IE47846B1 - Water purification - Google Patents

Description

It is well known known that polyvalent metal ions, particularly ferric and aluminium ions, are useful as flocculating agents to bring about flocculation of suspended solids in aqueous liquids. Normally relatively pure compounds such as ferric or aluminium sulphate or chloride are added to the liquid that is to be flocculated, although some references occur in the literature suggesting that, for instance, both ferric and aluminium salts nay be taken and mixed together and added to the aqueous liquid, the mixing either occurring before or in the liquid.

The general method of preparing the polyvalent metal flocculants generally comprises starting from relatively pure metal or metal compound and this naturally IS causes the flocculants to be relatively expensive, A method according to the present invention comprises incorporating into an aqueous liquid a flocculating composition, and thereby flocculating impurities in the liquid, and in this method the flocculating composition comprises the water soluble reaction product obtained by lixivating naturally . .occurring laterite in acid. Laterite is a naturally occurring inorgdn-ic deposit comprising a plurality of acid, soluble polyvalent metal ions. The flocculating composition may consist of the reaction product, in which .'evenl i$.can be used in the conventional manner of inorganic flocculants or it may comprise other ingredients, - 3 in which event it may be used in a different way, but again the overall effect is generally to cause flocculation. For instance it may contain detergent in which event the composition is primarily a detergent but the polyvalent metal compounds in it impart to the detergent a desirable but usually delayed flocculating action.

The effectiveness of the composition will naturally depend upon the inorganic deposit from which the water soluble reaction product is obtained. Naturally not all inorganic deposits containing a plurality of acid soluble polyvalent metal ions will give reaction products of equal effectiveness and simple routine experimentation is necessary to determine whether any particular composition is useful for any particular purpose.

The reaction product obtained from laterite comprises at least ferric, aluminium and titanium ions.

It appears that the presence of titanium is particularly beneficial on the flocculating effectiveness of the composition. The content of the laterite deposits being used can vary, and typically one can have from to 50Ϊ aluminium, measured as Α1π0-, 5 to 40’» ferric i 3 iron, measured as Fe2O3, 5 to 15% being found in some of the most suitable ores while others may contain 20 or 25¾, and 2 to 10% titanium measured as TiO2< However the preferred iron content in the form of Fe^j lies in the range of 20 to 25%. These proportions are all by weight. In the laterite deposits the solubility of the ions is such that the proportions in the reaction product fall within the same ranges as the proportion stated above for the deposits.

The laterite deposit and the reaction product preferably contain also additional polyvalent metal ions, especially ions selected from chromium, vanadium and manganese, and often preferably include also monovalent and divalent ions, especially magnesium and potassium but also often sodium.

In many flocculation processes flocculation - 4 using polyvalent metal ions is accompanied by the use of silica, generally in the form of activated silica.

A particular advantage of the invention is that not only can one obtain very simply and very cheaply a water soluble reaction product containing metal ions and suitable for use as a flocculant but also this reaction product can easily contain sufficient silica to give a beneficial effect on the flocculation, thereby eliminating or reducing the need for the use of a separate addition of activated silica. Silica and silicon compounds in the deposit do of course tend to be acid insoluble but their solubility is usually sufficient that an adequate amount, e.g. 0.1 to 2¾, silica measured as SiO2 is present in solution in the reaction product.

Such a reaction product may be made by use of a laterite deposit containing, preferably, 5 to 25¾ silica measured as SiO2· The balance of the silica remains with various other insoluble ingredients after dissolving the laterite deposit in acid. This solid residue may be separated from the reaction product and discarded or may be used with the reaction product.

The naturally occurring laterite deposit that is dissolved in acid may be the raw deposit in the form in which it is mined, optionally after physical treatments such as crushing and/or screening, or may be the deposit resulting from washing or leaching processes or from the addition of small amounts of chemicals. These processes may result in the removal or addition of chemicals from the deposit but it will still be naturally occurring in that it still contains a wide range of ions in approximately their naturally occurring proportions. Preferably the raw deposit is subjected only to physical treatments and/or washing before use, and does not have chemicals added to it.

Laterite is a naturally occurring mineral which has been formed by the decomposition of alumina silicate rocks and is defined on page 259 of Les Laterite de la Guinee 1913. Nouv. Arch, du mus. Paris 5th Seree Vol V. - 5 by Lacroix and also on page 460 of Geological Magazine 1911 What is Laterite by Fermor.

It occurs in natural deposits in north-east Ireland, for instance Antrim, in Asia, for instance Sri Lanka and the Draccan Plateau in India, in Saudi Arabia, Shasaj, Muscat and Oman, Syria, Ghana, Tanzania, Kenya, Thailand and Indonesia. It contains silica, iron oxides, aluminium in the form of oxides or silicates, and various other metals, e.g. as oxides or silicates, all in varying proportions, depending upon its source.

The mineral acid can be any mineral acid capable of forming a solution containing most at least of the metals in deposit. Thus it can be, for instance, phosphoric acid, nitric acid or hydrochloric acid, but preferably it is sulphuric acid. However it is not necessary to use acid of the purity normally used in the synthesis of chemicals and instead waste acidic effluent can be used provided that its acid concentration is sufficient that adequate solution of the deposit can be achieved using reasonable amounts of effluent. Typical waste acid effluents are spent iron pickle solutions and aluminium anodising waste acids. The acid content is preferably at least 5, usually about 101 or more, sulphuric or other acid. If reagent acid (i.e. acid that is substantially e.g. 801 purejthen the concentration may be greater, e.g. 15 to 301.

The reaction product may be formed merely by mixing the deposit with the acid. The amount of acid is preferably a substantially stoichiometric amount, in order that the reaction product does not contain a significant excess of free acid. Hydrogen peroxide or other suitable oxidising agent may be added during the process of manufacture of the flocculant to assist in converting any ferrous iron (Fe 0) contained in the raw laterite material into ferric iron (Fe,0,). Often it is preferred that the amount of acid should be less than stoichiometric and/or that alkali should be added - 6 to the reaction product to bring the pH to 7 or higher.

The contact between acid and the laterite deposit may be at room temperature but preferably is at elevated temperature, e.g. 30 to 110°C. It may be conducted under conditions that result in the formation of a liquid reaction product, i.e. a solution of the various ions, or it may be conducted under conditions that yield a solid reaction product. Such products can be made either by using an appropriate concentration of acid and appropriate amounts of acid solution and deposit so as to obtain a solid product which can be separated from any excess liquid and dried, or a liquid reaction product may first be made and this may then be evaporated to form a solid reaction product.

However if it is made,the solid product may be formed directly or indirectly into any convenient type of composition, e.g. a cake, powder or granules.

The reaction product solution is useful as a flocculant when contacted with an aqueous liquid containing flocculatable solids. The polyvalent metal ions which are present in the laterite,have to be in solution to bring about flocculation and so conveniently the reaction product is in liquid form when it is added to the aqueous liquid that is to be flocculated. However if desired the liquid flocculating composition may be generated in situ by adding a composition, generally a solid composition, containing solid reaction product to the liquid and allowing the reaction product to dissolve in the liquid.

When used in conventional manner as a flocculant the defined reaction products are capable of giving particularly beneficial reduction in phosphate, nitrate and/or organic suspended solids content. The reductions in phosphate and/or nitrate can be particularly beneficial and surprising and are very useful in view of the acute need in many areas to remove phosphates and nitrates from aqueous liquids. Although throughout this specification we refer to removing suspended solids of course some of the solids - 7 may be in colloidal or almost dissolved form.

The method of the invention can be used for purifying reasonably pure water, such as naturally occurring lake water, orfor purifying contaminated waters such as sewage. The purification can be accompanied by improvement in clarity, e.g. from 20° Hazen to less than 5° Hazen.

As mentioned above, it may be desirable to adjust the pH of the composition in order to improve optimum flocculation in any particular situation. Alternatively the aqueous liquid that is to be flocculated may have its pH adjusted by separate addition of another chemical either before, with or after addition of the composition of the invention. Generally it is necessary to increase the pH, by adding alkali, such as sodium hydroxide. The pH of the aqueous liquid is generally in the range 3 to 8, most preferably 3.5 to 7.

After adding the flocculant composition by conventional techniques to the liquid to be flocculated the liquid is preferably stirred, for instance for about 1 minute, e.g. half to 2 minutes, and the system then left to settle for an appropriate period, e.g. half to 4 hours usually about 2 hours. After the settling period it may be observed that a reddish orange colour sediment will have settled on the bottom of the vessel containing the liquid and the water .above this sediment will either be crystal clear or at least will be substantially clearer than it was before adding the composition.

Particularly preferred methods according to the invention are methods in which the flocculating agent is incorporated into the water to be treated together with a detergent, i.e. as a detergent composition.

In preferred detergent compositions the detergent itself and the reaction product are both in solid form e.g. granules or powder form and it is usually desirable - 8 that the particles sizes of the detergent and reaction product should be similar. Although the reaction product is solid, it may contain a certain proportion of water and the hardness of the product can be influenced by varying the extent to which residual water is driven off, the less water that remains, the harder being the product. This ability to control hardness is valuable since the harder products take longer to dissolve and, in use of the solid products as additives in detergents or as flocculating agent for purification of aqueous systems, it can be desirable to control the rate at which the products dissolve.

Phosphates are extensively used in the manufacture of detergents: phosphates will sequester calcium in hard IS water and thus soften the water and thereby improve its suitability for washing and heating. However, phosphorus compounds are one of the main types of substances responsible for eutrophication of natural waters. Conventional methods of treating raw sewage, particularly from domestic sewerage systems, have difficulty in satisfactorily treating effluents having a high content of phosphorus compounds.

If a significant proportion of phosphorus compounds and organic matter in raw Sewage can be removed by flocculation at or before the point of input to a treatment plant, the capacity of the plant may be increased dramatically. By use of the reaction products, the capacity may, for example, be increased by up to 70¾ without the need fox.any modification of the treatment containing a fToccuiarit prepared plant. Likewise, if detergents/in accordance with the invention rather than conventional detergents are used, part of the phosphorus compounds and organic matter in sewage will already be flocculated before reaching the input to a treatment plant. By achieving flocculation at an early stage, the bacteriological action in the treatment beds of the plant is enhanced in that the bacteria have less organic matter to deal with and the removal of phosphorus compounds at this stage, when they become part of the sludge, is advantageous in that it - 9 diminishes eutrophication of water courses into which the treated effluent flows.

The reaction products do not interfere with the washing action of conventional detergents and a practical advantage as far as washing operations are concerned is that complete flocculation and coagulation tend not to occur immediately but only when the discharged wash water ceases to he agitated e.g. on route to a sewage treatment plant or at the intake tanks of the treatment plant.

The reaction products can be incorporated into liquid detergents and in this case it is relatively unimportant whether the reaction product is initially in solid form or in solution form.

The reaction products are advantageous in detergents even in quite small proportions e.g. in amounts (solids content) as low as 0.51 by weight of the detergent itself, whilst preferred amounts are from 1 to 5¾.

An important preferred feature of the present invention is to add a small proportion of sodium chloride to the reaction product or, preferably, to include this with the reactants used to form the reaction product. Desirably the proportion of sodium chloride used is from 0.05 to 0.51 by weight of the laterite used, preferably about 0.11. This use of sodium chloride enhances the water-softening properties of the detergents and thereby renders phosphates in the detergents more effective.

It is thought that the effectiveness of the sodium chloride may be due to the formation of a sodium aluminosilicate structure having water-softening properties similar to the well known water-softening properties of zeolites.

For use in certain circumstances, e.g. in detergents for use in manually operated washing machines, it may be desirable to prolong the time taken for the solid reaction products to exert their flocculating action. - 47846 - 10 This object may be achieved by coating granules of the solid reaction products with a polymer which would gradually dissolve in warm water.

The following are some Examples.

S Example 1.

A flocculant solution is made by mixing 4.5 grams of powdered laterite and obtained by grinding laterite rock from the mountains north of Ballymena at the origin of the Maine River in N. Ireland with 100 ml of a solution obtained by mixing 65 ml of distilled water with 35 ml of 50% concentrated sulphuric acid. The mixture was stirred and after settling for 15 minutes it was seen that a reddish brown liquid had formed and a sediment had settled out.

Conveniently the sediment is separated from the liquid before this is used as flocculant.

Example 2 ml of the red-brown liquid obtained from the previous Example was added to 6 x 700 mis of Lough Neagh water. These samples were then stirred for 10 minutes, the pH was then adjusted with NaOH (approx.

N) in each sample to approximately 5, 6, 7, 8 and 9 respectively. Each of the above treated samples visually displayed the same characteristics i.e. once the pH . r flocculation exceeded 4.5 / appeared immediately and after 2j hours this floe had settled completely on the bottom leaving crystal clear water on the top.

Samples of Lough Neagh water containing approximately 0.1 ppm phosphorus were adjusted to approximately 0.4 ppm phosphorus and then treated with laterite solution in the above manner. Each sample was adjusted to a particular pH and after 2j hours was analysed for organic matter and phosphorus. mis of each of 5 clear samples of different pH and of the untreated water, was taken and to each 5 mis of acid molybdate.and 1 ml ascorbic acid were added, and the samples were then made up to 50 ml mark with distilled water. After the blue colouration had formed - 11 (10 mins) the absorbancy of each sample was measured in the SP600 spectrophotometer at a wavelength of 882 mp using distilled water as a reference and 4 cm cells. The results are in Table I.

Each of the above samples was analysed for organic matter content using the Perkin Elmer, Double beam, spectrophotometer at a wavelength of 320 mp - using distilled water as a reference and 4 cm cells. The absorbancy of each sample was recorded in Table II.

Table I pH Absorbancy or OD °s Efficiency Samples 5.05 0.008 97.8 15 treated 5.99 0.021 94.3 with 7.03 0.017 95.3 laterite 7.95 0.024 93.4 20 8.95 0.077 77.1 Untreated sample 8.927 0.365 - Ihe OD is proportional to ppm phosphorus.

Table II PH Absorbancy or 01 Samples 5.05 0.135 treated 5.99 0.120 with 7.03 0.130 laterite 7.95 0.145 8.95 0.180 Untreated sample 8.927 0.240 478 46 - 12 These results show that more than 93¾. e.g. 97.8¾ of the phosphorus is removed and a substantia] proportion of the organic matter.

Example 3 A dry composition was made by mixing together with stirring and heating 1 gram of finely powdered laterite and 4 mis of an acid solution obtained by mixing together one part by volume concentrated sulphuric acid and five parts by volume water. The resultant cake weighed about 3.3 grams and could then be used as a flocculant either by dissolving direct in the aqueous system to be flocculated or by being dissolved first in water and the resultant solution then being added to the system to be flocculated.

The laterite used in each of Examples 1 and is preferably from Clinty but good results can also be obtained with laterite from Evishacrow, also in County Antrim.

Example 4 The cake made in Example 3 is mixed with 57 parts by weight of a conventional solid detergent, e.g. a solid detergent designed for use in automatic washing machines, per part by weight of solid reaction product. Tests using such a detergent in an automatic washing machine have shown that a heavy flocc is formed soon after e.g. 30 minutes after, the final washing sequence and discharged of the wash water from the washing machine. Tests on the water discharged from the washing machine have shown that the use of the detergent according to the invention may result in removal, by flocculation, of over 90¾ of the content of phosphorus compounds in the discharged water and of about 70¾ of the organic matter in the water.

Claims (15)

1. A Hi^tjhod in which a flocculating composition is incorporated / an aqueous liquid and impurities in the liquid are thereby flocculated, in which the flocculating composition comprises a water soluble reaction product which has been obtained by lixiviating naturally occurring laterite in acid and which comprises at least ferric, aluminium and titanium ions.

2. A method according to claim 1 in which the laterite contains between 20 and 25% by weight of Fe^Oj.

3. A method according to claim 1 in which the laterite contains also silica or a silicate in a form such that part at least is dissolved into the reaction product.

4. A method according to claim 3 in which the laterite contains between 5 and 25% by weight silica.

5. A method according to claims 1 to 4 in which the deposit and the reaction product comprise also vanadium and chromium ions.

6. A method according to any preceding claim in which the acid is sulphuric acid.

7. A method according to any of claims 1 to 6 in which the acid is a waste acidic effluent.

8. A method according to any preceding claim in which the amount of acid is substantially stoichiometric.

9. A method according to any preceding claim in which an oxidising agent is added during the manufacture of the composition to convert any ferrous iron to ferric iron.

10. A method according to any preceding claim in which the flocculating composition is incorporated into the liquid medium while in the form of a solution.

11. A method according to any of claims 1 to 9 5 in which the flocculating composition is incorporated into the liquid medium while in the form of a solid.

12. A method according to any preceding claim in which the flocculating composition is incorporated into the liquid medium together with a detergent. 10

13. A method according to claim 12 in which the flocculating composition is incorporated into the liquid medium together with sodium chloride.

14. A method according to any preceding claim in which the aqueous liquid contains, as material that is removed 15. Hy flocculation, one or more of phosphate, nitrate and organic suspended matter.

15. A method according to claim 1 substantially as herein described.