Technical Field:

The present invention relates to a method of recovering alkaline chemicals from a material containing inorganic dissolved compounds, which contain sodium and/or potassium by which method the material is gasified in a reactor by an external heat source at a temperature of preferably over 1000 C thus producing gas in which the sodium and the potential potassium are substantially in a gaseous state, which gas is then cooled.

Background Art;

When spent pulping liquor containing sodium or potassium salts is combusted in order to recover the heat content of the organic material dissolved during the cooking and the- chemicals contained in the spent liquor, the alkaline metals begin to vaporize to a gas phase when the temperature exceeds 1000 C; the smaller the air coefficient and the higher ^* * the temperature is, the intenser the vaporization becomes. By supplying the strongly substoichiometric combustion with additional heat energy by preheating with plasma, auxiliary fuel or other combustion gas, it is theoretically possible to reach conditions where e.g. the combustion of sulfate black liquor produces gas the temperature of which is over 1000°C, even 1500°C, and which contains CO, CO-, H_0 and H-S and in which the alkaline metals are substantially in the form of a one-atomic gas. This is disclosed by the Swedish patent application no. 8302245.9.

The application also discloses a method of condensing the alkaline salts to a melt or to an aqueous solution from the gas phase by cooling the gas produced by the

combustion as described above whereby the cooking chemicals are directly recovered as NaOH and Na-,S.

In order to recover the cooking chemicals directly, it is necessary to cool the gas very quickly to prevent the CO-, contained in the" gas from reacting with NaOH or Na-,0 to form sodium carbonate. If sodium carbonate is formed, the advantage provided by the method, i.e. the direct production of cooking chemicals, e.g. NaOH, is lost.

In conventional cooling the NaOH/Na_,0/Na is condensed onto the cooling surface in the melt temperature range concerned and forms a layer the thickness of which increases until the surface temperature reaches the melting point. The condensed material is removed slowly from the cooling surfaces, providing the melt with a long contact time with the CO- in the "gas. The heat transfer is decreased. The melt layer is corroding and decreases the heat exchange capacity, it also shortens the endurance expectancy of the device. Finally, the gas is cooled slowly and the Na-CO, content in the melt increases with the result that it is no longer an advantage to use the melt directly as a cooking chemical.

Disclosure of the Invention;

The present invention provides a remarkable improvement to the above. The cooling of the vaporized alkaline metal compounds is controlled and rapid whereby the formation of sodium carbonate is minimized. No corroding melt condensates are produced.

The method of the invention is characterized by the feature of cooling the gas from the gasification reactor by bringing it into contact with an adequate amount of

cooled recirculated solid particles, which are separated from the gas, to decrease the temperature of the gas rapidly below the sublimation temperature of sodium or potassium compounds.

The present invention is particularly applicable to the recovery of alkaline chemicals from the spent liquor of cellulose cooking.

The method of the invention provides very rapid and efficient cooling of the gas. When the gas is mixed with the small circulating bed particles which move fast, the heat is quickly divided evenly between the particles and the gas. Further, the combined area of the cooled particles mixed with the hot gas flow is very large thus providing a large heat exchange area which results in very rapid cooling of the gas below the sublimation temperature. In general the cooling speed is of the order of 500 - 1000°C/s or greater.

When the cooling is carried out by mixing cold, cooled particles separated from the gas flow, to the hot gas flow to decrease the temperature of the suspension of gas and solid material rapidly enough below the sublimation temperature of the gaseous sodium or potassium compounds the above mentioned condensation takes place by sublimating the sodium or potassium compounds directly as a solid layer onto the surface of the particles. At the same time the gas is cooled past the critical temperature range (approx. 500 - 1000°C) so rapidly that the formation of carbonate is practically impossible.

By bringing the gas into contact with a large amount of circulating particles, the advantage of cooling the gas rapidly below the temperature (500 - 1000°C) , which is

kinetiσally advantageous for the formation of carbonate, is achieved, further the gas is rapidly cooled below the sublimation point of the solid material, which is an advantage for the operation of the device.

The method of the invention makes feasible a compact device in which the cooling of the gas is controlled without the formation of melt and in which the formation of carbonate is minimized.

In gasification of the spent pulping liquor from e.g. sulfate cellulose cooking, the circulating particles consist of sublimated alkaline compounds, mainly Na-0/NaOH and a ₂ S.

To reach balance, a part of the particles must be removed from the cooling circulation. For instance, by dis- _^ solving the above particles in water, a strong NaOH solution containing Na-,S is produced which can be used for washing out the H_,S remaining in the gas phase. Alternatively the gas can also be treated following the after-burning in which case S0 ₂ is removed from the gas.

Brief Description of the Drawings;

The invention is described further below with reference to the drawing which illustrates schematically a method according to the invention for the recovery of alkaline chemicals.

Mode of Carrying out the Invention;

In the figure, the spent liquor from a sulfate cellulose cooking is introduced through an inlet 1 to a gasification chamber 2 in which the spent liquor is gasified at a temperature of over 1000 C by an external heat source 3, such as a plasma generator. Before the

gasification the spent liquor has preferably been evaporated or e.g. spray dried by a method not illustrated in the figure. Auxiliary fuel or e.g. oxygen can be introduced into the gasification chamber through an inlet 4. The gasification of sulfate spent liquor is carried out at an oxygen deficit in order to cause the sulfur to form sulfur hydrogen and not sulfur dioxide. If liquor that do not contain sulfur is gasified the gasification can be carried out in an excess of air.

The gas produced by the gasification is guided to a sublimation chamber 5 where the gas is mixed with the cooled recirculated particles brought to the sublimation chamber through conduit 6. The sodium contained in the spent liquor, which after the gasification is substantially in the form of a one-atomic gas, is rapidly sublimated and in a solid state passed on with the gas and the circulating particles to a heat exchanger 7. The sodium is sublimated substantially to NaOH, Na_0 and Na-S. After the heat exchanger the cooled solid particles are separated from the gas in a cyclone 8. The solid particles separated from the gas are transported through pipe 9 either to the conduit 6 and back to the sublimation chamber or to a conduit 10 and therefrom to a dissolving chamber 11. The cooling of the gas in the sublimation phase in the circulating bed cooler can be controlled by controlling the amount of particles recirculated through conduit 6. When alkali is sublimating onto the surface of the particles the size of the particles increases and at a predetermined size the particles are removed from the circulation into the dissolving. chamber.

In the dissolving chamber, NaOH and Na„S is dissolved from the separated particles in water. The water is brought to the chamber through a pipe 12. A part of the

aqueous solution containing alkaline chemicals is transferred through a pipe 13 to the digester to be used as a cooking chemical, and a part of the water solution is guided through a pipe 14 to a gas washer 15.

The gas separated in the cyclone is conducted through a heat exchanger 16 to the scrubber 15, where sulfur containing compounds and the solid material, which was not separated in the cyclone, are removed from the gas. The obtained solution is added through a pipe 17 to the aqueous solution fed into the digester.

After washing the gas is after-burned in a chamber 18. Air or oxygen in supplied through a channel 19. After this the gases are guided through heat exchangers 20 to a gas outlet 21.

Instead of a cyclone, several other methods of separating the particles can be used, e.g. an electric filter which provides the advantages of a small pressure drop and high separation rate if necessary.

The solid particles can alternatively be cooled in a separate device disposed after the cyclone e.g. by air. In this case the heat exchanger 7 is not needed. Cooling of the gas can be controlled by controlling the flow velocity of the particles or by controlling the temperature of the particles by menas of the heat exchanger 7.

Example

A test run was performed in a pilot plant device, in which alkaline compounds, mostly potassium and sodium chlorides, were recovered from the flue gases of a cement clinker furnace by cooling the gas in a circulating bed

cooler. The flue gases, which leave the furnace at a temperature of over 1000 C, were supplied to the sublimation chamber or the mixing phase of a cooler through a pipe of approx. 2 meter. The flow velocity in the pipe was approx. 30 m/s. The temperature of the gas when it came to the mixing phase was approx. 860 - 880 C and when it left the mixing phase approx. 265 - 285 C. The cooling speed in the test run was 2070 - 2425 C/s.

The gas cooling speed and the temperatures at different phases were

1 2 3

Temperature after cement furnace, approx. approx. approx. i.e. inlet temperature °C 1000 1000 1000

temp, before mixing °C 860 880 ,870

temp, after mixing phase C 265 275 282

temp, after cooler before cyclone C 215 230 235

3 gas volume n/s 0,100 0, 119 0, 125

time in suction channel and sublimation chamber s 0,355 0,308 0,296

cooling speed °C/s 2070 2350 2425

Industrial Applicability;

The invention is not limited to the embodiment presented here as an example only but it can be modified and applied in several ways within the scope of protection defined by the patent claims , e . g . in particular the recovery of chemicals after cellulose cooking is possible by this method .