GB2117098A

GB2117098A - Dewatering of peat or peat slurries

Info

Publication number: GB2117098A
Application number: GB08119555A
Authority: GB
Inventors: John Thomas Mccullins
Original assignee: Individual
Current assignee: Individual
Priority date: 1981-06-25
Filing date: 1981-06-25
Publication date: 1983-10-05

Abstract

The peat is dewatered by the addition of trivalent metal ion salts, which neutralise negative electrical charges in the fibrous matrix, and the water thus released, is removed by mechanical means or simply by allowing air drying to take place. The moisture content of peat, as taken from the bog, can be reduced to 50- 55% w/w without the application of any energy other than that required to mechanically compress the treated slurry.

Description

SPECIFICATION Dewatering of peat or peat slurries Peat as it occurs naturally in the ground, or when mechanically dug and macerated so that it can be pumped out of a bog, seldom contains less than 95% w/w of water. Insitu bog peat, or slurries prepared thereof, are therefore not unlike aqueous gel phases and as such should be amenable to many of the theories of Colloidal Chemistry and Physics. For example one would expect that peat slurries should show great shrinkage when drying, and yield quantities of run-off water when thawing, after a freezing cycle; and such is the case.

If peat in the bog, or as a macerated slurry, is colloidal in nature then it must be retaining its very high percentage of water either in the form of a hydrophyllic shell (solvation layer) or by means of an electrical charge (Helmholtz Double Layer) or by a contribution from both.

Experiments which I have conducted have shown that the solvation layer can be destroyed by the addition of hydrophyllic solvents to peat slurries (alcohols, ketones) and when this occurs the peat shrinks greatly and releases much of its bound water content.

Solvents like the first three alcohols in the aliphatic monohydric series will dewater peat, but the process is uneconomic because the solvent losses are great and a large amount of energy and processing is required to return the solvents to a relatively anhydrous state so that they can be used again in the dewatering process. My researches, after the solvent dewatering trials, proceeded to dewatering via the reduction, or destruction, of the electrical double layer in the colloidal peat and I began treating peat and its slurries with varying quantities of electrolytes, especially those whose metal ions contained more than one ionic charge, especially those represented by the salts of iron (ferric) and aluminium.

My experiments have shown that, with care, peat slurries can be centrifuged down to moisture contents of 70% (intially 95%) but that if the slurry is treated with 0.25 to 1 kG of ferric sulphate or aluminium sulphate per cubic metre of peat then the moisture content will readily fall to 50 to 55% in an eighth of the previous centrifuging time.

In another experiment the peat slurry was treated with 1 Kg of a liquor prepared from a reaction of native bauxite or latterite with sulphuric acid and in essence this liquor was a mixture of ferric and aluminium sulphates with up to about 2% of titanium sulphate contained therein. The latterites of N Ireland contain a varying quantity of titanium dioxide but in this dewatering process it behaves in the same way as the other polyvalent metals described.

The above described liquor contains 60% of total water content and the 40% of solids constitute the aluminium, ferric iron and titanium sulphates content of the regent. In this case the negative electrical charge on the colloidal peat particles is neutralized and on coagulating, the peat shrinks and releases its water. In this second experiment the water was removed with a filter press and gave a solid of 54% moisture content.

In all these experiments and applications the peat slurries are acidic, pH value 3.5, and at no time does the pH value rise to a level where autohydrolysis of the ferric or aluminium sulphates yields the corresponding hydroxides; as would be the case in dewatering other sludges e.g. digested or raw sewage sludge.

This natural mixture of iron and aluminium sulphates, together with small percentages of titanium sulphate, is more efficient than aluminium sulphate on its own and this is attributed to the formation of some iron salts of humic acids in the peat matrix. The iron salt (ferric) of peat humic acids is more hydrophobic than the free polyphenol complex of the natural peat and hence iron electrolytes aids dewatering.

Divalent salts like the chlorides of calcium and magnesium help dewatering but as would be expected their efficiency is much lower than the trivalent effects of iron and aluminium.

In the above description I have indicated that the novel way to dewater peat, to at least a moisture content of 55% w/w, is to employ trivalent electrolytes so that the electrical double layer in the colloidal matrix is reduced, or destroyed, thereby reducing the water holding capability of both the electrical layer and the associated solvation shell.

This is the basis of the invention which I have described and the reagents which can best be employed to effect this peat dewatering comprise: (a) The salts of ferric iron or aluminium in the form of any soluble electrolyte e.g. the cheaper ones like the sulphates and chlorides.

(b) A reagent, which is on sale in N Ireland under the trade name of F.A.S. and which is prepared by reacting native bauxite, ferruginous bauxite or latterites with concentrated sulphuric acid and thereafter diluting the reagent to a solids content of 40% w/w. The reagent consists essentially of the sulphates of aluminium, ferric iron and small quantites (up to 2% of titanium sulphate).

(c) Divalent reagents like the soluble salts of calcium and magnesium can be used to effect the dewatering but the efficiency is low and they are not preferred in this dewatering context.

After the colloidal structure of the peat slurry has been affected the water can be removed by either centrifuging or by the use of a filter press.

Claims (filed 24-6-82) 1. A process for the treatment of peat, or peat slurries, whereby the peat is treated with a solution of a trivalent metal salt so that the water holding capacity of the fibrous matrix is destroyed, or greatly reduced, and dewatering is then achieved by filtration under vacuum or by pressing or centrifuging.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. SPECIFICATION Dewatering of peat or peat slurries Peat as it occurs naturally in the ground, or when mechanically dug and macerated so that it can be pumped out of a bog, seldom contains less than 95% w/w of water. Insitu bog peat, or slurries prepared thereof, are therefore not unlike aqueous gel phases and as such should be amenable to many of the theories of Colloidal Chemistry and Physics. For example one would expect that peat slurries should show great shrinkage when drying, and yield quantities of run-off water when thawing, after a freezing cycle; and such is the case. If peat in the bog, or as a macerated slurry, is colloidal in nature then it must be retaining its very high percentage of water either in the form of a hydrophyllic shell (solvation layer) or by means of an electrical charge (Helmholtz Double Layer) or by a contribution from both. Experiments which I have conducted have shown that the solvation layer can be destroyed by the addition of hydrophyllic solvents to peat slurries (alcohols, ketones) and when this occurs the peat shrinks greatly and releases much of its bound water content. Solvents like the first three alcohols in the aliphatic monohydric series will dewater peat, but the process is uneconomic because the solvent losses are great and a large amount of energy and processing is required to return the solvents to a relatively anhydrous state so that they can be used again in the dewatering process. My researches, after the solvent dewatering trials, proceeded to dewatering via the reduction, or destruction, of the electrical double layer in the colloidal peat and I began treating peat and its slurries with varying quantities of electrolytes, especially those whose metal ions contained more than one ionic charge, especially those represented by the salts of iron (ferric) and aluminium. My experiments have shown that, with care, peat slurries can be centrifuged down to moisture contents of 70% (intially 95%) but that if the slurry is treated with 0.25 to 1 kG of ferric sulphate or aluminium sulphate per cubic metre of peat then the moisture content will readily fall to 50 to 55% in an eighth of the previous centrifuging time. In another experiment the peat slurry was treated with 1 Kg of a liquor prepared from a reaction of native bauxite or latterite with sulphuric acid and in essence this liquor was a mixture of ferric and aluminium sulphates with up to about 2% of titanium sulphate contained therein. The latterites of N Ireland contain a varying quantity of titanium dioxide but in this dewatering process it behaves in the same way as the other polyvalent metals described. The above described liquor contains 60% of total water content and the 40% of solids constitute the aluminium, ferric iron and titanium sulphates content of the regent. In this case the negative electrical charge on the colloidal peat particles is neutralized and on coagulating, the peat shrinks and releases its water. In this second experiment the water was removed with a filter press and gave a solid of 54% moisture content. In all these experiments and applications the peat slurries are acidic, pH value 3.5, and at no time does the pH value rise to a level where autohydrolysis of the ferric or aluminium sulphates yields the corresponding hydroxides; as would be the case in dewatering other sludges e.g. digested or raw sewage sludge. This natural mixture of iron and aluminium sulphates, together with small percentages of titanium sulphate, is more efficient than aluminium sulphate on its own and this is attributed to the formation of some iron salts of humic acids in the peat matrix. The iron salt (ferric) of peat humic acids is more hydrophobic than the free polyphenol complex of the natural peat and hence iron electrolytes aids dewatering. Divalent salts like the chlorides of calcium and magnesium help dewatering but as would be expected their efficiency is much lower than the trivalent effects of iron and aluminium. In the above description I have indicated that the novel way to dewater peat, to at least a moisture content of 55% w/w, is to employ trivalent electrolytes so that the electrical double layer in the colloidal matrix is reduced, or destroyed, thereby reducing the water holding capability of both the electrical layer and the associated solvation shell. This is the basis of the invention which I have described and the reagents which can best be employed to effect this peat dewatering comprise: (a) The salts of ferric iron or aluminium in the form of any soluble electrolyte e.g. the cheaper ones like the sulphates and chlorides. (b) A reagent, which is on sale in N Ireland under the trade name of F.A.S. and which is prepared by reacting native bauxite, ferruginous bauxite or latterites with concentrated sulphuric acid and thereafter diluting the reagent to a solids content of 40% w/w. The reagent consists essentially of the sulphates of aluminium, ferric iron and small quantites (up to 2% of titanium sulphate). (c) Divalent reagents like the soluble salts of calcium and magnesium can be used to effect the dewatering but the efficiency is low and they are not preferred in this dewatering context. After the colloidal structure of the peat slurry has been affected the water can be removed by either centrifuging or by the use of a filter press. Claims (filed 24-6-82)

1. A process for the treatment of peat, or peat slurries, whereby the peat is treated with a solution of a trivalent metal salt so that the water holding capacity of the fibrous matrix is destroyed, or greatly reduced, and dewatering is then achieved by filtration under vacuum or by pressing or centrifuging.

2. A process according to Claim 1 in which the trivalent metal salt can be any water soluble salt of aluminium or ferric ion, preferably the sulphate or chloride.

3. A process according to Claims 1 and 2 in which the aluminium and ferric iron salts are used together in a natural ratio which occurs when any bauxite or latterite is reacted with sulphuric acid.

4. A process according to Claims 1 to 3 in which the peat is treated with the trivalent metal solution during digging and maceration and then set aside in the bog for air drying.

5. A process according to Claims 1 to 3 in which the peat is treated with a trivalent metal salt solution and then dewatered by pressing.

6. A process according to Claim 5 in which the pressure on the treated peat is maintained for a time, preferably 5 to 10 seconds, before it is released and the compressed cake expelled from the hydraulic or screw press.

7. A process for the treatment of peat, substantially as hereinbefore described, where the colloidal structure of the organic matrix is modified by the addition of a trivalent metal ion and the released water removed by mechanical procedures.