Claims:We Claim:
1. A method of preparing stable poly aluminium chloride (PAC)liquid by reacting alumina trihydrate with hydrochloric acid (HCl) wherein a salt of a mineral acid other than HCl and H2SO4 is added during the reaction.
2. The method as claimed in claim 1, wherein the salt is selected from the group comprising polyaluminium phosphate, Al salt of nitric acid, metal phosphate, sodium methexaphosphate, Al salt of phosphoric acidand combinations thereof.
3. A method of preparing stable poly aluminium chloride (PAC) liquid by reacting alumina trihydrate with hydrochloric acid (HCl) wherein a salt of a mineral acid other than HCl and H2SO4is added to the PAC formed at the end of the reaction.
4. The method as claimed in claim 3, wherein the salt is selected from the group comprising poly aluminium phosphate, Al salt of nitric acid, metal phosphate, sodium methexaphosphate, Al salt of phosphoric acid and combinations thereof.
5. A method of improving the stability of poly aluminium chloride (PAC) powder dissolved in water, the method comprising adding 0.1 to 5% of a mineral acid other than HCl and H2SO4 to the solution of powdered PAC dissolved in water.
6. The method as claimed in claim 5, wherein the acid is selected from the group comprising nitric acid, phosphoric acid and combinations thereof.
7. A method of improving the stability of poly aluminium chloride (PAC) powder dissolved in water, the method comprising adding a salt of a mineral acid other than HCl and H2SO4to the solution of powdered PAC dissolved in water.
8. The method as claimed in claim 7, wherein the salt is selected from the group comprising Al salt of nitric acid, metal phosphate, sodium methexaphosphate, Al salt of phosphoric acid and combinations thereof.
9. A method of preparing stablepoly aluminium chloride (PAC) powder comprising spray drying PAC liquid prepared by any one of the methods claimed in claims 1 or 3.
, Description:FIELD OF THE INVENTION
This invention relates to amethod of preparing stable Poly aluminium chloride (PAC) liquid and a method of improving the stability of PAC powder.

BACKGROUND OF THE INVENTION
Polyaluminium chloride (PAC) is manufactured in both liquid and powder forms. PAC is widely used as a coagulant/ flocculant in water purification, in treatment of drinking / potable water, and in wastewater treatment. Liquid PAC is manufactured by reacting alumina trihydrate with hydrochloric acid (HCl) at higher temperature and pressure.

General formula of PAC is [Al(OH)aClb]nwhere a and b are variable and a> 1.05. Ordinarily liquid PAC contains 17-18% Al represented as Alumina. This percentage of Al can be varied and compounds can be added to improve the efficiency of the PAC. For logistical reasons, often PAC is spray dried into powderform which contains around 30% Al represented as Alumina. Prior to use, the powdered PAC is dissolved in water such that the end user has a solution of PAC having 10-18% Al represented as Alumina.

Both forms of PAC, i.e. the liquid and the powder dissolved in water, show a white gelatinous material which settles at the bottom after about 1 to 10 days of the production of a batch of PAC. In fact, this settling tendency is higher for PAC powder dissolved in water due to partial decomposition of PAC during drying.

WO 2001044110 A1teaches the addition of boric acid, sodium metaborate and sodium tetraborate for stabilization of PAC. Sodium gluconate, glucose, glycerol, mannitol, tartaric acid, citric acid, sodium citrate L-ascorbic acid, lactic acid, glacial acetic acid and potassium dihydrogen orthophosphate have also been evaluated at 1% concentration for improving stability of PAC but the turbidity of the PAC solution was found to exceed 100 NTU (Nephelometric Turbidity Units) in less than 2 weeks.

There is a need for delaying the settling of the white jelly-like material in PAC and thereby improving the stability of the PAC liquid and PAC powder.

SUMMARY OF THE INVENTION
According to an embodiment of the invention there is provided a method of preparing stable PAC liquid by reacting alumina trihydrate with hydrochloric acid (HCl) wherein a salt of a mineral acid other than HCl and H2SO4 is added during the reaction or is added to the PAC formed at the end of the reaction.

According to yet another embodiment of the invention there is provided a method of improving the stability of PAC powder, the method comprising adding a0.1 to 5%of a mineral acid other than HCl and H2SO4 to the solution of powdered PAC dissolved in water.

According to yet another embodiment of the invention there is provided a method of preparing stable PAC powder, the method comprising adding a salt of a mineral acid other than HCl and H2SO4 in the PAC liquid before or after the reaction and spray drying the same to powdered PAC.

According to yet another embodiment of the invention there is provided a method of improving the stability of PAC powder dissolved in water, the method comprising adding a salt of mineral acid other than HCl and H2SO4 to the solution of powdered PAC dissolved in water.

DETAILED DESCRIPTION OF THE INVENTION
In the method of preparing PAC according to an embodiment of the invention, salt of a mineral acid other than HCl and H2SO4 is added during the reaction or is added to the PAC formed at the end of the reaction. Alternatively, the stability of PAC powder dissolved in water can be improved according to an embodiment of the invention by adding a salt of mineral acid other than HCl and H2SO4 to the solution of powdered PAC dissolved in water.

The salt of acid is selected from the group comprising Al salt of nitric acid, Al salt of phosphoric acid, metal phosphate, sodium metahexaphosphate, poly aluminium phosphate and combinations thereof.

The PAC prepared according to an embodiment of the invention shows improved stability i.e. settling of white gelatinous matter does not occur after 20 to 30 days as seen conventionally. PAC liquid stability of up to 4 to 6 months, PAC powder stability up to 8-12 months and stability of solution of PAC powder dissolved in water up to 4-6 weeks has been observed in accordance with any embodiment of the invention. The added stabilizer reacts with aluminium species and forms a complex with polymeric species. Addition of said stabilizer prevents crystallization of aluminium species by keeping them in suspension.

The following experimental examples are illustrative of the invention but not limitative of the scope thereof:

Example 1:
80 g of alumina trihydrate was suspended in 700 g of phosphoric acid and heated on hot plate for 2 hours at near to boiling temperature. The material was further heated on steam bath for 3-4 hours. A small amount of water was also added carefully while heating to maintain the concentration of solution.
The amount of alumina trihydrate was also varied from 200 g per liter of phosphoric acid to traces. This allowed to have varying degree of polymerization and molecular weight of poly aluminium phosphate.

Example 2:
Approximately10500-10700 kg of 33% HCl was taken in glasslined reactor, 165 to 200 kg of poly aluminum phosphate as prepared in Example 1was taken and 4500-4700 kg of alumina trihydrate powder was added. Reaction was performed at 160 °C for 3 hours and 800-1000 kg of water was added. Material was cooled to 60 °C and filtered using filter press. Turbidity of PAC liquid with and without poly aluminium phosphate with time is given in Table 1.
Table 1: Turbidity (NTU) of PAC liquid samples with time
Day
Sample 1 11 22 35 52 69 79 94 108 126 136 149
Control Batch 01 38 45 54 63 89 102 112 124 167 186 239 320
Control Batch 02 14 22 21 35 44 53 58 71 127 150 153 165
Stabilizer Batch 01 16 28 24 30 38 30 33 40 43 48 49 73
Stabilizer Batch 02 10 21 19 28 26 23 22 29 38 45 45 54

Similarly, metal phosphate, sodium methexaphosphate, were also found to have a similar effect as poly aluminum phosphate mentioned above.
Example 3:
PAC samples were prepared as per Example 2, however thepoly aluminium phosphate was added after the PAC was formed, not during the reaction. The concentration of poly aluminium phosphatewas varied from 0.4 to 3.4 %. After addition of poly aluminium phosphate, sample and control were heated at 60-80 ºC for 30 minute in stoppered flasks. The same were incubated at ambient temperature for stabilization study. Turbidity of the samples was measured at different time intervals for evaluating the stability of samples (Table 2). There was higher decomposition of the polymer in control sample and PAC with poly aluminium phosphate was very stable even after 233 days.
Table 02: Turbidity (NTU) of PAC liquid samples with time
Days
Samples 01 15 27 45 56 71 86 106 130 154 180 207 225 233
Control 2.1 2.4 2.7 2.8 5.6 12 12 34 45 74 125 530 903 1024
0.4% 2.4 2.3 2.0 2.3 4.2 5.7 8.1 10 12 27 25 29 32 40
0.9% 2.5 2.4 2.3 2.2 3.3 5.8 7.8 9.1 12 25 10 10 11 10
1.7% 2.4 2.4 2.3 2.1 4.8 6.0 6.1 7.3 9.1 23 9 9 9 10
3.4% 2.7 2.2 2.0 2.0 4.8 6.0 6.2 6.2 8.1 9 9 10 9 10

Example 4:
PAC powder having ~30% alumina content was dissolved in water to have 17.1% alumina solution. The solution was spiked with phosphoric acid to evaluate the effect of the same (Table 3) where the dosages of phosphoric acidvaried between 0.55 to 2.22% of powder. There was increase in turbidity in all the samples except those with highest dosage of phosphoric acidand samples with phosphoric acidwere more stable. It revealed that there is stabilization of PAC powder with increase in dosages of phosphoric acid.
Table 3: Impact of phosphoric acid on stability of solution from PAC powder
Sample Stabilizer dosages Turbidity, NTU
Day 1 Day 7 Day 12
Control Nil 38 59 139
Sample 1 0.55 g/100 g Powder 36 45 82
Sample 2 1.11 g/100 g Powder 36 44 68
Sample 3 1.66 g/100 g Powder 36 41 56
Sample 4 2.22 g/100 g Powder 36 30 42

Example 5:
PAC powder having ~30% alumina content was dissolved in water to have 17.1% alumina solution. The solution was spiked with poly aluminium phosphate to evaluate the effect of stabilizer (Table 4) where the dosages of stabilizer varied between 0.55 to 2.22% of powder.There was increase in turbidity in all the samples except those with highest dosage of poly aluminium phosphate, Samples with poly aluminium phosphate were more stable. It revealed that there is stabilization of PAC powder with increase in dosages of poly aluminium phosphate.
Table 4: Impact of poly aluminium phosphate on stability of solution from PAC powder
Sample Stabilizer dosages Turbidity, NTU
Day 1 Day 7 Day 12
Control Nil 38 59 139
Sample 1 0.55 g/100 g Powder 36 48 99
Sample 2 1.11 g/100 g Powder 36 44 74
Sample 3 1.66 g/100 g Powder 36 40 57
Sample 4 2.22 g/100 g Powder 36 31 49

Example 6:
Stable PAC liquid produced as described in Example 2 was introduced to spray drier and dried. Thereafter, this dried stable PAC powder was reconstituted with water and made into a solution. Turbidity of PAC solution made using stable PAC powder and control sample is given in Table 5. Initial turbidity of control PAC samples (i.e. without poly aluminium phosphate) was very high and crossing the target level of 100 NTU. There was negligible decomposition of PAC polymer with poly aluminium phosphate during drying and turbidity of PAC solution remained less than 100 NTU even upto 40 days of observation, whereas the same for control samples was around 1000 NTU.
Table 5: Impact of poly aluminium phosphate on stability of solution from PAC powder
Sample Day 01 Day 03 Day 07 Day 16 Day 23 Day 27 Day 34 Day 40
Control Batch 01 244 240 242 302 634 1008 1835 2012
Control Batch 02 150 151 155 222 373 461 628 1017
Control Batch 03 139 140 139 187 268 362 516 817
Stabilizer Batch 01 54 55 54 56 59 62 71 79
Stabilizer Batch 02 85 89 86 86 87 88 91 94
Stabilizer Batch 03 50 51 52 53 52 56 54 59

It is clear from the above examples that the stability of PAC powder, when dissolved in water, and PAC liquid is greater when the PAC liquid is made or PAC powder is reconstituted as per an embodiment of the present invention. The white gelatinous material causing turbidity takes much longer to show up in PAC prepared according to an embodiment of the present invention, and even when this turbidity does appear, the examples show that this is significantly less than the turbidity appearing in PAC solutions which are prepared by conventional methods.

The above examples are non-limiting. The invention is defined by the claims that follow.