WO1995011197A1

WO1995011197A1 - Flocculants

Info

Publication number: WO1995011197A1
Application number: PCT/GB1994/002181
Authority: WO
Inventors: Neil Clarke; John Harris Newton
Original assignee: Laporte Industries Ltd
Current assignee: Evonik LIL Ltd
Priority date: 1993-10-21
Filing date: 1994-10-06
Publication date: 1995-04-27
Anticipated expiration: 1996-04-21
Also published as: GB9321734D0; AU7789994A

Abstract

A process for the production of a flocculant from an acidic aqueous liquor containing dissolved salts of aluminium and/or iron such as a waste liquor from the acid activation of a montmorillonite clay mineral comprises contacting the liquor with a basic material such as magnesite capable of reacting with the dissolved salts to precipitate aluminium and/or iron values, separating the precipitate in the form of a cake or slurry from the remaining liquor, treating the cake or slurry with hydrochloric acid to redissolve aluminium and/or iron values, preferably introducing into the digestion liquor metallic aluminium and/or iron, in the case where the solution contains ferrous iron preferably contacting the solution with an oxidising agent to convert iron to the ferric form, and separating suspended solid matter from the resulting solution. The flocculant product, which may comprise a solution of basic poly(aluminium and/or iron) halide, gives a low residual aluminium value when used to treat water. A novel magnesium-containing poly(aluminium/iron) halide product is also provided.

Description

FLOCCULANTS

This invention relates to flocculants, to a process for manufacturing them from waste acidic liquors and to a process for the treatment of water, aqueous effluents, such as sewage effluents, or other aqueous liquids using the flocculants.

There are current European Economic Community Standards for potable water which set out stringent guide and mandatory levels for a wide range of inorganic contaminants. The levels set for aluminium, which are of importance in relation to the possible long term deleterious biological effect of aluminium, are a guide level of 0.05 mg/1 and a mandatory level of 0.2 mg/1, calculated as Al . The corresponding levels for iron are respectively 50 micrograms/1 and 200 micrograms/1, calculated as Fe. It is usual practice for the water producers to require adherence to the guide levels in their treatment processes. This imposes a performance requirement on flocculants and other water treatment products.

The quality of bathing water is also subject to a current European Community Directive and this has stimulated research into means for increasing the quality of sewage outfall discharges and similar effluents. As a result, in the United Kingdom, the ultraviolet irradiation of sewage effluents has been approved as a means of reducing the bacterial content of such discharges or effluents. The practise of such irradiation requires the use of ultraviolet sources, such as quartz lamps, which are prone to masking by the deposition on them of organic matter from the effluent. If an iron salt such as ferric sulphate has been used to decrease the organic content of the sewage prior to irradiation there will, usually, also be plating out of residual iron values onto the lamps. There is, therefore, a requirement for a flocculant which will give low residual organic and iron contents in sewage effluents.

It is known to treat water with flocculants comprising solutions or colloidal suspensions of basic polyaluminium halides , otherwise known, in the case of the chloride, as PAC. The term "solution" is used herein to include such suspensions. The effect of such treatment is to cause finely divided particulate matter present in the water to be enveloped in floes and to be caused to sediment, enabling a relatively purified supernatent water to be withdrawn. Such a treatment may be applied to the production of potable water, to the treatment of industrial effluents, for example to reduce the content of metallic contaminants in such effluents, to the treatment of sewage effluents, for example to reduce the content of organic matter or to the treatment of other aqueous liquids requiring purification.

The requirement for a low residual aluminium content in potable water gives rise to operational problems in the use of certain flocculants, such as polyaluminium halides or iron salts, in that there is a restricted water pH range of 6.2 to 6.8 within which low residual aluminium levels may be achieved. The iron salts are inherently acidic and tend to contribute to a lowering of the pH of generally alkaline potable water sources towards the required range, although some further pH adjustment may be required. Polyaluminium halide flocculants, due to the hydrolysis of some of their acidic sites, are less acidic in overall character and usually require the use of a substantial separate acid dosing procedure as part of the flocculation process.

The present invention relates to the production of a flocculant which may give an acceptable performance at pH levels outside the range referred to above and which may be produced from liquors which are currently processed at considerable expense to render them suitable for waste disposal.

The present invention provides a process for the production of a flocculant from an acidic aqueous liquor containing dissolved salts of aluminium and/or iron, comprising contacting the liquor with a basic material capable of reacting with the dissolved aluminium and/or iron salts to precipitate aluminium and/or iron values, separating the precipitate in the form of a cake or slurry from the remaining liquor, treating the cake or slurry with hydrochloric acid to redissolve aluminium and/or iron values and separating suspended solid matter from the resulting solution. In the case where the solution contains ferrous iron the solution is preferably treated with an oxidising agent to convert iron to the ferric form as hereafter indicated.

The flocculant product of the invention may be a "poly (aluminium and/or iron) halide" which term is used to encompass the alternatives that iron and aluminium are present as a single polymeric species or at separate polymeric species as well as the alternative that either iron or aluminium is absent. The iron content the product of the invention is anticipated to provide a dense floe blanket and has shown excellent storage stability. The term liquor is used herein in a practical sense to refer to liquid plant effluent and includes any suspended solid matter with may be present in the effluent. Waste acidic liquors containing dissolved aluminium and/or iron compounds and which are suitable for use according to the present invention are produced on a large scale in various industries. The treatment of aluminosiliceous or aluminoferrosiliceous minerals such as smectites, kaolinites, attapulgites, sepiolites or the like and particularly, within the smectites, the montmorillonites, with acids such as, for example, sulphuric acid or hydrochloric acid, to increase the surface area of the minerals and to activate them for various uses, for example for edible oil purification or for paper coating applications, is practised on a large scale. The acidic liquor so produced contains a substantial proportion of dissolved aluminium and/or iron compounds. This liquor may be neutralised with lime to produce a cake or slurry which is usually disposed to land¬ fill. Acidic mine drainage liquor may contain a substantial proportion, for example up to about 1000 mg/1 of dissolved iron compounds calculated as Fe and up to about 4000 mg/1 of sulphate. Such liquor may likewise be neutralised with lime to produce a slurry which may be aerated to convert the iron to the ferric form and then allowed to settle to produce a solid waste product for disposal. Waste pickling liquors contain substantial quantities of dissolved iron compounds. The disposal of such waste products represents a considerable expense and is increasingly considered to be undesirable ecologically and it is of considerable benefit to industry, and environmentally, to provide means for utilising them as a raw material.

The montmorillonite clay minerals, which term is used to include bentonites, are layered minerals composed of an octahedrally coordinated aluminium layer connected by the sharing of oxygen atoms to two adjacent tetrahedrally coordinated silicon layers. The aluminium in the octahedral layer is partially substituted by iron and may also be substituted by magnesium or other atoms and the silicon in the tetrahedral layer may be partially substituted by aluminium. Such minerals have appreciable surface area which is increased by acid treatment for a variety of applications. The effect of acid treatment is to dissolve aluminium and iron preferentially from the octahedral layer. Where the treating acid has been sulphuric acid, as is usual, the liquor resulting from the acid treatment contains dissolved aluminium and iron sulphates. The treating acid may alternatively be hydrochloric or other mineral acid or a suitable organic acid and the composition of the resulting liquor will differ correspondingly. A typical acidic liquor may contain from 0.5% to 4% for example 1.5% of aluminium compounds calculated as AI2O3, 0.2% to 3% for example 0.8% of iron compounds calculated as Fβ2θ3 and may have a free acidity of 0.5% to 2% for example 1%.

According to the present invention an acidic liquor produced as described above may be contacted with sufficient of the basic material, and preferably with an excess of up to 100% thereof, to precipitate aluminium and iron values, usually as the hydroxides or hydrous oxides. The basic material preferably is or comprises one or more magnesium compounds, such as magnesium carbonate, suitably as magnesite, magnesium hydroxide or other suitable magnesium compounds. In this case the majority of the sulphate anions remain in solution in the form of magnesium sulphate. The precipitation may be conducted at ambient temperature or at the temperature of the acidic liquor which may, for example, be from 10°C to 50°C. It is convenient to separate the precipitate by filtration although the separation may be accomplished by other means such as by settlement and decantation to produce a slurry which may c may not be further dewatered to produce a cake. It ^ found that basic calcium compounds are relatively unsuitable for use in the practise of this invention because calcium sulphate is sparingly soluble and gives separation problems. In the case of magnesium compounds the presence of an appreciable amount of magnesium which is present in the cake or slurry contributes to the characteristic composition of the product.

A typical composition for the filter-cake so produced from the sulphuric acid treatment of a montmorillonite mineral may be:

Remainder water

Some of the above constituents may be absent. For example there may be substantially no iron present if the acidic liquor is wholly derived from a kaolinite or no aluminium if the acidic liquor is derived from steel pickling. The silica may be absent if the acidic liquor is not derived from the treatment of a siliceous mineral. In practice, the iron may be present substantially in the ferrous form unless such has been oxidised to the ferric form by an addition of an oxidising agent at some preceding point in the history of the liquor. The magnesium values indicated above, or values derived from other basic materials which may be used in the process in place of a magnesium compound, or a proportion of them, persist in the eventual product, in for example at least 0.2% and up to 2% or more calculated as MgO, to produce a new or useful product comprising a poly(aluminium and/or iron) magnesium (or other) halide. The magnesium content of this product may itself be in polymeric form. This product may be used to reduce the phosphate content of a water by precipitation as magnesium phosphate. According to the present invention the cake or slurry is digested by treatment with hydrochloric acid in, for example, a 1:1 to 2:1 cake:acid weight ratio. The digestion may be conducted at a somewhat elevated temperature, for example at 40°C to 90°C, for, for example for 10 minutes to 2 hours. To obtain a basic product the pH of the digestion liquor should be at least 2 preferably at least 2.5 and, for example, up to 4.

It is an important preferred feature of the present invention that, depending whether the product is an aluminium or an iron flocculant or a mixed aluminium/iron flocculant, metallic aluminium and/or iron may be introduced either during the digestion of the filter cake or slurry or to the digestion product. It has been found that while a basic poly(aluminium and/or iron) chloride product may be produced directly from the digested cake this has a relatively low charge density and could not be expected to be of particularly high effectiveness as a flocculant. The inclusion of a relatively small quantity of metallic aluminium or iron considerably improves the product and results in a product having a charge density in the range of about 400 to 700, or to 900, meq/g. The improvement in the product appears to be greater than would be expected having regard to the quantity of metallic aluminium and/or iron added.

The aluminium and/or iron used in the present process may be in the form of powder, pellets, swarf or the like. Very suitably the aluminium may be in the form of shot and the iron in the form of powder. The dissolution in the hydrochloric acid digestion liquor is preferably assisted by elevated temperature. The use of pressure and the higher temperatures normally associated with this is not necessary and a temperature slightly below reflux temperature, eg up to 20°C below reflux temperature, will suffice. The use of reflux temperature is not excluded but it may be preferred to avoid it for practical operational reasons. It is found that the dissolution of the aluminium metal proceeds relatively rapidly for example in from about 15 minutes to 2 hours although if iron metal is present this duration may have to be extended. The quantities of aluminium and/or iron are preferably controlled to achieve the desired degree of improvement in the final product and may be from 10% to 100% by weight based on the aluminium/iron content of the digestion liquor.

It has been found that suitable control of the hydrochloric acid digestion/metal dissolution process can enable the separation of iron from the liquor to produce a basic polyaluminium halide product or a basic poly(alumunium/iron) halide product containing a decreased proportion of iron. If the hydrochloric acid digestion liquor produced as described above is aged for at least 0.25 hours, for example for from 0.5 to 24 hours at a temperature of for example ambient temperature to 60°C before the addition of aluminium metal the dissolution of that metal is accompanied by the precipitation or crystallisation of iron in the metallic form. This metallic iron may readily be separated from the product. This finding is of importance to the present process since it would enable a polyaluminium halide product to be produced directly from a mixed aluminium/iron containing initial acidic liquor. It is also of wider significance as an expedient for separating iron from mixed aluminium halide/iron halide solution.

Accordingly, there is also provided, as an independent aspect of the present invention, a process for the treatment of mixed aluminium halide/iron halide solutions to separate iron therefrom comprising ageing the solution, adding aluminium metal to the aged solution, allowing metallic iron to precipitate or crystallise and separating the remaining aluminium halide from the metallic iron. The aluminium halide/iron halide solution preferably has some or all of the features of the hydrochloric acid digestion liquor described above.

It is found, according to the present invention, that some of the minor constituents of the original acidic liquor are still present in the liquor obtained from the dissolution of the hydroxide/hydrous oxide cake or slurry. If the original acidic liquor was derived from the treatment of siliceous minerals a portion of the silicon is present in the cake or slurry and persists in the product solution in the form of finely divided solids in a quantity, usually, of from 0.5% to 5%. Such ε -lids are preferably separated, for example by filtration. This siliceous material has been found to be an effective oil bleaching agent in its own right and provides a substantial extra added value to the invention. The poly(aluminium and/or iron)halide product solution also contains silicon in tl^s dissolved form. It may be present, typically, in at leas_^ 0.02 mg/kg, more generally in at least 0.05 mg/kg and, for example, up to 0.5 mg/kg or more, calculated as Si and based on the weight of the solution. Soluble silicon may help to reduce the biological effect of aluminium in water and may therefore be advantageous in a water treatment product.

In the case where basic poly (aluminium/iron) halide product is produced according to the invention there may be a substantial proportion of the iron, or all of it, in the ferrous form. This would detract from the performance of the product. Accordingly, it has been stated to be preferred for the liquor containing the mixed aluminium/iron product is preferably treated to achieve the oxidation of the iron, or of a proportion of it, to the ferric form. Preferably this may be achieved by the addition of a quantity of an oxidant, such as hydrogen peroxide, or by bubbling oxygen gas through the solution. In the case where the oxidant is hydrogen peroxide it is preferred to use a quantity of from above 1 to 2 times the quantity theoretically required to convert the ferrous iron content of the solution, or the desired proportion of it into the ferric form. Ambient temperature or an only slightly elevated temperature of, for example, up to 50° is preferred to reduce decomposition. It may be found adequate from the point of view of flocculant properties to oxidise less than all, for example from 50% to less than 100%, of the ferrous iron and in this case the quantity of the hydrogen peroxide may be adjusted accordingly.

The product of the present invention may also be sulphated if desired, either by introducing a water soluble sulphate or a solution containing sulphate values into the hydrochloric acid digestion liquor, into the reactants at a stage in the manufacture of that liquor, into the reactants during the digestion of the metallic iron and aluminium, into the polymeric product of the invention or at any other stage in the process. It has been found by the Applicants that the quantity of sulphate is preferably controlled for optimum performance at not more than 4.0% by weight calculated as SO4. A preferred range is from 0.5% to 3.5%.

Above about 4% there is a marked instability.

The invention will now be more particularly described by reference to the following Examples and Tests. It is understood that these Examples and Tests are illustrative of particular embodiments of the invention and are not limiting on the scope of the invention.

Example 1 Preparation of basic poly (iron/aluminium) chloride

A filter cake prepared by the neutralisation with magnesite of an acidic liquor derived from the sulphuric acid treatment of a montmorillonite clay mineral was mixed with hydrochloric acid in a cake:acid weight ratio of 1.5:1 giving a pH of 2.5. The cake was digested at 70°C for 30 minutes with stirring and the liquor separated. The liquor was then aged for 2 hours and 1.5% of aluminium metal was added to it and the temperature was them maintained at

105°C for a further 2 hours. The iron content of the solution was already in the ferric form. The product of the example was a dark brown solution containing 78.3g/l AI2O3,

19.0g/l Fe₂0₃/FeO, 146g/l Cl- and lOg/lMgO and had a basicity in the range 25% to 50%.

Example 2

Example 1 was repeated with the sole difference that the aluminium addition was ommitted. The product had a similar basicity to that of Example 1.

Tests 1 to 18 - Purification of Water

In the following Tests raw river waters were treated with the product of Examples 1 or 2 or with a comparative PAC flocculant under varying conditions. The supernatant so obtained was tested for floe quality after 10 and 20 minutes treatment time, colour, turbidity, and % transmission and was analysed for residual Al and Fe values. The turbidity test is a standard test for the content of colloidal particles using a light scattering technique and is expressed in NTU units. The transmission test is a test for the content of organic substances in a sample conducted using a 1 cm quartz cell by determining the transmission of light having a wavelength of 254 nm through the sample and is expressed as a percentage (Trans

%). Floe quality was observed visually and allocated a grading of from A (unsatisfactory) to G (excellent).

Colour was measured in Hazen Units (H) .

The following raw water was used in the Tests taken from the River Dee at Huntington:

pH 7.9

Colour (H) 31.8 Turbidity (NTU) 3.5 Trans % 71.5

Al (mg/dm3) 0.02

Fe (mg/dm3) 0.03

The following comparative flocculants were used in the Tests

(A) Tests 1 to 6.

Basic Poly (aluminium/iron) magnesium chloride of Example 2 produced without any addition of metallic aluminium.

(B) Tests 7 to 12.

Basic Poly (aluminium/iron) magnesium chloride of Example 1 produced with an addition of aluminium content of the product.

(C) Tests 13 to 18.

Polyaluminium chloride (PAC) standard commercial product.

Test No 10 11 12

Claims

1. A process for the production of a flocculant from an acidic aqueous liquor containing dissolved salts of aluminium and/or iron, the process being characterized by the combination of steps comprising contacting the liquor with a basic material capable of reacting with the dissolved aluminium and/or iron salts to precipitate aluminium and/or iron values, separating the precipitate in the form of a cake or slurry from the remaining liquor, treating the cake or slurry with hydrochloric acid to redissolve aluminium and/or iron values and separating suspended solid matter from the resulting solution.

2. A process as claimed in Claim 1, wherein the resulting solution comprise a basic poly(aluminium and/or iron)halide.

3. A process as claimed in Claim 1 or Claim 2, wherein the resulting solution contains ferrous iron and is contacted with an oxidizing agent to convert ferrous iron to the ferric form.

4. A process as claimed in anyone of the preceding Claims, wherein the acidic liquor is the acid-treatment product of an aluminosiliceous or aluminoferrosiliceous material.

5. A process as claimed in anyone of the preceding Claims, wherein the acidic liquor is a pickling liquor or a mine drainage liquor.

6. A process as claimed in anyone of Claims 1 to 4, wherein the acidic liquor contains from 0.5% to 4% of aluminium compounds calculated as A1₂0₃, from 0.2% to 3% of iron compounds calculated as Fe₂0₃, and has a free acidity of from 0.5% to 2%.

7. A process as claimed in anyone of the preceding Claims, wherein the basic material is or comprises one or more magnesium compounds.

8. A process as claimed in anyone of the preceding Claims, wherein metallic aluminium and/or iron is included with the cake or slurry during its dissolution or in the resulting solution.

9. A process as claimed in Claim 8, wherein metallic aluminium is included in the resulting solution after that solution has been aged.

10. A process as claimed in anyone of the preceding Claims, wherein the suspended solid matter comprises silica having oil bleaching activity.

11. A basic poly-(aluminium and/or iron)halide flocculant solution containing 4.5% to 8% of aluminium compounds calculated as A1₂0₃, from 1% to 4% of iron compounds calculated as Fe₂0₃, and 0.5% to 3% of magnesium compounds calculated as MgO.

12. A flocculant solution as claimed in Claim 11, having a charge density of 400 to 900 meq/g.

13. A flocculant solution as claimed in Claim 11 or Claim 12, additionally containing from 0.02 to 0.5 mg/kg of soluble silicon calculated as Si.

14. A flocculant solution as claimed in anyone of Claims 11 to 13 additionally containing sulphate.

15. A process for the treatment of a mixed aluminium halide/iron halide solution to separate iron therefrom comprising ageing the solution, adding aluminium metal to the aged solution, allowing metallic iron to form and separating the metallic iron.