WO2001044110A1

WO2001044110A1 - Flocculants

Info

Publication number: WO2001044110A1
Application number: PCT/EP2000/012870
Authority: WO
Inventors: Neil Clarke; John Newton; Annette Halliwell; Wendy Wood
Original assignee: Feralco Ab
Priority date: 1999-12-16
Filing date: 2000-12-07
Publication date: 2001-06-21
Also published as: GB2358186A; WO2001044110B1; GB0030092D0; GB2358186B; GB9929637D0; GB0030024D0

Abstract

Polyaluminium chloride solutions may tend to be unstable in storage particularly those having a high basicity or charge density. This may markedly reduce the efficiency of the flocculant. The invention provides an aqueous polyaluminium chloride flocculant solution containing a stabilising quantity of a dissolved boron compound or compounds such as, for example, the boric acids or salts. The stabiliser may be included during the production of the focculant or to an existing flocculant.

Description

Flocculants

This invention relates to flocculants, more particularly to polyaluminium chloride flocculants.

If an aqueous solution of aluminium chloride is treated by the addition of a base it tends to polymerise and, to some extent at least, is thought to form the Alι₃0₄ (OH) ₂a (H₂0) ₈ ³⁺ ion. This basic aluminium chloride material, known as polyaluminium chloride or "PAC" in industry, has been used as a flocculant in a wide range of water or aqueous effluent purification processes. A flocculant in which the sulphate ion is included in addition to the chloride ion is also in common use and is understood to be included herein under the terms "polyaluminium chloride" or ^Λ,PAC". A flocculant in which a proportion of the aluminium is replaced by one or more divalent elements such as magnesium, calcium or divalent iron and/or trivalent elements such as trivalent iron are also understood to be included within the terms referred to. Some such flocculants are disclosed in USP 4566986.

Typical of the industrial processes in which effluents or process liquids which may be treated with flocculants of the types identified above are paper manufacture, in which the cationic charge density of the flocculant is of importance since negatively charged colloidal particles, known in the industry as

"anionic trash" may require to be removed from a liquid, food preparation processes, the effluents from which may be difficult to purify due to the presence of traces of carotene, mining, industrial and power generation in which cooling liquids may require purification, sewage treatment and other processes or treatments in which aqueous liquids contain suspended or colloidal particles which require to be removed.

Polyaluminium chloride solutions may tend to be unstable in storage. This may be due to the presence of impurities deriving from the raw materials used in its manufacture. Also, high basicity or high cationic charge density polyaluminium chloride may tend to gel or to precipitate aluminium hydroxide. This may markedly reduce the efficiency of the flocculant. Instability may show itself by an increase in the turbidity of the flocculant solution and may manifest itself either over months of storage or, in the case of high basicity or high charge density flocculants, in days.

According to the present invention the stability of at least some polyaluminium chloride solutions may be improved by the use of a dissolved boron compound as a stabiliser. This improvement may manifest itself as a lowering of the rate of onset of instability and/or as a lengthening of the usual or optimum storage life of the flocculant solution.

The present invention therefore provides an aqueous polyaluminium chloride flocculant solution containing a stabilizing quantity of a dissolved boron compound or compounds. The invention also provides a low energy process for the production of such a stabilized flocculant for example specifically, a charge- augmented stabilized flocculant, and a process for the purification of waters or aqueous effluents by the use of such a flocculant. By a stabilizing quantity is meant a quantity of stabilizer which will achieve a turgidity of below 100 NTU, preferably below 50 NTU and particularly preferably below 10 NTU for at least 2 weeks, preferably substantially longer, of ambient temperature storage.

The boron compound or compounds used according to the invention may be any which are soluble in the polyaluminium chloride solution at the intended concentration, which are capable of yielding B3+ ions in that solution and which do not contain anions which are unacceptable in the water or effluent to be treated by the polyaluminium chloride flocculant. It is noted that there are in many countries, including the UK, Regulations limiting the maximum content of boron in water intended for human consumption and these may limit or prohibit the purification of potable water using flocculants stabilised according to the invention. The oxy-acids of boron or salts of such acids are preferred for use as stabilisers according to the present invention for example orthoboric acid or the metaborate or tetraborate acids or salts. Suitable salts giving suitable solubility may be the alkali metal or ammonium salts. Another boron compound which may have stabilisiing properties is sodium pyroborate ("borax") or other pyroborates.

The effective quantity of the boron stabiliser may vary according to the particular composition and concentration of the polyaluminium chloride solution is preferably in the general range of 0.25% to 10%, or even up to 15% by weight of the polyaluminium chloride solution, or more, expressed as boric acid. In some cases, for example when the content of calcium in the flocculant is low, as hereafter described, the quantity of boron compound may be below 0.25% for example down to 0.01%, although preferably above 0.05% or, more preferably, above 0.1% on the same basis. For reasons of cost it may be desired to limit the quantity of boron compound to a maximum of 2.5% by weight. The present invention is applicable to the stabilisation of existing commercial polyaluminium chloride flocculant solutions by the inclusion in the solution of a suitable quantity of the boron compound. More usually the invention may be used in conjunction with a treatment of an existing commercial polyaluminium chloride solution to augment its cationic charge density, or in conjunction with the production of a high cationic charge density polyaluminium chloride, for example, preferably that described below, both of which processes might otherwise produce a relatively unstable solution. By high cationic charge density is understood a density of more than 500, but often at least 750, for example at least 1000 and possibly up to 1750 or more m.eq/kg of flocculant solution. The charge density may be measured by the streaming current method with higher charge densities indicating increasing polymeric sizes. The compositions of the invention preferably also have a relative basicity of from 20%, particularly preferably from 25% to 45%, and suitably up to 50% or more.

International Patent Publication WO 99/35090, the content of which is incorporated herein by reference, describes a process for the production of polyaluminium chloride flocculants by the inclusion of one or more magnesium and/or calcium carbonate- containing materials in the aluminium chloride solution. The cationic charge density was found to be considerably augmented when a suitable quantity of the carbonate material had been added. It is envisaged herein that this charge augmentation process may be modified by the use of other soluble carbonates, such as alkali metal or ammonium carbonates, and/or by the use of other basic materials for example alkali metal oxides and, also, that the process may be used to upgrade existing polyaluminium chloride flocculant solutions by the suitable addition of magnesium carbonate and/or calcium carbonate-containing materials, or of other basic materials. In the practice of the above charge-augmentation process magnesium carbonate may be used alone or may be used in conjunction with up to a major proportion of calcium carbonate, or suitable sources, for example mineral sources thereof. For example, calcium carbonate or a suitable source thereof may be in combination with only 0.1% or 0.2% of magnesium carbonate or a suitable source thereof. Any relative proportions of magnesium to calcium carbonate between these extremes may be used for example at lease 20%, very suitably at least 30%, of the total quantity of these carbonates, of magnesium carbonate.

Preferably, the content of impurities in the magnesium carbonate is less than 20%, particularly preferably less than 10% by weight. The quantity of the carbonate material is preferably at least sufficient, in theory, to react with the aluminium chloride. More suitably, the carbonate material is in at least

5% excess, preferably at least 20% excess, and possibly up to

100% excess or more as required at the reaction conditions used to achieve complete conversion of the aluminium chloride.

Preferably a proportion of the calcium carbonate, if used, is replaced by sodium carbonate. This also helps to reduce sludge formation during the use of the flocculant, as a result of the precipitation of calcium compounds. If a content of sodium is required in the flocculant composition this may preferably be provided by an addition of soda ash or other sodium source or sodium carbonate material.

In the preparation of the flocculants of the invention the solution of the aluminium chloride may be derived directly from synthetic raw materials such as, for example from anhydrous aluminium chloride or from a mixture of alumina trihydrate and hydrochloric acid or may be a by-product of another industrial process. The aluminium chloride has a concentration of at least 5%, particularly preferably at least 8% and preferably up to 25% or more, particularly preferably up to 20%, calculated as A1C1₃. If the concentration of the aluminium chloride solution as supplied is towards the lower end of, or below, the range stated above its aluminium content may be augmented by the addition of further aluminium compounds or aluminium metal together with an addition of HC1 if required to ensure the conversion of the same to the chloride. The pH of the solution of aluminium chloride is preferably from 0.5 to 1.5. It is preferred to add the carbonate compounds and/or other basic compounds to the acidic solution of the aluminium chloride, to maintain the resulting mixture with stirring or other means of agitation for the desired reaction period to allow the carbonate compounds to be sufficiently consumed having regard to any decrease in the reaction rate with decreasing concentration of reactants . If a carbonate other than magnesium carbonate is to be used it is found that this may materially reduce the reactivity of the magnesium carbonate, even to the extent that it is difficult to achieve the consumption of the last 5 of 10% by weight thereof, unless the magnesium carbonate is added to the aluminium chloride first and allowed to react, at least to a major extent, for example to more than 75%, preferably to more than 90% by weight, before the other carbonate is added.

The polyaluminium flocculant solutions of the invention preferably contain less than 3.5%, preferably less than 2%, suitably less than 1%, and, for example at least 0.25% by weight of the flocculant solution of calcium, calculated as the element, and/or at least 0.5% preferably at least 0.75%, for example, very suitably, at least 1%, and preferably up to not more than 3%, particularly preferably not more than 2.5% by weight calculated as the element of sodium, also calculated as the element. Where the quantity of boron compound is below 0.25% the content of calcium in the flocculant is preferably below 2% by weight.

The charge-augmentation process is preferably conducted by maintaining contact between the aluminium chloride and the carbonate compound or compounds at a temperature less than 60°C for a duration of more than 4 hours so as to encourage a slow rate of carbonate decomposition. The temperature may very suitably be at least 10°C and/or less than 50°C. The duration of the contact may, very suitably, be more than 8 hours and/or up to

36 hours although the effect anticipated may be attained in a lesser time than that, for example in no more that 24 hours.

Where the aluminium chloride is sufficiently pure for use to produce a specific flocculant, a temperature of up to the boiling temperature at the prevailing pressure, and a correspondingly shorter time of contact, may suitably be used. If a content of sulphate is required in the flocculant according to the invention a proportion of sulphuric acid and/or one or more sulphates may be added to the reaction mixture or to the flocculant product.

On completion of the reaction to the desired extent the resulting solution may be filtered or otherwise separated from any residue and forms the product of the invention.

The charge-augmentation process described above is particularly suitable for the conversion of aluminium chloride contaminated with organic impurities into a flocculant. Aluminium chloride is used on a large scale as a catalyst for organic transformations. For example, it is used as a catalyst in the alkylation of aromatic or aliphatic compounds by means of the Friedel-Krafts reaction. After use for this purpose the aluminium chloride may be contaminated with compounds such as, for example, benzene, chloropropanol, alkyl substituted dioxolanes, dioxanes or methylene chloride, up to individual concentrations which may vary from as little as 4 micrograms/litre to as much as 200 micrograms/litre aluminium chloride solution or even up to a level as high as 1% by weight, in the case of methylene chloride. Commonly, the total organic carbon content of used catalytic aluminium chloride solutions may be over 100, often over 200 milligrams/litre. Such contaminated aluminium chloride solutions are usually deep yellow in colour. It may be undesirable to use a flocculating agent containing organic or aromatic impurities for particular clarification applications, depending on the nature of the impurities, and for this reason organics- contaminated aluminium chloride may be considered to be unsuitable for use as a raw material for the production of flocculants .

If the reaction with the magnesium or magnesium and calcium compounds is conducted at the relatively low temperature and extended period of time disclosed above a proportion of the organic impurities may be stripped from the solution. The stripping agent is believed to be carbon dioxide generated by the reaction of carbonate with chloride and emitted from the solution. This stripping agent is generated 'in situ' on a molecular level of contact with the other constituents of the solution in contrast to externally sourced gaseous stripping agents. Thus the process has the dual benefits of requiring only a low energy input and being adapted to the use of impure raw materials .

The polyaluminium chloride flocculant according to the invention may advantageously be used in combination with a cationic polymer such as, for example, a polyethyleneamine, polyamidoamine or a polyvinylamine or a polydiallyl ammonium compound. In some cases the presence of the polymer enhances the flocculation ability of the polyaluminium compound. The polymer preferably has a cationic charge density of from 4 to 24 m.eq./g and is water soluble. A suitable quantity of polymer is from 1% to 10% by weight of the flocculant solution. When in combination with a cationic polymer, to preserve the stability of the polyaluminium compound in solution, the content of calcium, calculated as the element, in the polyaluminium chloride solution is suitably reduced and is preferably less than 3.5% and particularly preferably less than 2% and at least 0.25%, by weight. To enhance stability the polyaluminium chloride solution, as a separate feature or in combination with the control of the calcium level, preferably has a sodium content of at least 0.5%, particularly preferably at least 0.75%, for example, very suitably, at least 1%, and preferably up to not more than 3% by weight calculated as the element. The present invention will now be illustrated by means of the following specific and non-limiting Examples.

Examples 1 - 10.

The reactant materials used were:

Aluminium Chloride lOOOg

10% Wt. As A1₂0₃ ex Friedel-Krafts catalyst

Deep yellow colour

0% Wt as free HC1

Basic material 140g Dolomite (44.2% MgC0₃, 55.8% CaC0₃)

A polyaluminium chloride flocculant solution was produced by adding the basic material to the A1C13 aqueous solution and maintaining the resulting mixture with stirring at 20°C for 8 hours. The quantities of the constituents gave a relatively high calcium content in the solution produced. The mixture was then filtered to recover the product. The charge density in m.eq/kg and the turbidity in NTU of the product was measured.

1% by weight of each of the following potential stabilising compounds was added separately to a sample of the flocculant solution namely sodium gluconate, glucose, glycerol, mannitol, tartaric acid, citric acid, sodium citrate L-ascorbic acid, lactic acid, glacial acetic acid and potassium dihydrogen orhophosphate. The turbidity of the flocculant solution, in all cases, was found to exceed 100 NTU in less than 2 weeks, and often in less than 1 week, of ambient temperature storage. This was quite unacceptable.

Quantities of boric acid, sodium metaborate and sodium tetraborate were added to further samples of the same polyaluminium chloride solution in the quantities indicated below. The charge density of each sample was measured. The stabilising effect of the various additions is tabulated below in terms of a test of turbidity giving a result in NTU and performed after the indicated number of days of storage of the polyaluminium chloride solution at ambient temperature.

o-Boric acid Charge Density Turbidity

Ex.No.l- -7 . eq/g NTU Days

Nil -1200 2.5 1

126 6

\\ 633 9

0.1% 1176 >2000 5

0.25% 997 3 5 w > 200 11

R9- 1113 2.1 6

ΛΛ 2.4 20

0.75% 1200 1.8 1

2.1 15

1.0% 820 0.39 1

5.3 15

1.0% 1228 1.8 6

2.1 20

Sodium Charge Density Turb: Ldity tetrabor ^•ate m.eq/g NTU Days

(Ex.No.8, 9)

0.5% 1077 2.9 14

1.0% 1130 2.0 5

2.2 19

Sodium metaborate (Ex. No. 10)

1.0% 1095 2.1 14

Example 11.

lOOOg of a solution of polyaluminium chloride at a concentration of 10% by weight A1₂0₃, which contained 2% by weight of sulphate ion, was established. This solution had a cationic charge density of 980 m.eq./kg. lOg of boric acid and 50g of magnesium carbonate were added and allowed to react to completion at room temperature. The charge density of the resulting charge- augmented solution was found to be 1220 m.eq./kg.

Example 12.

The reactant materials used were: Aluminium Chloride lOOOg

10% Wt. As A1203 ex Friedel-Krafts catalyst

Deep yellow colour

0.7% Wt as free HC1

Basic material 170g Dolomite (44.2% MgC03, 55.8% CaC03)

Stabiliser lOg Boric acid

lOOOg of a solution of polyaluminium chloride at a concentration of 10% by weight A1₂0₃, which contained 0.7% of free HC1, was established. This solution had a cationic charge density of 10 m.eq/g. lOg of boric acid and 170g of dolomite were added and allowed to react to completion at room temperature. The charge density of the resulting charge augmented solution was found to be 1235 m.eq/g, confirming that charge augmentation had been achieved.

Example 13.

An effluent from a fabric dyeing process having a relatively low turbidity and suspended solids content was treated with a range of doses of a standard polyaluminium chloride flocculant, denoted as "A" hereafter, having a charge density of 300 m.eq/kg and of a charge-augmented polyaluminium chloride solution according to the invention, denoted "B" hereafter, having a charge density of 1250 m.eq./kg and stabilized with 1% by weight of boric acid, both at a flocculant dilution of 25g/500ml.

The initial characteristics of the effluent were:

PH 5.22

Turbidity (NTU) 5.68 COD (mg/1 as 0) 1230 Absorbance at 300nm 1.420 Suspended solids (mg/1) 174

Certain characteristics of the treated effluents are summarised in Figures 1 to 4 inclusive. Figure 1 is a graph plotting turbidity in NTU against flocculant dose in mg/1. Figure 2 is a graph plotting chemical oxygen demand (COD) in mg 0/1 against flocculant dose in mg/1. Figure 3 is a graph plotting colour removal. Measured at 300 nm, against flocculant dose in mg/1. Figure 4 is a graph plotting residual aluminium, in g Al/1, against flocculant dose in mg/1. The data summarised in the Figures shows the considerable improvement obtained by using the flocculant of the invention in respect particularly of turbidity and residual aluminium values.

Example 14.

An effluent from a food manufacturing process having a very high initial turbidity and suspended solids load was treated with 400 and 500 mg/1 of a standard polyaluminium chloride flocculant denoted as "A" hereafter, having a charge density of 300 m.eq/kg, and of a charge-augmented polyaluminium chloride solution according to the invention, denoted as "B" hereafter, having a charge density of 1250 m.eq./kg and stabilised with 1% by weight of boric acid, both at a flocculant dilution of 25g/500ml. Additionally, 15 mg/1 of a cationic polymer (POLYDADMAC) was added. The initial characteristics of the effluent were:

PH 5.2

Turbidity (NTU) >1000 COD (mg/1 as 0) 3640 Absorbance at 300nm>2.0 Suspended solids (mg/1) 174

Tests conducted at a controlled pH of 7.1 to 7.3 showed approximate overall parity between the two flocculants over a range of tests. In tests conducted at a pH controlled at 5.9 to 6.3 the flocculant of the invention, "B", showed considerably the better performance. The test results are set out in the following Table 1. Preferably, in the practice of flocculation of this invention the pH is controlled at below 7, for example from above 5 to below 7.

Table 1

Flocculant A A B B

Dose (mg/1) 400 500 400 500

PH 6.0 6.3 6.1 5.9

Turbidity (NTU) 12.5 11.7 9.2 4.6

COD (mg/1 as O) 970 960 940 900

Absorbance at 400nm) 0.478 0.384 0.252 0.363

Suspended solids 659 671 670 678

(mg/1)

Claims

Claims :

l.A composition comprising an aqueous polyaluminium chloride flocculant solution containing a stabilising quantity of a dissolved boron compound or compounds.

2. A composition as claimed in claim 1 wherein the boron compound or compounds are selected from oxy-acids of boron, and their soluble salts.

3. A composition as claimed in claim 1 or claim 2 wherein the boron compound or compounds are selected from boric acid, salts of boric acid, metaborates, tetraborates and pyroborates .

4. A composition as claimed in any preceding claim wherein the boron compound or compounds are present in from 0.05% to 10% by weight of the flocculant solution calculated as boric acid.

5. A composition as claimed in any preceding claim wherein the polyaluminium chloride is a polyaluminium magnesium chloride having a cationic charge density of at least 750 m. eq/kg.

6. A process for the production of a flocculant solution comprising dissolving in a polyaluminium chloride solution, or in a basified aluminium chloride solution, or in an acidic aluminium choride solution containing free HC1, a stabilising quantity of a boron compound or compounds .

7. A process as claimed in claim 6 wherein there are also dissolved magnesium carbonate and/or calcium carbonate thereby to augment the cationic charge density of the flocculant to at least 750 m.eq/kg.

8. A process as claimed in claim 7 wherein the polyaluminium chloride or aluminium chloride solution has a total organic carbon content of over 100 micrograms/litre.

9. A process for the treatment of water or an aqueous effluent by mixing with the water or effluent a composition as claimed in any one of claims 1 to 5, allowing flocculation to occur and separating the supernatant liquid.

10. A process as claimed in claim 9 wherein the water or effluent has a pH of below 7.

11. A composition as claimed in claim 1, or a process as claimed in claim 6 or claim 9, and substantially as described herein.

12. A composition or process as claimed in claim 11 and substantially as described in any one of Examples 2 to 13 herein.