CA1293097C - Method of concentrating sludges - Google Patents

Abstract

Abstract Method of concentrating sludges In a method of concentrating sludges, liquor is heated under pressure indirectly with flue gases from the combustion of previously concentrated sludge after which the sludge is caused to expand or subjected to reduced pressure which results in evaporation of water and concentration of the sludge to A
dry-substance content of over 55 %.

Description

Method of concentrating sludges The present invention relates to a method of concentrating various sludges such as waste liquor from pulp production.

The waste liquor from pulp production must be concentrated to a dry-substance content of greater than 55 % before combustion in a the combustion chamber of a soda recovery system is satisfactorily possible. The heat contained in the flue gases resulting from the combustion hitherto is only used for pro-duction of steam.

Many soda recovery boilers, however, have been designed for a final concentration by direct flue gas evaporation in a flue gas evaporator. The following, for example, have been the reasons for this:

- direct flue gas evaporation does not require expensive apparatus - the evaporation plant itself can be smaller - some waste sludges form heavy deposits on heat surfaces in dry-substance contents of over 50 % which means that they are very difficult to treat in indirect evaporators.

Flue gas evaporation, however, also has its drawbacks:

- evaporation takes place in one stage and the secondary heat can not be utilized - smell and emission problems - steam production efficiency of the boiler is low :1293097 For these reasons many recovery boilers built according to the flue gas evaporation principle have been modernized by removing the flue gas evaporator, providing a new preheater for water for final cooling of the flue gases and by building a further evaporating unit for final concentration of the liquor.
Such modernization can sometimes be quite extensive. In the case of heavily contaminating liquors, such as bagasse soda liquor, modernization can not be done.

The object of the invention is to provide a substitute method for flue gas evaporation.

The method according to the invention is characterized in that the final concentration or evaporation of aqueous waste sludge is carried out in a way to utilize the heat produced in the combustion chamber of a boiler and contained in the flue gases leaving the boiler for heating under pressure the waste sludge in an indirect heat exchanger whereby the pressu-rized liquor expands to a lower pressure and water evaporates from the liquor so as to concentrate the liquor. The produced secondary steam can be used to replace the primary steam in a series evaporation plant or other apparatus operating with low pressure.

The appropriate temperature range in substituting the flue gas evaporator for black liquor is 200 - 400 oc for flue gases, and the final temperature 120 - 180 oc when the temperature of the supplied liquor is 80 - 120 oc and the temperature of the discharged liquor 160 - 250 oC.

Depending on the properties of the liquid to be treated the temperatures can be different from the ones given above.

The advantages of the method of the invention are as follows:

- when a flue gas evaporator is dispensed with, the building ~29309~

of a pressure heating unit does not require changes in the existing steam generation and heating systems of the boiler or modifications in the existing series evaporation plant - the steam produced in single or multi stage flash evapora-tion plants can be utized in the evaporation or other pro-cesses for substituting primary steam - pressure heating takes place in a concentrating apparatus having no heat surfaces, in a concentration in which no de-posits on heat surfaces are created, yet, and further treatment of the liquor is possible in existing apparatus.

The method according to the invention is described further, by way of example, with reference to the accompanying drawing which is a schematic representation of an embodiment of the invention.

A recovery boiler unit 1 illustrared in the drawing comprises a combustion chamber 2 into which concentrated waste liquor is supplied via duct 3 from a vessel 4. When waste liquor is combusted the combustion gases produced flow to the upper part of the boiler and pass through heating zones S, 6 and 7, and through evaporating zones 8. When the combustion gases have passed over heat transfer surfaces 9 to the lower part of a channel lO leading the gases downwards, which lower part is provided with a pressurized heating unit 11 for the waste liquor, the temperature of the gases is 200 - 400 oc Waste liquor, which has been concentrated to have a dry-sub-stance content of approx. 50 % in multi-stage evaporators 12, 13, is supplied by a pump 14 through s pipe 15 under a pressure of e.g. approx. lO bar to the heating unit 11 in which it is indirectly heated with by the combustion gases and from which it is then supplied via a pipe 16 to a flash-evaporator 17 for expansion to a lower pressure whereby water is evaporated from the liquor and the liquor is thus concentrated. The expansion steam i6 supplied via a duct 18 to the evaporator 12 the operation principle of which is based on indirect heat exchange, and the evaporation steam is used there as heating steam. The liquor which has been concentrated to a dry-sub-stance content of over 55 %, is discharged from the lower part of the flash-evaporator 17 and is supplied via vessel 4 to the combustion chamber 2 of the boiler unit 1.

The following examples illustrate the invention.

Example 1 (compar ion example) ~he final concentration of black liquor is carried out in a flue gas cyclone evaporator according to the following balance:

In M (kg/t d.s.) dry-substance T (oC) content (%) Black liquor 52.0 % 95 - dry substance 1000 - water 923 Flue gas 210 - dry substance 6200 - water 1012 Total 9235 Out Black liquor 5~.6 95 - dry-substance 1000 - water 706 Flue gas 140 - dry-substance 6200 - water 1229 Total 9135 1293(~97 Example 2 When the flue gas evaporator is replaced by a pressurized heating unit and a flash-evaporator for liquor the corre-sponding readings are as follows:

In M (kg/t d.s.) dry-substance T (oC) content (%) Black liquor 1923 52.0 95 Flue gas 7212 210 Total 9135 Out Black liquor 1923 S2.0 183 Flue gas 7212 140 Total 9135 Flash In Black liquor 1923 183 Out Black liquor 1678 59.6 112 Evaporation 245 100 Volume of secondary steam 0.245 t/t dry substance The invention is no limited by the described embodiment exam-ples but it can be modified in various ways within the scope of protection defined b~y the appended patent claims.

Claims (4)

1. A method of concentrating sludges to an adequate dry-substance content for combustion, comprising indirectly heating the sludge under pressure with flue gases produced by combustion of previously concentrated sludge and evaporating water from the thus heated sludge by expansion thereof at a lower pressure.

2. A method as claimed in claim 1, wherein the sludge to be treated is waste liquor from the production of pulp for the manufacture of paper or board.

3. A method as claimed in claim 2, wherein the waste liquor is concentrated to a dry-substance content of over 55%.

4. A method as claimed in one of claims 1, 2 or 3, wherein the expansion steam is used as primary steam in a series evaporation plant.