Background of the Invention When recovering pulping chemicals in connection with pulping according to the sulphate method, the sulphate waste liquor or black liquor remaining after pulping is evaporated, yielding what is generally referred to as con¬ centrated waste liquor of a dry solids content of about 60-65%. The concentrated waste liquor is burnt in a soda recovery boiler at a temperature of 700-1000°C to form a melt which is subsequently mixed with weak liquor in a melt dissolver, yielding green liquor. The green liquor is conducted to what is generally called a green liquor clarifier, producing a clear green liquor and a viscous green liquor sludge. This sludge mainly consists of com¬ bustion residues formed when burning the concentrated waste liquor in the soda recovery boiler, and weak liquor which can be recycled to the above-mentioned melt dissol- ver. In order to separate these constituents, the green liquor sludge is passed through a filter with a pre-coat- ing of lime sludge which is taken in a small amount from the container where the green liquor is converted to white liquor by the addition of lime. This filter, which advan- tageously is a rotary filter, separates what is commonly referred to as black sludge or residual lime sludge. Apart from various impurities, this sludge contains, inter alia, sodium, calcium, and sulphur. In practice, the sludge is a gluey and viscous mass of substantially semi-plastic con- sistency. In average pulp mills with a production capacity of some 100,000 tonnes of pulp per year, about 20 tonnes

3 or 10 m black sludge or residual lime sludge is produced every day. Since this product is unusable, and thus has to be deposited on special refuse dumps, this is a consider- able inconvenience. Furthermore, the viscosity and gluey consistency of the product makes it difficult to handle and transport to the dump. Thus, the gluey residual lime

sludge has a pronounced tendency to adhere to all kinds of surfaces, «hich makes it difficult or even impossible to transport in conventional goods vehicles, this being also a considerable inconvenience. In fact, if transported on a lorry, the residual lime sludge will adhere to the vehicle platform to such an extent that an essential amount there¬ of remains on the platform even if this is tipped to a substantially vertical position. Consequently, large amounts of -sludge must be removed from the platform by hand, e.g. by means of scrapers or spades. A temporary solution has therefore been to put the residual lime sludge in special containers which, owing to their limited size, arehowever not very well suited for intermediate storage. This is particularly inconvenient when the mill is run at week-ends, when either a few containers must be emptied several times or a large number of empty con¬ tainers must be procured. Finally, it should be observed that the residual lime sludge, despite all precautions taken, easily spills on the route where it obviously becomes an environmental hazard, as it is on the dumps where it -±s deposited, for which reason these dumps need a special authority permit. Summary of the Invention

The present invention aims at obviating the above- mentioned inconveniences linked with the handling of re¬ sidual lime sludge or black sludge in the recovery of chemicals in the cellulose industry, as well as creating opportunities for simple, expedient and environment- friendly handling thereof. According to the invention, this is ^' achieved by mixing the sludge with a drying agent in the form of soot in a mixing ratio, counted in units of volume, of 1:2-1:4, suitably 1:3, the soot absorbing liquid from the residual lime sludge and forming with the sludge a dry, particulate and/or granular mixed product. The soot -obtained as a residual product in the combustion of waste liquors in the boiler house or a solid-fuel boiler can be advantageously used. At present, this soot

is collected in an outdoor container which is emptied at regular intervals. The soot, however, is volatile and has a tendency to dust, and thus, like the residual lime sludge, constitutes an environmental hazard and poses considerable problems of handling.

Mixing residual lime sludge and soot as prescribed by the invention results in a product with both particulate and granular constituents, the largest particle size being about 25 mm. The dry consistency of the product makes it easy to handle and transport since it has no tendency whatsoever to stick to, for instance, the platform of a lorry.

In practical tests, one part by volume of residual

3 lime sludge having a volumetric weight of 1.90 tonne/m was mixed with 3 parts by volume of soot having a volu-

3 metric weight of 0.70 tonne/m , yielding a mixed product having the above-mentioned qualities. This product had an earth-like appearance and a consistency resembling that of so-called industrial earth. Further, the product was brown/black and virtually odourless. With a larger propor¬ tion of residual lime sludge, giving a mixing ratio above 1:2, the mixed product became gluey and viscous. In other tests where the proportion of soot was increased at the expense of the residual lime sludge, giving a mixing ratio less than 1:4, the mixed product became so dry as to give off dust, i.e. dry soot particles were easily set free into the air. Thus, the mixing ratio of residual lime sludge to soot should principally be 1:3, although satis¬ factory results can be expected also within the range of 1:2-1:4.

Further Description of the Prior Art

SE Patent 78493 discloses a method for treating lime sludge by admixing a drying agent. This method is based on the assumption that the lime sludge is a practically use- less waste product entailing substantial costs and prob¬ lems when to be removed. Therefore, this prior art method aims at recovering the lime content of the sludge by burn-

ing, after the sludge has been dried and formed into brick-like pieces suited for burning. The lime sludge is dried, optionally by the addition of particular drying agents, e.g. pulverised coal or coke, peat or saw-dust (see the specification, p 2, col 1, ultimate lines). All these drying agents are combustible, i.e. have an intrin¬ sic value by containing combustible substances that can produce thermal energy. As stated in the specification of the SE patent, these materials must be combustible because they should be combusted when the lime sludge is burnt, thus contributing to bringing about the temperature neces¬ sary for the burning. Furthermore, these materials should increase the porosity of the pieces of lime sludge as the combustion thereof proceeds. The SE patent also states the possibility of admixing a non-combustible material, namely limestone powder, evidently for increasing the lime con¬ tent of the burnt end product. Thus, the reburning of lime sludge which is standard procedure in today's recovery of chemicals is based on this patent. The present invention aims at solving a completely different problem than SE 78493, namely how to improve the handleability and transportability of the black sludge or residual lime sludge obtained when filtering green liquor sludge through a lime sludge layer. Furthermore, the solu- tion of the invention is essentially different from that of SE 78493 in that soot is a material having no intrinsic value since, unlike coal, coke, peat or saw-dust, it is a combustion product which cannot be further burnt. Thus, the invention does not aim at providing a combustible mixed product or a mixed product having an increased con¬ tent of lime to be recovered. On the contrary, the object of the invention is to achieve, at minimum costs, expe¬ dient and simple handling of black sludge or residual lime sludge, and to this end nothing but soot is used, as explicity stated in the appended claim. More precisely, the soot is mixed with the black sludge in a mixing ratio, counted in units of volume, of 4:1-2:1, suitably 3:1.

It goes without saying that the product obtained by the inventive method can be used and refined in different ways. Thus, it may be used as an earth filler. Since the mixture is markedly alkaline owing to its content of sodium and calcium, it is advantageously used on acid soils. The product may be refined, e.g. by admixing small amounts of peat, bark, saw-dust or the like, for instance when it is to be used as an earth filler.