BACKGROUND OF THE INVENTION Aluminum hydroxide (A1 (OH) 3) is generally prepared by the Bayer process which comprises the steps of dissolving bauxite in concentrated sodium hydroxide (NaOH) at a high temperature and a high pressure to produce a sodium aluminate (NaAl02) solution, filtering the solution to remove insoluble red mud impurities of bauxite, adding gibbsite as a seed in the filtered sodium aluminate solution and then hydrolyzing sodium aluminate to obtain aluminum hydroxide as a white solid.

In the step of removing insoluble solid impurities, if red mud particulates having a diameter of about 1 to 5, um are not precipitated and remain in the pregnant liquor (PL), the whiteness of the aluminum hydroxide product becomes poor and insoluble precipitates are formed when aluminum hydroxide is redissolved in an alkaline solution during a zeolite preparation process. Accordingly, insoluble solid impurities are generally removed by filtration using a polishing filter.

However, a large amount of solid impurities, e. g. , 2 to 10mg/9 of solid impurities, still remain in PL after the filtration using a polishing filter and further filtration is required to produce aluminum hydroxide of a chemical grade. Also, during such filtration using a polishing filter, a large amount of calcium hydroxide (Ca (OH) 2) is used to improve the filtration efficiency.

The use of calcium hydroxide adversely affects the product quality due to

contaminantion of fine particles which can cause to form calcium iron aluminate substance in a poly aluminum chloride (PAC) or zeolite preparation process. As a result, polishing filter process brings about the poor whiteness of the product and thus is not preferred to a PAC and zeolite preparation process as well as Bayer process.

Accordingly, there has existed a need to develop an improved process for filtering solid impurities including fine red mud and calcium hydroxide particles from a strongly alkaline solution such as PL of the Bayer process.

SUMMARY OF THE INVENTION It is, therefore, an object of the present invention to provide an efficient process for filtering PL of the Bayer process to remove solid impurities therefrom.

In accordance with one aspect of the present invention, there is provided a process for filtering a sodium aluminate solution containing solid impurities comprising feeding the sodium aluminate solution containing solid impurities to a porous ceramic micron filter to filter said solution under an applied pressure ; collecting the filtrate and storing a portion of the filtrate in a storage tank; and discharging the retentate from the ceramic micron filter.

BRIEF DESCRIPTION OF THE DRAWINGS The above and other objects and features of the present invention will become apparent from the following description thereof, when taken in conjunction with the accompanying FIG. 1 which shows a schematic view of a process for filtering sodium aluminate solution containing solid impurities in accordance with the present invention.

DETAILED DESCRIPTION OF THE INVENTION The present invention provides a process for filtering Bayer pregnant liquor containing insoluble impurities such as suspended red mud particles having an average diameter of about Igm using a ceramic micron filter (CMF) to efficiently remove insoluble impurities. The filtration process of the present invention may be applied to various other processes requiring the removal of solids, from various solutions under severe conditions such as those encountered during zeolite synthesis and disposal of acid and alkaline wastes.

In accordance with the present invention, CMFs are used to remove solid impurities suspended in high-temperature, strongly alkaline PL without the aid of additives such as calcium hydroxide, and thus, the filtration using a CMF is an environmental-friendly process and gives good filtering efficiency to improve the final products quality, e. g. , improved whiteness of the aluminum hydroxide product.

The CMFs used in the present invention may be of various forms which may be porous or nonporous, hollow, planar, honeycomb-shaped, tubular or a combination thereof. A CMF having a porous composite membrane having a tubular shape and good mechanical strength is preferably used in the present invention.

The CMFs used in the present invention have pores of 0.1 to 1, um, preferably 0.5 to 1 am.

The CMFs used in the present invention may be made of various materials including alumina, zirconia, glass, etc. Preferably, a CMF made of alumina is used in the present invention in order to prevent possible changes in the pore size and structure during prolonged use at a high temperature, and such CMF preferably has an asymmetrical, porous shape which can be produced by coating a porous ceramic membrane on the porous surface of partially fused alumina.

The filtering process of the present invention is shown schematically in

FIG. 1. In order to prevent the feed from becoming concentrated during circulation, a constant flow mode is preferably adapted in the inventive process in which the inflow and outflow rates are constantly controlled to equalize the flow rate of the feed (QT) with the sum of the flow rates of the filtrate (QF) and retentate (QR) (QT = QR + QF) As is shown in FIG. 1, the feed 00 is sent by pump 11 to CMF. The total flow rate (QT) can be controlled by valve 22. The flux and pressure into CMF 77 can be measured with pressure gauge 33. The pressure in the CMF is controlled with flow valve 55 since a proper pressure must be maintained in the CMF to increase the removal efficiency of solid impurities. Also, the flow rates of QR and QF are determined by flow valve 55, and preferably, QR and QF are controlled at a same flow rate with two valves 22 and 55. A portion of the retentate can be recycled into the feed. Valve 44 is closed during normal operation and opened only for recycling. The 3-way valve 66 is used to send the feed to the CMF and drain accumulated solid impurities therefrom by washing the CMF. A portion of the filtrate from the CMF is stored in storage tank 88 for back washing CMF. If necessary, the retentate (QR) can be passed to a secondary CMF.

Valves 22 and 55 play important roles in that the removal efficiency of solid impurities is affected by the pressure and flow rate changes in the CMF.

Accordingly, it is preferred to control the initial pressure applied to the CMF to 1 to 3 kgf/cmf by simultaneously adjusting valves 22 and 55.

Also, it is preferred to establish a cleaning intense point (CIP) for prolonged durability of the CMF when the pressure gauge 33 shows 4.0 kgf/cd.

At the CIP, back washing of the CMF is carried out using the filtrate stored in tank 88 and it is preferred to pump about 0.005 to 0.01 m'of the filtrate per wash under an air pressure of 3 to 10 kgf/cm"for back washing of the CMF.

The present invention will be described in further detail with reference to Examples. However, it should be understood that the present invention is not restricted by the specific Examples.

EXAMPLE 1 The removal efficiency of solid impurities in PL was evaluated at various initial pressures applied to the CMF of 1.0, 1.5, 2.0, 2.5 and 3.0 kgf/cd, using the system shown in FIG. 1. The CMF was a multi-tubular CMF made of alumina having an average pore size of lam (R 4mmxl9 channel, R 30mmxl024mm). The flux of water through the CMF was about 3. 3x103 Q/m2. hr (LMH) under 1 atm. The pressure was controlled with valves 22 and 55, and the flow rate ratio of the filtrate (QF) to the retentate (QR) was maintained at a constant value. Feed PL was the PL (96°C1 °C) of Bayer process containing 0.690 (i 0.01) of Al203/Na2CO3 ratio, 215g/Q (i 5g) of caustic and 4 to 5mg/ of solid impurities. The removal efficiency of solid impurities after filtration was expressed as the concentration of solid impurities calculated when the filtrate passed through CMF was refiltered through GF/C (Whatman, 1. 2, um) commercially used as a standard filter of Bayer PL. The target concentration of solid impurities after filtration was no more than 0.10 mg/9. The cleaning intense point (CIP) was set at 4.0 kgf/cm2 of the pressure of gauge 33. At the CIP, 0. 008 m'of the filtrate stored in tank 88 was sent back to CMF under 7 kgf/cm2 air pressure, to carry out the back washing of CMF. The results obtained are shown in Table 1.

TABLE 1 Initial Flow rate of PL Operating Total Solid impurities Pressure (QT=QR+ QF) time (CIP) yield after filtration (kgM) (m'/hr) (min) (m') (mg/Q) 1.0 1.02 176 2.992 < 0. 20 1.5 1.18 164 3.224 < 0. 10 2.0 1.35 148 3.330 0.00 2.5 1.55 140 3.616 0.00 3.0 1.66 112 3.098 < 0. 10

As can be seen from Table 1, as the initial pressure increased by 0.5 kgfcd, the flow rate of PL increased while the operating time was reduced.

When the initial pressure was 2.5 kgf/cm2, the maximum allowable CIP of 4.0 kgf/cm2 was reached after 140 minutes operation and a total yield of 3.616 ruz of solid impurity-fiee PL was obtained.

EXAMPLE 2 The procedure of Example 1 was repeated except that the initial pressure applied to CMF was set at 2.5 kgf/cm2 and the changes in the pressure in CMF and the flow rate of PL with operating time were evaluated.

The results are shown in Table 2.

TABLE 2 Operating Pressure Flow rate of PL (m3/hr) Solid time of CMF impurities after (min) (kgflcd) Filtrate Retentate Total PL filtration (QF) (QR) (Or) (rng/ 1 2. 5 0. 82 0. 83 1.65 0. 10 10 2.5 0.82 0.82 1.64 0.00 20 2.5 0.81 0.83 1.64 0. 00 40 2.5 0. 83 0. 82 1.65 0.00 60 2.5 0.81 0. 83 1.64 0.00 80 2.7 0. 80 0.78 1. 58 0.00 100 3.1 0. 78 0.76 1.54 0.00 120 3.5 0.75 0.76 1. 48 0.00 140 3.9 0.69 0.71 1.40 0.00 average 0.79 0.79 1. 58

As can be seen from Table 2,0. 1 mg/ of solid impurities remained in the filtrate in the early stage of operation. However, no solid impurity remained after 10 minutes operating time. Also, the pressure applied to

CMF was constant until the operating time reached 60 minutes and then suddenly increased after 100 minutes of operation. The maximum allowable CIP of 4.0 kgf/cmZ was reached at 143 minutes of operation, at which the average flow rate of PL was 1. 58m3/hr.

EXAMPLE 3 The procedure of Example 1 was repeated except that the initial pressure applied to CMF was set at 2.5 kgf/cm2 while varying the concentration of solid impurities among 2,5 and 10 mg/, and the removal efficiency of solid impurities was evaluated. The results are shown in Table 3.

TABLE 3 Solid impurities Operating time Total flow rate of Solid impurities suspended in PL (min) PL (QT) (m'/hr) after filtration (mg/. ) 2.0 185 1.67 < 0. 10 5.0 143 1.55 < 0. 10 10 86 1. 28 < 0. 10 As can be seen from Table 3, the concentration of solid impurities after filtration was less than 0.10 mg/ despite of increased amounts of solid impurities suspended in PL.

EXAMPLE 4 The procedure of Example 1 was repeated except that the concentration of solid impurities suspended in PL was set at about 4 to 5 mg/ and the back washing efficiency was evaluated at various air pressures.

The results are shown in Table 4.

TABLE 4

Air pressure Operating time Total flow rate of (kgf/cmz) (min) PL (m3/hr) 3 133 1.50 5 145 1.57 7 143 1.55 10 146 1.54 As can be seen from table 4, when the air pressure was 3 kgf/cd, the operating time was reduced but the solid impurities were not completely removed from the filter. However, when the air pressure was 5 kgf/cm2 or more, the operating time and the total flow rate of PL remained more or less constant, which suggests that the CMF was satisfactorily free of solid impurities.

EXAMPLE 5 The procedure of Example 1 was repeated except that the CMF having an average pore size of 0. 5 um was used. The flux of water through this CMF was about 2. 6x103 t/m2. hr (LMH) under 1 atm. The results are shown in table 5, which are compared with the results evaluated with the filter having an average pore size of 1 cKm.

TABLE 5 The size Flow rate Operating Total Solid b value of pore of of PL (QT= time (CIP) yield impurities CMF QR + QF) (min) (m') after filtration , um mg/. 0.5 0.92 85 1.303 < 0. 00 1.56 1 1.55 140 3.616 < 0. 00 1.61

As can be seen from Table 5, when the filter having an average pore size of 0. 5, can was used, the total yield and operating time was reduced while the whiteness index was improved showing a reduced b value (Whiteness index (WI) = L*-3b*). That is, the removal efficiency of solid impurities in PL increases as the pore size of the filter is reduced.

EXAMPLE 6 The procedure for precipitating aluminum hydroxide using the filtrate collected from Example 1 was carried out using 40 g/ of a seed (average diameter 53, am) for 48 hours under the conditions of start temperature of 75C, final temperature of 55 °C and agitating speed of 140rpm.

Whiteness index (WI) of the aluminum hydroxide thus precipitated was evaluated by calculating Hunter equation,"L-3b"from L and b* values measured with X-rite (SP-68 Series, X-rite Co. Ltd.).

COMPARATIVE EXAMPLE 1 The procedure for precipitating aluminum hydroxide using PL which was not filtered through CMF was repeated 3 times. The whiteness index (WI) of the aluminum hydroxide thus precipitated was evaluated as in Example 6.

The results thus obtained are shown in Table 6.

TABLE 6 Start Precipitated aluminum hydroxide pressure Avearge L a b WI particle size (, cem) Comparative 88.54 93. 98 0.67 2.69 85. 91 Example 1 85. 44 93.80 0.62 2.64 85. 88 86. 01 94.52 0.59 2.70 86. 42 1.0 86. 70 95.37 0.57 1.69 90.30 Example 1 1.5 88. 80 95.92 0.51 1.61 91.09 2.0 86.50 96.20 0.49 1.61 91.37 2.5 85. 44 96.13 0.52 1.61 91. 81 3. 0 87. 36 96.15 0.50 1.64 91.23

I a* serves as a reference As can be seen from Table 6, in the case that aluminum hydroxide was precipitated from the filtrate passed through CMF according to Example 1, the whiteness indexes markedly increased as compared with those of Comparative Example 1.

While the invention has been described with respect to the above specific embodiments, it should be recognized that various modifications and changes may be made to the invention by those skilled in the art which also fall within the scope of the invention as defined by the appended claims.