work with a seed wash stream (“B filtrate”) showed only slight degradation for most membranes after 15-20 weeks in 20 g/L free caustic. These results are summarised in FIG. 3. Six out of eight membranes still demonstrated good rejection (&gt;25%) of NOOC which was used to differentiate the impact on sodium oxalate, which is at an unnaturally high level in seed wash, from all other organic carbon in solution. Maximum NOOC rejection was 88% for membrane (E). The two different initial (“zero” time) NOOC rejections for membrane (E) in FIG. 3 indicate that membrane (E) may have deteriorated in the supplier&#39;s storage container prior to testing.  
       [0072] Overall fluxes varied quite significantly over the range of membranes, 15-600 L/m 2 /hr for both LSOF, and B filtrate feed streams. Storage in the caustic solutions usually increased these fluxes but only marginally. Those membranes with higher TOC or NOOC rejections generally had lower fluxes, indicating “tighter” porosity. Fluxes were usually slightly higher for LSOF compared with B filtrate.  
       [0073] Rejections of sodium carbonate for B-filtrate were variable, ranging from 5 to 46%. This is believed to also give a good indication of the relative “tightness” of the various nano-filtration membranes. For process viability lower soda rejection (or more soda permeating) is useful if the retentate is being purged, because the sodium carbonate in the permeate can be subsequently causticised for caustic recovery. The flexibility to tailor/select a membrane with respect to both TOC (or NOOC) and sodium carbonate rejections affords significant advantages when the present invention is coupled with downstream processing. All membranes had low to moderate alumina rejections, 6-27%, with lower values being preferred for process viability.  
       [0074] The results of oxalate rejection from B filtrate are summarised in FIG. 4. The results were substantial for two membranes, 70% for membrane (E) and 50% for membrane (L). The other membranes had rejections of less than 30%. On storage in caustic solution, oxalate rejection decreased for all membranes.  
       [0075] The rejection of oxalate facilitates oxalate removal and destruction as well as recovery of soda. The benefit from the nano-filtration process of the present invention for organics removal or soda recovery may be enhanced by supplementing or replacing existing oxalate removal processes such as lime treatment or microbiological treatment of seed wash solution, or some other such oxalate removal process.  
       Example 4  
       [0076] Relative Caustic Durability and Performance of Nano-Filtration Membranes for Medium Free Caustic (50-100 g/L) Streams  
       [0077] Medium free caustic streams in a Bayer plant would typically include first washer overflow liquor, pregnant liquor or pregnant liquor diluted with water or a low caustic Bayer solution, or spent liquor or liquor to digestion diluted with water or a low caustic Bayer solution, or any other such combination in this free caustic concentration range.  
       [0078] A caustic aluminate synthetic Bayer liquor of medium free caustic strength was used to store membranes prior to performance testing with diluted liquor-to-digestion (L-D). This storage liquor was deliberately made to be under-saturated in alumina at 60° C., using accordingly sodium hydroxide pellets, aluminium metal wire, sodium carbonate and sodium chloride. Liquor analysis gave a caustic of 99 g/L, an A/C of 0.168 and a C/S of 0.875, such that free caustic was 82 g/L.  
       [0079] For performance runs with diluted L-D as feed, L-D was diluted to 140 g/L caustic using DI water. This gave a similar free caustic (80 g/L) to the caustic aluminate storage liquor used for durability testing.  
       [0080] Maximum TOC removal from the diluted L-D stream was 30% with fresh membranes (“zero” storage time).  
       [0081] Storage in the medium free caustic aluminate solution slightly decreased performance of all membranes to give a marginal decrease in TOC rejection. Four of the eight membranes tested however had TOC rejections greater than 20% after 11 weeks of storage. These results are illustrated in FIG. 5.  
       [0082] Fluxes were lower with diluted L-D than for the LSOF and B filtrate feed cases, ranging from 6-200 L/m 2 /hr. Storage in caustic aluminate solution increased these fluxes by 70-200% across the time period studied.  
       [0083] Rejections of sodium carbonate were variable, ranging from 0 to 16%, and is believed to give a good indication of the relative “tightness” of the nano-filtration membranes. Alumina rejections were also low (1-6%). That is, as desired, most soda and alumina permeate the membranes for recovery to the Bayer process.  
       [0084] Studies using pregnant liquor were undertaken using only membrane (F) with results summarised in Table 5 below. Pregnant liquor was heated and evaporated sufficiently to re-dissolve any precipitated gibbsite or other material and then re-diluted to original concentration (220 g/L caustic, 0.72 A/C, 55 g/L free caustic).  
       [0085] Under crossflow conditions 21% of organic matter was rejected within the first 3 hours of operation when no spacers were used in the membrane cell. In this experiment done in batch operation, some precipitation of gibbsite occurred during membrane filtration, with organic rejection decreasing, from 21% down to 45% after 20 hours operation. This is believed to be consistent with reverse osmosis behaviour. If particles of precipitate collect on the surface of the nano-filtration membrane, they have the effect of creating an unstirred boundary layer. As the concentration of retained solutes at the surface increase with time then rejection would decrease. The flux decreased in this test where gibbsite was precipitating and trended from 10 down to 2 L/m 2 /hr.  
       [0086] Further testwork was performed with the incorporation of membrane spacers in order to cause turbulence at the surface of the membrane and thereby improve rejections and flux. This test was undertaken over 48 hours in 6 hour batches. There conditions allowed a longer run time at overall more stable organic rejections. The level of organic rejections also improved with the incorporation of spacers. An organic rejection of 25% was achieved within 23 hours of operation, and this decreased slightly as precipitation of the gibbsite occurred.  
       [0087] Testwork with pregnant liquor was also performed under stirred cell conditions, however, the flux was extremely low and during six hours of operation only enough permeate was collected for organic analysis. Even under these conditions a 21% organic rejection with respect to the initial feed was achieved prior to the precipitation of gibbsite.  
                                       TABLE 5                                           Run                   Flux   Pressure   Time   % TOC           (L/m 2 .hr)   (kPa)   (hrs)   Rejection   Conditions                                                                ˜0.8   800   6   21 (f)   Stirred           10 −&gt; 2   1000   20   21 (c)   Crossflow            7 −&gt; 4   1000   48   25 (c)   CrossfLow                           with spacers                      
 
       [0088] (f)=Calculated with respect to organics concentration in the initial feed, (c)=Calculated with respect to organic concentration in the concentrate at the time of sampling.  
       [0089] This example demonstrates that pregnant liquor (liquor to precipitation) can be treated by nano-filtration membranes with similar organic removal as for diluted spent liquor or washer liquor and that it is desirable to minimize gibbsite auto-precipitation.  
       Example 5  
       [0090] Relative Caustic Durability and Performance of Nano-Filtration Membranes for High Free Caustic (&gt;100 g/L) Streams  
       [0091] High free caustic streams in a Bayer plant would typically include spent liquor or liquor to digestion, or any other liquor combination in this free caustic concentration range.  
       [0092] A caustic aluminate solution of 184 g/L free caustic was used to store membranes prior to performance testing with liquor to digestion (“L-D”).  
       [0093] The storage solution was made under-saturated in alumina at 60° C. using sodium hydroxide pellets, aluminium metal wire, sodium carbonate, and sodium chloride. Analysis of the liquor gave a caustic of 265 g/L, an A/C of 0.295, and a C/S of 0.883; equivalent to a free caustic of 183.7 g/L.  
       [0094] TOC rejections of &lt;20% were achieved after 9 weeks of storage with the results illustrated in FIG. 6.  
       [0095] Fluxes with fresh membranes and L-D feed liquor were low (1-46 L/m 2 /hr). After storage in the caustic aluminate synthetic liquor, the fluxes increase to 6-151 L/m 2 /hr.  
       [0096] Rejections of sodium carbonate were all low, ranging from 0 to 8%, and indicate again that as the ionic strength of feed increases, then generally rejections decrease, for example for both TOC and sodium carbonate. For process viability lower soda rejection (or more soda permeating) is useful if the retentate is being purged, because the sodium carbonate in the permeate can be subsequently causticised for caustic recovery. All membranes had low rejections for alumina, 1-7%.  
       [0097] These results indicate that free caustic in Bayer liquors is the main cause of degradation of membrane performance. This is illustrated graphically in FIG. 7 which plots TOC rejection against free caustic for membrane (H) at zero and 0.2 years storage. The effect is marginal at low to medium free caustic. A similar trend is evident for flux versus free caustic, with the flux decreasing with increasing free caustic.  
       [0098] It is also evident that at lower caustic strength there is a much greater rejection of organics than there is at medium/high free caustic, that is more organics are passing through the membrane at higher caustic strength. This indicates that the membrane process of the present invention could be designed for selective application for desired TOC removal using a modification to free caustic/ionic strength of feed streams, and membranes formulated to different “tightnesses”. There are also other levers available to one skilled in the membrane technology arts, such as pressure, temperature and crossflow velocity—optimisation of these variables are anticipated to improve rejection.  
       [0099] It is anticipated that membrane operation with commercial spiral wound or tubular elements will exhibit enhanced fluxes compared to those in the foregoing examples which utilised a flat sheet mode. 
     
    
    
     BRIEF DESCRIPTION OF DRAWINGS  
     [0100] FIGS.  1 - 7  of the accompanying drawings are referred to in the foregoing Examples. FIG. 8 of the accompanying drawings is a schematic representation of a prior art Bayer process with purge Preferred embodiments of the present invention are described, by way of example only, with reference to FIGS  9 - 14  of the accompanying drawings, in which:  
     [0101]FIG. 9 is a schematic representation of a Bayer process using the nano-filtration membrane organic removal process of the present invention in respect of lime treated seed wash, “LSOF” feed;  
     [0102]FIG. 10 is a schematic representation of a Bayer process using the nano-filtration membrane organic removal process of the present invention in respect of seed wash feed;  
     [0103]FIG. 11 is a schematic representation of a Bayer process using the nano-filtration membrane organic removal process of the present invention in respect of washer overflow, “WOF” feed;  
     [0104]FIG. 12 is a schematic representation of a Bayer process using the nano-filtration membrane organic removal process of the present invention in respect of spent liquor, “SL” feed;  
     [0105]FIG. 13 is a schematic representation of a Bayer process using the nano-filtration membrane organic removal process of the present invention in respect of liquor to precipitation, “L-P” feed; and  
     [0106]FIG. 14 is a schematic representation of a Bayer process using the nano-filtration membrane organic removal process of the present invention in respect of liquor to digestion. “L-D” feed feeding liquor purification.  
    
    
     DRAWING RELATED DESCRIPTION  
     [0107] In a typical Bayer process, controlling the organics concentration level in the main process liquors (pregnant liquor and spent liquor) must take into account the cyclical nature of the Bayer process. Organic carbon originates mainly from the incoming bauxite ore feed to digestion. A proportion of the bauxite organic carbon is extracted during digestion and degraded to be soluble in the process liquor, as sodium salts of organics including oxalate and as sodium carbonate. The degree of extractive degradation and dissolution may range from 50 to &gt;90% depending on the type of bauxite and the temperature and other conditions of digestion.  
     [0108] Since the Bayer process is cyclical the organic contamination of the process liquor will keep increasing with each digestion cycle unless organic removal processes are practised. Further, control of liquor organics to a desired level in Bayer liquor requires design of organics removal processes of a sufficient scale and of an appropriate type to meet the required removal capacity, and the desired selectivity of organics removal for required plant productivity and operational efficiency.  
     [0109] The Bayer design engineer of usual experience can utilise established organic removal processes which balance the above factors based on specific knowledge of the bauxite, the particular Bayer flow sheet and those organics which have specific impacts on the process, viz oxalate seed poisons, hydrate active organics, etc as well as the bulk organic soda components.  
     [0110] In any Bayer process some dilute soda solution and associated organic carbon will be inadvertent losses from the process, for example with last washer underflow solution entrained with red mud, and dilute liquor entrained with product hydrate to calcination (so called “leachable soda”). Nevertheless, to at least balance organic carbon input with the bauxite and moreover to keep liquor contamination to a manageable level with respect to productivity and operational efficiency, tailored organic removal processes must generally be implemented in Bayer plants, especially for bauxites with medium to high organic carbon levels. The simplest form of these is purge, as illustrated in FIG. 8, where a dilute liquor stream is discarded generally to the red mud waste dam. This can be practised only if allowable by local environmental legislation. Such a practice is assisted by other processes such as using neutralisation by addition of sea-water to the waste slurry. Nevertheless, the cost of such a practice is in the loss of soda from the process, which must be made up with extra addition of caustic soda reactant.  
     [0111] In FIG. 8, the notional Bayer refinery is balancing organics input mainly from bauxite by deliberate purging seed wash solution (termed “B-filtrate”). Another organics loss is via lime causticisation of seed wash solution where some of the Non Oxalate Organic Carbon (“NOOC”) is removed as precipitated calcium salts and the entrained lime treated seed wash solution, herein referred to as lime settler underflow (“LSUF”).  
     [0112] Operation of a nano-filtration membrane process of the present invention will now be described as a forerunner to a variety of applications of the present invention.  
     [0113] A dedicated feed tank of appropriate material is preferably arranged to off-take the desired flow-rate of fresh feed, and also any recirculating retentate flow if the system is run to a desired volume concentration factor. Preferably, the feed tank is cone shaped or some other design to allow operation to low volumes co achieve high volume concentration factors which will typically be the desired operation. The feed is maintained at any desired temperature for efficient separation (eg. 60° C.), taking into account the pressure-temperature envelope for operation of the membrane. Temperature control is generally maintained by a suitable heat exchanger in the retentate return line or by a suitable contact or other heater in the feed tank itself.  
     [0114] Feed is preferably pumped via a suitable low pressure pump to a microfiltration pre-filter to remove any solids which may be present in the feed stream. A suitable micro-filtration system would be a cartridge type filter removing down to 10 micron solid material, preferably a design with parallel filter lines, cartridges, isolation valves and drain and vent valves that allow cleaning and cartridge replacement while the process remains on-line.  
     [0115] The feed from the pre-filtration unit is preferably pumped via a suitable high pressure pump to a high pressure housing containing the nano-filtration membrane elements. The high pressure pump is preferably of a design to allow easy change of flow at constant pressure, for example, with a variable frequency drive or some other suitable control. The high pressure pump preferably allows modification and control of pressure up to at least 3000 kPa. Actual pressure will depend on the pressure-temperature operational envelope for the membrane in question.  
     [0116] The nano-filtration membranes are preferably arranged as multiple 4 inch or 8 inch elements in series arrayed in parallel banks which enables a variety of capacity requirements to be met. The membranes can be tubular or spiral wound or some other suitable fabrication, although capital costs are generally minimised with spiral wound elements.  
     [0117] When arranged in cross-flow mode, the retentate or concentrate stream is generally recirculated to the feed tank to a desired volume concentration factor (VCF) while the permeate stream is recovered for subsequent use as a purified stream.  
     [0118] A simple application of a nano-filtration membrane process in the notional Bayer plant of FIG. 8 is to purify the lime treated seed wash solution (“LSOF”) since this is a low ionic strength, low free caustic and low aluminate solution which has some of the organic salts already removed by calcium complexation. This application is illustrated in FIG. 9.  
     [0119] The process illustrated in FIG. 9 runs at relatively high flux with high TOC rejection such that organic removal and soda recovery is sufficient for economic operation, notwithstanding the relatively high capacity needed to offset the dilute concentration of contaminants in the LSOF feed stream.  
     [0120] In the embodiment of FIG. 9, the retentate from nano-filtration membrane treatment of LSOF would be purged to remove organics from the Bayer process, replacing the usual practice of purging seed wash solution (“B-filtrate”) as illustrated in FIG. 8, or operated at greater organics removal to purify Bayer process liquor and improve productivity. This is believed to be a more cost affective development since the purged stream from the membrane process has a higher NOOC/S ratio than seed wash solution. The permeate stream is forwarded to the washer circuit thereby decreasing the organics contribution of washer overflow (WOP) back into liquor to precipitation. The process can be operated with as much flexibility as the prior art process of FIG. 8 with B-filtrate purge, by manipulating the feed flow to the unit, the membrane operating conditions for a given organics rejection/soda recovery and the VCF for a given retentate split. The lower level of oxalate purge with the embodiment of FIG. 9 as compared with the prior art process of FIG. 8 can be compensated by operating the oxalate causticisation unit at greater efficiency or throughput using well-known and practised knowledge in this art, or some other suitable process modification.  
     [0121] A further embodiment of the present invention is illustrated in FIG. 10 which utilises a nano-filtration membrane process on seed wash liquor, with the added selectivity of soda recovery from organics and in some cases from sodium oxalate as well. The retentate stream is purged to waste at a lower soda loss than in the prior art process of FIG. 8. The permeate is returned as feed to an oxalate and NOOC causticisation unit (or other organics processing unit) for removal of remaining oxalate or NOOC. For nano-filtration membranes showing high oxalate rejection, and/or for particular process designs and/or for certain Bayer plant requirements, the permeate may be directed straight co the LSOF stream (not shown in FIG. 10), thereby freeing up capacity in the oxalate and NOOC causticisation unit. The embodiment of FIG. 10 may also be operated at sufficient capacity organics removal to improve liquor purity and plant productivity.  
     [0122] A further embodiment of the present invention is illustrated in FIG. 11 which utilises a nano-filtration membrane process to remove organics from washer overflow (“WOF”) dilution liquor thereby reducing the organic contamination of liquor to precipitation. The permeate stream which has lowered organic contamination is used as dilution liquor to settler feed replacing some or all of WOF as dilution stream, and the retentate is purged for overall organics control. Such a process can be designed and operated conservatively to replace seed wash purge such that soda losses are decreased. Alternatively the nano-filtration membrane process can be designed and operated more aggressively so as to decrease the level of organic contamination in washer overflow and thereby in liquor to precipitation for increased yield and plant productivity.  
     [0123] A further embodiment of the present invention is illustrated in FIG. 12 in which a nano-filtration membrane processes spent liquor feed producing a retentate rich in organics but with lowered soda for purge or for further processing for more cost efficient organics removal. The permeate is sent to evaporation for ultimate return to digestion with lower organics contamination back into the next cycle of the Bayer process.  
     [0124] The embodiment of FIG. 12 has the advantage of requiring a smaller scale nano-filtration membrane process unit and therefore lower capital cost, since the feed stream has higher organics contamination than in the previous embodiments. If operating cost is unduly impacted by membrane fouling or degradation at the higher free caustic level of this feed stream, then a diafiltration mode can be used whereby dilution water or some other suitable low caustic stream can be used for improved performance.  
     [0125] A further embodiment of the present invention is illustrated in FIG. 13 in which a side-stream of liquor to precipitation is the feed stream to the nano-filtration membrane whereby the permeate stream having usual caustic aluminate concentration but depleted organic contaminants is returned to the precipitation process resulting in higher liquor productivity in the Bayer process. It is believed that the organics removed from this stream will be the higher molecular weight organics some of which are known to impact precipitation of gibbsite and/or sodium oxalate in the Bayer process. Additionally, it is believed that a reasonable proportion of the medium molecular weight organics will also be removed which are known from various studies to be prominent in reducing precipitation rate and sizing and impacting morphology of precipitating gibbsite.  
     [0126] The retentate from the embodiment of FIG. 13 can be purged but the economics of doing so will depend very markedly on the degree of soda and aluminate rejection by the nano-filtration membranes. Alternatives to purging may be more economical including using the retentate as a feed stream to an existing organics removal process such as wet oxidation or liquor burning or some other suitable organics removal process, as already described in the literature or practiced in the industry.  
     [0127] In applying the embodiment of FIG. 13 it is desirable to consider the propensity for auto-precipitation of gibbsite or some other solid product during the nano-filtration membrane process. Various means are available to minimise or manage this issue including means to minimise holding time in the membrane elements, means to minimise degree and rate of change/stress in supersaturated solutions, means to control fluctuations in supersaturation during operation, means to minimise contact with surfaces to promote auto-precipitation, and the use of selective chemical additives (such as precipitation inhibitors, sometimes referred to as “precipitation poisons”) to stabilise the supersaturated sodium aluminate liquor prior to contact with the nano-filtration membrane with the precipitation inhibitor chemical being essentially removed by the nano-filtration membrane prior to return to the main liquor to precipitation stream; all of which are well established design or operating principles or are known from general research literature.  
     [0128] A further embodiment of the present invention is illustrated in FIG. 14 which uses liquor to digestion (“L-D”) (or some other liquor stream which is concentrated in organic contaminants) as the feed stream. The embodiment of FIG. 14 has the advantage of requiring a smaller scale nano-filtration membrane process unit and therefore lower capital cost because the feed stream has higher organics contamination than in various previous embodiments. If operating cost is unduly impacted by membrane fouling or degradation at the higher free caustic of this feed stream, then a diafiltration mode can be used to improve performance.  
     [0129] Retentate produced by the embodiment of FIG. 14 is desirably used as a feed stream to prior art organics removal process such as wet oxidation or liquor-burning or some other suitable organics removal process.  
     [0130] Each of the embodiments described in relation to FIGS.  9 - 14  has been described in isolation of the other embodiments. It is to be clearly understood that the present invention includes a plurality of the embodiments being used in the same Bayer plant.  
     [0131] The nano-filtration membrane process of the present invention is preferably selected with a tailored degree of rejection of organics and other contaminants relative to rejections of the main liquor components such as sodium hydroxide, sodium aluminate, sodium carbonate, etc. In this way a combination process can be designed to give an overall optimum cost-and efficiency of organics removal.  
     [0132] Prior art methodology for removing organics (including oxalate) that is deliberately used in Bayer plants may involve a combination of purging, for example of seed wash and accepting soda loss, causticisation of seed wash to recover soda and dispose of organics including oxalate as calcium salts, natural losses for example with washer underflow, wet oxidation of organics to sodium carbonate and sodium oxalate and causticising to recover soda, liquor burning whereby organic carbon is destroyed and soda returns as sodium aluminate. These prior art processes can suffer from high capital or high operating costs, including a loss of valuable soda reactant in the Bayer process. The present invention enables flexibility to be introduced to organics removal management with applicability over a range of caustic strengths found in Bayer plants and the option of using the present invention is combination with prior art methodology.