WO1994027917A1

WO1994027917A1 - Water purification composition and its use

Info

Publication number: WO1994027917A1
Application number: PCT/SE1994/000485
Authority: WO
Inventors: Halina Wnukowska
Original assignee: Admix Scandinavia Ab
Priority date: 1993-05-25
Filing date: 1994-05-24
Publication date: 1994-12-08
Also published as: EP0701536A1; SE9301775D0; SE9301775L; SE501569C2

Abstract

A water purification composition is disclosed, which comprises an aqueous solution of (A) an alkali metal or alkaline earth metal hydroxide, (B) an alkali metal or ammonium sulphide or sodium, potassium or calcium polysulphides, (C) an alkali metal disulphite, (D) an alkali metal dithionite, and (E) an alkali or alkaline earth metal thiosulphate or ammonium thiosulphate. The composition is used for the treatment of polluted water containing heavy metal ions, e.g. polluted scrubber water from the purification according to the wet method of flue gases from refuse incineration.

Description

Water purification composition and its use

The present invention relates to a water purification composition based on sulphur-containing compounds and the use of said composition. More particularly, this invention is related to a composition for the removal of heavy metals from aqueous liquids, which contain these metals in the form of ions, such as industrial waste waters.

The heavy metals which are especially contemplated in this connection are copper (Cu) , silver (Ag) , mercury (Hg) , cadmium (Cd) , zinc (Zn) , lead (Pb) , tin (Sn) , nickel (Ni) , cobalt (Co) , iron (Fe) , chromium (Cr) , molybdenum (Mo) , tungsten (W) and vanadium (V) .

The development and growth of industry has involved an increase in the amounts of industrial waste waters containing heavy metals. These effluents can be derived from various chemical and electrochemical processes within, for instance, metal finishing industry, tanneries, chloralkali factories, organic process industry and refuse disposal units.

Heavy metals do not decompose in nature but are accumulated in the living organisms.

The dangers associated with wastes containing heavy metals have been paid attention to during the last decades resulting in demands for purification. Particularly rigorous demands are nowadays put on effluents of poisonous heavy metals such as Hg, Cd, Pb, Zn, Ni, Cu, Cr, Sn, Mo, W and Co. This in turn puts very high demands on the purification techniques which are used.

Chemical treatment of industrial waste waters includes a great number of processes belonging to the following principal groups: hydroxide precipitation, sulphide precipitation, oxidation reactions, and reduction reactions.

The chemical treatment is often combined with physical- chemical processes, such as electrodialysis, ultrafiltration and ion exchange technique, along with a mechanical treatment, e g sedimentation or flotation.

Chemical treatment involves the addition of a reagent, which precipitates the polluting substance as a slightly soluble or insoluble compound. Usually sodium hydroxide (NaOH) , calcium hydroxide [Ca(0H)₂] or sodium carbonate (Na2C03) is added as the precipitation chemical. Then the pH-value is adjusted to the value at which the solubility of the metal hydroxide (the amount which is in solution) exhibits a minimum.

An equilibrium is established between metal ions in solution and the metal hydroxide.

Me²⁺ + 20H^" «*Me(OH)₂ Me³⁺ + 30H-_^Me(OH)₃

Precipitation differs from flocculation in that the polluting substance participates itself in the reaction instead of being adsorbed or enclosed in a flock.

In those cases where a pure precipitation reaction is at hand the course can be determined from the solubility product and one can decide how great residual amount of the pollution which can be accepted after the treatment.

However, the solubility of the hydroxide precipitates is too high which results in too high residual amounts of the pollutions after this treatment.

Another disadvantage of this method is that the solubility of the metal hydroxide is greatly depending on the pH-value. Different metal hydroxides exhibit different solubility minima at different pH-values. This means that effective precipitation of an individual metal only can be achieved within a very narrow pH range.

This renders precipitation more difficult, especially when complexing agents such as ethylenediamine tetraacetic acid (EDTA) , tartaric acid or citric acid are present in the waste water. Accordingly, in case of metal containing waters of varying composition, a multi-step precipitation is required which results in high costs. These hydroxide precipitates are often gelatinous and difficult to remove by filtration.

Hydroxide precipitation is often supplemented with ion exchangers, reversed osmosis or electrodialysis for that reason. However, these processes require high investment costs. Nor do they cope with greater variations in the loading.

The alternative to hydroxide precipitation in order to obtain process stable solutions by means of precipitation is for chemical and economical reasons only the use of precipitation agents based on sulphur.

Purification of waters contaminated with heavy metals by precipitating the metals as metal sulphides is a process which has been known and used for a long time. A plurality of courses of action are used, e g with sodium sulphide, organic sulphur compounds, polysulphides and different combinations of organic and inorganic sulphur compounds.

In contrast to the hydroxides, the sulphides have such a low solubility product that a high-grade purification is attained also in case of water containing strong complexing agents.

9

Hydrogen sulphide, H-^S, l e the sulphide ion S~^~, forms slightly soluble metal sulphides with mono-, di-, and trivalent metal ions, 2Me - +^" + S :2^z-^' → Me₂S

Me²⁺ + S^{2 ■} MeS

2Me^{3 +} + 3S^2" Me₂S₃

Depending on the precipitating agent and the type of metal ion the pH range for the precipitation is chosen. In case of metal sulphides having such low solubility products that these are exceeded already at a minimal

concentration the precipitation can be carried out in acid solution. This applies to mercuric sulphide (HgS) , lead sulphide (PbS) , cupric sulphide (CuS) and cadmium sulphide (CdS) whereas nickel sulphide (NiS) and zinc sulphide (ZnS) , the solubility products of which are comparatively high, in contrast are precipitated in alkaline solution.

Sulphide precipitation is very seldom practised today. The handling of the nasty-smelling, poisonous, explosive and combustible hydrogen sulphide is encountered with problems on one hand and most metal sulphides are difficult to separate from an aqueous phase on the other.

In order to avoid these disadvantages of sulphide precipitation EP-A1-0349671 suggests the use of a precipitation composition comprising alkaline earth metal polysulphides. It is alleged in this reference that polysulphides are very unstable in alkaline surroundings and are readily decomposed under emission of hydrogen sulphide. Based on this allegation this precipitation composition is initially used at an acid pH-value, whereafter a neutralization is carried out. The precipitation composition thus used has a low solubility (about 3-4% based on dry substance) which is mentioned as an advantage because the precipitation composition can never be added in high concentration to the waste water to be treated and accordingly formation of hydrogen sulphide is avoided. At the same time, however, the great amount of water which must be conveyed when the precipitation composition is not prepared on the place for its use, will constitute a significant disadvantage from transportation point of view.

According to the present invention there is now provided a water purification composition which utilizes the advantages of the sulphide precipitation (low solubility products) and at the same time avoids the disadvantages thereof and in itself combines the properties that it - suppresses emission of hydrogen sulphide (the Wackenroder reaction) , - gives extremely low residual contents of metal in the waste water, has a very high solubility in water, - precipitates heavy metal ions also in the presence of complexing agents, gives a precipitate with much better sedimentation properties than that obtained by traditional precipitation of metal sulphides, - functions over a wide pH range, viz 0.5-14.

The composition according to the invention is characterized in that it comprises an aqueous solution of A) an alkali metal or alkaline earth metal hydroxide, B) an alkali metal or ammonium sulphide or sodium, potassium or calcium polysulphides,

C) an alkali metal disulphite,

D) an alkali metal dithionite and

E) an alkali or alkaline earth metal thiosulphate or ammonium thiosulphate.

The term "alkali metal" as used in this connection preferably designates sodium and potassium, especially sodium, and the term "alkaline earth metal" preferably designates calcium.

Accordingly to the preferred embodiment of the composition according to the invention all components A) to E) comprise a sodium compound.

Purification of water contaminated with heavy metals is carried out by adding the precipitation composition according to the invention to the polluted water.

The components B) - E) included in the composition according to the invention give the following effects:

Component B) (sulphide or polysulphide)

The sulphide ion S 9 , forms practically insoluble sulphides with divalent and trivalent metal ions:

In neutral and basic environment:

2Me⁺ + S^2" → Me₂S Me²⁺ + S²- → MeS 2Me^{3 +} + 3S^2- → Me₂S₃

In acid environment:

On precipitation with

in acid environment not only the S~9^' ion but also the HS^" ion will react with the metal ions. This leads to the formation of hydrogen sulphide salts, which are unstable and are transformed into the corresponding metal sulphides.

Me²⁺ + 2HS^~ → Me(HS)₂ Me(HS)₂ → MeS + H₂S

The evolution of hydrogen sulphide arising in acid environment is, however, intercepted by the presence of thiosulphate (component E) in the so-called Wackenroder reaction (vide below) . Precipitation with sodium polysulphides (Na2S_χ) in acid environment takes place according to the reaction:

(l-y)Me²⁺ + yNa₂S_χ + 2(l-y)H⁺ → (l-y)MeS + yNa₂S_n + +[x(l-y)-yn]S + (l-y)H₂S

wherein x is the number of moles of S associated with Na in the starting polysulphide, n is the number of moles of S associated with Na in the product mixture, and y is the stoichiometric coefficient.

Like in the case above the evolution of hydrogen sulphide arising in acid environment is intercepted by the presence of thiosulphate in the Wackenroder reaction.

Component C) (disulphite)

9 . .

The disulphite ion S-°5 ^{1S nere} utilized as an reducing agent, e g for the reduction of chromium (VI) to chromium (III) .

Component D) (dithionite)

The dithionite is subject to disproportionation to the formation of thiosulphate ions

according to the reactions:

^S2°4^{2" + 2H}2° ^→ 2S0₃ ^2" + 4H⁺ + 2e^~ oxidation ^S2°4^{2" + 2e~ + H}2° ^{→ S}2°3^{2" + 20H"} reduction

2S₂0₄ ^2' + H₂0 2SC ²- + S 2Oc3^2" + 2H⁺

By the incorporation of component D) a regeneration of thiosulphate ions is obtained which can be used in the precipitation of metal ions or for the binding of hydrogen sulphide. Component E) (thiosulphate)

The thiosulphate ion (S2O3^2") forms practically insoluble metal sulphides with divalent and trivalent metal ions:

In neutral and basic environment:

Me²⁺ + S₂0₃ ^2" + H₂0 → MeS + S0₄ ^2" + 2H⁺ 2Me³⁺ + 3S₂0₃ ^2" + 2H₂0→Me₂S₃ + 3S0₄ ^2" + 6H+

In acid environment:

S₂0₃ ^2" + 2H+ → HS0₃ ^" + S + H⁺ HS0₃ ^" + S + H⁺ → S0₂ + H₂0 + S

Furthermore, it binds hydrogen sulphide in the so-called

Wackenroder reaction. The solution thus formed (Wackenroder liquid) comprises a mixture of polythionic acids and colloidal sulphur.

S₂0₃ ²- + 2H⁺ → HS0₃ ^' + S + H⁺ HS0₃ ^" + S +H⁺→S0₂ + H₂0 + S

2H₂S₂0₂ + H₂S → H₂S₅0₂ + 2H₂0

In the case when all components A) - E) in the composition according to the invention consists of sodium compounds the composition will usually contain

2.5 - 4.5%, preferably 3.3 - 3.7 % by weight of NaOH as component A) , 8.5 - 16,1%, preferably 9.5 - 12.1% by weight of Na₇S as component B) ,

1.1 - 3.2%, preferably 1.7 - 2.5% by weight of Na2S₇C>5 as component C) ,

3.5 - 6.7%, preferably 5.1 - 5.8% by weight of Na₂S2θ as component D) , and

1.0 - 3.0%, preferably 1.5 - 1.9% by weight of Na->S2θ as component E) , calculated on the total weight of the composition, the balance to 100% by weight being water.

As an alternative to sodium hydroxide as component A) potassium hydroxide or calcium hydroxide can be mentioned.

As an alternative to sodium sulphide as component B) ammonium sulphide or potassium sulphide can be mentioned.

Furthermore, as an alternative to sodium sulphide as component B) there can be used a mixture of sodium, potassium or calcium polysulphides, preferably a mixture of sodium or calcium sulphides which is prepared in the way disclosed in Ullman, Enzyklopadie der technischen Chemie, l:st Edition, 1964, pp 527 and 529.

An an alternative to sodium disulphite as component C) potassium dithionite can be mentioned.

As an alternative to sodium thiosulphate as component E) ammonium thiosulphate, potassium thiosulphate or calcium thiosulphate can be mentioned.

When any of these compounds is substituted for the corresponding sodium compound the amount thereof is calculated to correspond to the amount on molar basis of the compound that it should replace.

The composition according to the invention can be prepared by dissolving the different components in arbitrary order in water.

The invention also relates to the use of a composition according to the invention for the treatment of polluted water containing heavy metal ions.

The composition according to the invention can be used as the sole precipitation agent or in a supplemental precipitation step subsequent to a hydroxide precipitation. In the latter case no separate separation of sludge is normally required between the addition of hydroxide and the composition according to the invention.

The composition according to the invention is added to the polluted water while stirring, the amount to be dosed of a composition based on sodium compounds having a content of the different components within the preferred range as set forth above generally being within the range of from 20 to 150 ml

1 . . . . per irr of polluted water depending on the initial content of heavy metal ions of the polluted water. The optimum amount in each individual case can be established by simple pre- experiments.

When the composition according to the invention is added to waste water in order to separate heavy metal ions therefrom it is preferred to adjust the pH-value of the waste water to 2-11 or especially to 5-8.

Optimum pH-values for the precipitation of some generally occurring heavy metals by means of the composition according to the invention are given below: Metal ion Optimum pH

Cd 2+ 3.8-3.9

Hg^ 3.3-3.4

Ni 2 + 5.6-5.7

Pb 2 + 6.1-6.2

Sn 2 + 6.9-7.0

Zn 2+ 7.1-7.2 Any suitable acid or alkali can be used as the acid or alkali for the adjusting of the pH-value. However, generally hydrochloric acid (HC1) , sulphuric acid (H₂S0₄) or, nitric acid (HNO3) is used as the acid whereas sodium hydroxide (NaOH) , potassium hydroxide (KOH) or calcium hydroxide [Ca(0H)2] is used as the alkali.

The composition according to the invention may advantageously be used for the treatment of polluted scrubber water from the purification according to the wet method of flue gases from refuse incineration.

Hence, according to another aspect of the invention there is also provided a method for the purification of flue gases from refuse incineration, wherein the flue gases are washed in a scrubber to give polluted scrubber water, which is purified in a water cleaning plant, which method is characterized in that a) the pH of the scrubber water is in a first stage of its purification adjusted to a value of about 0.8 to 2.0 with calcium carbonate, whereafter carbon dioxide thus formed is removed in a stripper, b) the scrubber water in a second stage is further treated by the addition of a composition according to the invention, c) the pH-value is raised in a third stage to about 9 to 9.5 by the addition of calcium hydroxide and d) a flocculant is added in a fourth stage, whereafter e) the sludge thus formed is separated in a conventional way.

The invention will in the following be further illustrated by means of a number of working example which shall not be construed as limiting the invention.

EXAMPLE 1

A composition for the purification of water was prepared by dissolving the substances listed below in the specified amounts in water and then filling up to 100.00 parts by weight of solution with water.

Component Parts by weight

NaOH 3 . 61

Na₂S 10 . 20

Na₂S₂0₅ 2 . 21

Na₂S₂0₄ 5 . 60

( corresponding to Na2S2U3 1 . 8 )

EXAMPLE 2

The composition prepared according to Example 1 was used for the purification of a waste water containing heavy metal ions and having a pH-value of 1.43.

The pH-value was first raised with Na2C03 to pH = 3 and then with NaOH to pH = 9.35. Then the composition was added in an amount of 0.1 1/ιrr of waste water.

The contents of the different metal ions in the water before ("Water IN") and after the treatment ("Water OUT") can be seen from the following Table.

Metal in Water IN Water OUT mg/1 mg/1

Zn2+ 70.32 <0.0041

Claims

C L A I M S

1. Water purification composition based on sulphur containing compounds, characterized in that it comprises an aqueous solution of

A) an alkali metal or alkaline earth metal hydroxide,

B) an alkali metal or ammonium sulphide or sodium, potassium or calcium polysulphides,

C) an alkali metal disulphite, D) an alkali metal dithionite and

E) an alkali or alkaline earth metal thiosulphate or ammonium thiosulphate.

2. Water purification composition according to Claim 1, characterized in that all components A) - E) comprise a sodium compound.

3. Water purification composition according to Claim 2, characterized in that it contains 2.5 - 4.5%, preferably 3.3 - 3.7 % by weight of NaOH as component A) ,

8.5 - 16,1%, preferably 9.5 - 12.1% by weight of Na₂S as component B) ,

1.1 - 3.2%, preferably 1.7 - 2.5% by weight of Na₂S₂0₅ as component C) ,

3.5 - 6.7%, preferably 5.1 - 5.8% by weight of Na₂S₉0₄ as component D) , and

1.0 - 3.0%, preferably 1.5 - 1.9% by weight of Na_?S-_>0 as component E) , calculated on the total weight of the composition.

4. The use of a composition according to any of Claims 1-3 for the treatment of polluted water containing heavy metal ions.

5. The use according to Claim 4, characterized in that the polluted water to be treated is polluted scrubber water from the purification according to the wet method of flue gases from refuse incineration.

6. Method for the purification of flue gases from refuse incineration, wherein the flue gases are washed in a scrubber to give polluted scrubber water, which is purified in a water cleaning plant, which method is characterized in that a) the pH of the scrubber water is in a first stage of its purification adjusted to a value of about 0.8 to 2.0 with calcium carbonate, whereafter carbon dioxide thus formed is removed in a stripper, b) the scrubber water in a second stage is further treated by the addition of a composition according to the invention, c) the pH-value is raised in a third stage to about 9 to 9.5 by the addition of calcium hydroxide and d) a flocculant is added in a fourth stage, whereafter e) the sludge thus formed is separated in a conventional way.