State of the art and problems When pulp is produced in accordance with the sulphate method, a spent liquor, gen- erally termed black liquor, is obtained which contains organic material and the re- sidual chemicals which have been used when cooking the fibre raw material. In general, this black liquor is evaporated and conveyed to a separate process for re- covering of the energy content of the organic material and recovering the cooking chemicals as so-called green liquor. For a long time, the so-called Tomlinson pro- cess has been the commercially dominant method used for this recovery of energy and chemicals. However, a disadvantage of this process, which is now very old, is that it requires very large combustion ovens which are complicated both from the technical point of view and as regards their operation.

Swedish patent SE 448,173 describes a more recent process which, besides re- quiring process equipment which is appreciably simplified, achieves an improved recovery of both energy and chemicals. This process is based on a pyrolysis reaction in which the black liquor is gasified in a reactor, resulting in the formation of an en- ergy-rich gas, principally comprising carbon monoxide, carbon dioxide, methane, hydrogen and hydrogen sulphide, and of inorganic chemicals in the form of small

drops of smelt, principally comprising sodium carbonate, sodium hydroxide and sodium sulphide. The resulting mixture of gas and smelt drops is rapidly cooled, in a first stage, by means of direct contact with a cooling liquid consisting of water and green liquor, which is formed when the smelt chemicals dissolve in the cooling li- quid. The gas is subsequently washed, in a second stage, in a gas-washing apparatus of the scrubber type. The gas is then used as a fuel for generating steam and/or electric power. The physical calorific value of the gas can also be utilized when the gas is cooled down from the gasification temperature to the saturation temperature for aqueous steam at the selected pressure. At a saturation temperature of 252"C, corresponding to 40 bar, for example, steam having a pressure of 3-8 bar can be generated when the green liquor is cooled and when the gas is cooled and its water content is condensed downstream of the gas-washing tower.

Nevertheless, this process too, despite being appreciably simpler and smoother than the Tomlinson process, still leaves room for improvement. For example, unwanted carbonate and hydrogen carbonate is formed in the green liquor when carbon di- oxide in the pyrolysis gas comes into contact with the green liquor when the gas and smelt droplets are quenched and dissociated in the aqueous solution of the first stage. Hence, pH of the green liquor is decreased. It also leads to formation of H2S which leaves together with the rest of the gases.

Consequently, there is a need for a process where the amount of time is minimized during which it is possible for carbon dioxide to come into contact with the green liquor and gaseous and/or liquid hydroxide salts.

Summary of the invention Now it has turned out that the above mentioned problems can be solved by conduct- ing the superheated gases from the gasifier (1) through a downcoming tube (2) which ends above the liquid surface of a quench vessel (3) containing a receiving liquor. One or more cooling media (13), preferably comprising condensate and satu-

rated steam, is injected into the stream of superheated gas within or near said tube in such an amount that the resulting mixture remains superheated and as a result solid salt particles are formed in the hot gas. The gaseous components of the flow from the gasifier including the injected cooling medium/media, will leave the quench ves- sel without any substantial contact with the receiving liquor, so that the contact time with the receiving liquor is prevented or minimized. The solid salt particles are, however, forced by gravity to continue down into the receiving liquor of the quench vessel for dissociation.

Brief description of the figures The invention will now be described below in more detail on the basis of a preferred embodiment and with reference to the attached figures, where: Figure 1 shows a preferred embodiment of the concept according to the invention.

Figure 2 is a simplified sketch illustrating the need of causticizing the salts of green liquor produced by gasifying spent liquor as a function of the degree of superheating in the zone where the salt particles are separated from the gaseous compounds in the gasifier product.

Detailed description of the invention As disclosed herein, the term "receiving liquor" relates to an aqueous liquid com- prising alkali metal salts, such as green liquor or white liquor.

As disclosed herein, the term "cooling medium" relates to steam and/or an aqueous liquid substantially free from alkali metal ions, such as a condensate from a counter- current condensor or a sulphur-containing condensate from evaporation of spent liquor. Several different types of cooling media may be used at the same time.

Steam is preferred.

As disclosed herein, the term "washing liquid" relates to an aqueous liquid which preferably comprises a condensate from indirect cooling.

As disclosed herein, the term "quench vessel" relates to a vessel containing the re- ceiving liquor. The quench vessel is arranged in such a way that salt particles soli- difying or close to solidifying as a result of injection of cooling medium in the stream from the gasifier, can be separated and dissolved in the receiving liquor.

As disclosed herein, the term "gasifier product" relates to the hot mixture of gases and salt smelt that enters the down-coming tube from the gasifier.

As disclosed herein, the term "superheated gas" relates to a gaseous mixture con- taining steam and does not contain any condensed water, i.e. steam of a certain pressure which is heated above the condensation/boiling temperature of that pres- sure.

All pressure values in the following description and the appended claims refer to ab- solute values.

The following equilibrium reactions in the quench are of special importance for the composition of the aqueous solution of alkali metal salts produced according to the present invention.

By contacting a process gas containing carbon dioxide with an alkaline solution us- ing prior art quench vessels, the gas tends to absorb into the solution and the resul- tating pH value of green liquor is decreased.

In accordance with reactions (1) and (6), alkali is partly converted to alkali hydro- gen carbonate, which is a highly undesirable compound in green liquor as it puts an extra load on the causticizing system. Twice as much lime is needed for conversion of alkali hydrogen carbonate to hydroxide compared to conversion from alkali car- bonate.

The concept behind the method which has been devised is to bring about the pos- sibility of producing green liquor and/or white liquor without unwanted hydrogen carbonate being formed in this liquor, and unwanted hydrogen sulfide being formed in the gas.

The principle is that the gasifier product leaving the reactor is cooled by means of direct contact with one or more cooling media, causing mostly solidification of the salts. Any extensive contact between the gas and the receiving liquor is avoided to the greatest extent possible. According to the present invention carbon dioxide in the gasifier product is prevented from reacting with sodium carbonate (reaction (6) above), and carbon dioxide is prevented from reacting with sodium hydroxide (reaction (3) above) and forming sodium carbonate. Furthermore, carbon dioxide is prevented from reacting with sodium hydrogen sulfide (reaction (4) above) and forming sodium hydrogen carbonate and also hydrogen sulfide desorption from the reaction between sodium hydrogen carbonate and sodium hydrogen sulfide is avoided. It is advantageous if the sodium hydroxide which has been formed is not converted to sodium carbonate, since sodium hydroxide is the desired final product following causticizing of the green liquor. During the causticizing, sodium carbon- ate is converted to sodium hydroxide by reacting with slaked lime.

The need of causticizing of green liquor produced according to the present invention is illustrated in fig. 2. The figure discloses a simplified sketch showing the need for causticizing the produced green liquor as a function of the temperature in the quench region where the alkali metal salts in the gasifier product are separated from the gaseous compounds in said gasifier product.

Preferably the above mentioned superheated gaseous compounds are quenched in a second stage, which is configured so that the maximum degree of contact is achieved between the gas and the washing liquid. Since the alkali metal salts were separated from the gas in the frrst stage the above mentioned equillibrium reactions cannot occur in this stage, and thus the intense contact between gas and liquid does not impair the result. The gas is quenched by being allowed to bubble through a liquid bath which principally consists of washing liquid. In this way, the originally superheated hot gas can be efficiently cooled and moist saturated.

Fig. 1 discloses a preferred embodiment of the present invention. Spent liquor is gasified in a ceramically lined gasification reactor (1). The reactor (1) is provided with an inlet for black liquor (not shown) and an inlet for oxygen or oxygen-con- taining gas (not shown), and a burner (not shown). The temperature of the gasifier is kept within the range of 500"C-1600"C, preferably 800°C-1200°C. The pressure in the gasifier is held in the range of 1-150 bar. There are two separate kinds of gasifi- cation processes, namely low-pressure (or atmospherical) gasification or high- pressure (or pressurized) gasification. Low-pressure gasification (e.g. sulphide reac- tor) is carried out at 1.5-5 bar, preferably 1.5-3 bar. High pressure gasification (e.g.

green liquor reactor), which is preferred, is carried out at 1.5-150 bar, preferably 10- 80 bar, and most preferably 25-40 bar. The bottom of the reactor opens as a down- coming tube (2), which in turn ends above the resting surface of the receiving liquor (14) in the quench vessel (3). The down-coming tube (2) ends (19) more than 0.5 m, preferably more than 0.7 m and most preferably more than 1.0 m above the surface of the receiving liquid. One ore more cooling media are transported in at least one conduit (8) to at least one nozzle (7) and is then injected into the hot gasifier pro-

duct.The cooling medium is injected at a pressure above the pressure in the down- coming tube (2). It is necessary that the temperature of the resulting mixture formed by the injected cooling medium and the gasifier product remains superheated in or- der to prevent dissociation of the salts and unnecessary formation of carbonates and hydrocarbonates, and that this temperature preferably is lower than the smelt tem- perature of the alkali metals in order enable the smelt to solidify or partially sodifi- fied or form sufficiently sized drops of smelt in the separation zone before reaching the surface of the receiving liquor (14). Preferably said mixture temperature is within the range of 250-800"C, preferably 250-600"C and most preferably 250- 400"C. When the pressure in the down-coming tube (2) is atmospheric the tempera- ture of the mixture of the cooling medium and gasifier products in the down-coming tube (2) is in the range of 150-800°C, preferably 150-600°C and most preferably 150-400°C. The injected cooling medium may at least partially be in liquid state, provided that the resulting mixture is superheated. The cooling effect is stronger if a liquid cooling medium is injected because evaporation of the liquid requires energy.

The skilled person can easily determine the amount of cooling medium that can be mixed with the above mentioned hot gasifier product so that this superheating re- quirement is fulfilled. In some cases it may be advantageous just to inject a minimal amount of cooling medium, especially when the gasification is carried out at a rela- tively low temperature or when a higher degree of superheating is desired. Prefera- bly the cooling medium/media is/are injected perpendicularly in relation to the gasifier product stream, or at an angle towards the quench vessel. Furthermore, it sometimes might be advantageous to inject the steam tangentially in order to create a rotational movement. In an alternative embodiment, cooling medium is introduced into a space (annulus, tubes) around the reactor in order to cool it before being supplied. The solidified or partially sodified salt particles (5) are moved downwards by the resulting gas stream and into the liquid by means of gravity and are finally dissolved in the receiving liquor (14). The concentration of the receiving liquor in quench vessel (3) is controlled by adding water or a suitable aqueous solution through pipe (18) and transporting receiving liquor away through conduit (6). The gaseous constituents of the mixture of cooling medium and gasifier products are

passed through at least one opening (15) after the down-coming tube (2). The outlet (15) is arranged at least 0.5 m, preferably 0.7 m and most preferably 1 m down- wards from nozzle (7) in order to obtain a suitable level of superheating of the gase- ous compounds/smelt drops within the tube (2).

In a preferred embodiment of the present invention the superheated gases are subse- quently quenched, and optionally cooled and moist-saturated in a second stage after passing through opening (15). The gases are forced into at least one preferably an- nular cavity (12) at least partially filled with washing liquid (11). Washing liquid is introduced into the side cavity (12) by means of at least one conduit (10) and at least one nozzle (9). It is preferable to inject the washing liquid in a manner so as to not dilute the receiving liquor. When the gases are passed through the side cavity (12) an intense contact with the washing liquor (11) is obtained. The gas is let out via conduit (16) together with at least a part of the washing liquid. It is also possible to let out excess washing liquid via pipe (17).

The present invention can, of course, be applied to recovering alkali metal salts from different steps in different pulping processes, such as recovering alkali metal salts from spent bleach liquor, or spent liquor from production of CTMP, or spent liquor from a pulping process based on potassium hydroxide.

The invention will now be further illustrated by the following example.

Example Gasified black liquor was quenched by a process according to the present invention.

Gaseous compounds and small smelt drops were let in a down-coming tube ending 1 m above the resting surface of green liquor. The temperature of the gas/smelt mixture was 950 "C and the pressure was 32 bar. As cooling media were injected condensate downstream followed by moist saturated steam of 40 bar, 250"C, in the down-coming tube, whereby the temperature of the resulting superheated mixture

became 410"C. The superheated steam and the gaseous compounds were let out through the down-coming tube to a second quenching step and the solidified smelt or partially solidified salt particles were separated from the gas stream by means of gravity and subsequently dissolved in green liquor. The results are disclosed in the table below.

Comparative example Gasified black liquor was quenched according to the teachings of SE-C-448, 173.

The gasification process were performed in the same manner as in the example. The results are disclosed in the table below.

Table Compound Example Comparative example NaHCO3 40 g/l Na2Co3 105 g/l 108 g/l NaHS 15 g/l 12 g/l NaOH 40 g/1 0 Total 160 g/l 160g/l Read g/l counted as NaOH.

The results show that green liquor produced according to the present invention is more alkaline than green liquor produced according to the method of the state-of- the-art. Causticizing of the green liquor of the state-of-the-art requires 120 % more slaked lime than the green liquor produced according to the present invention.