FI81139C - Process and apparatus for the recovery of chemicals from black liquor - Google Patents

Description

1 81139

The present invention relates to a method for recovering chemicals from black liquor. In particular, this invention relates to a process for reducing pyrolyzed black liquor with a carbonaceous substance by heating and at the same time stirring a suspension of sodium carbonate-containing melt and carbon in the reaction zone and removing sodium oxide-containing melt and gases therefrom.

The present invention also relates to an apparatus for carrying out the above process, comprising a reactor having means for pyrolysing black liquor and feeding the pyrolysis product to the reactor, keeping the melt hot and possibly stirring and removing gases and reduced melt.

Black liquor concentrated to a sufficient dry matter content is incinerated under carefully controlled combustion conditions to recover the heat and chemicals contained in the black liquor in such a way that they can be reconstituted in subsequent processing steps. During incineration, the water remaining in the evaporator contained in the black liquor evaporates and the organic matter contained in the black liquor is allowed to burn, while the inorganic matter is released and drained away from the bottom of the furnace as a melt. Combustion of black liquor in a recovery boiler yields a melt containing sodium sulfide and sodium carbonate, usually dissolved in a lean white liquor obtained from a lime wash to give green liquor, which is an aqueous solution of sodium carbonate and sodium sulfide. However, such green liquor cannot be used for cellulose pulp soup and the sodium carbonate it contains must first be converted to sodium hydroxide by causticization. The sodium sulphide in green liquor does not react with the calcium hydroxide used in the causticisation, but the causticisation results in an aqueous solution containing sodium hydroxide and sodium sulphide, called white liquor.

However, the calcium carbonate precipitated in the causticisation must be regenerated by burning it in a so-called lime kiln to calcium oxide, from which calcium hydroxide is again obtained by slurrying in water.

There are several drawbacks to this conventional method currently in use. First, the concentration of the resulting sodium hydroxide solution is relatively low, as is the causticization efficiency, in addition to which the calcium carbonate obtained from causticization must be regenerated to calcium hydroxide, which increases the cost of the process.

It is therefore an object of the present invention to provide a process for pyrolyzing black liquor and reducing the sodium carbonate of the product obtained directly to sodium hydroxide, by means of which a very concentrated sodium hydroxide solution is obtained without causticization and associated mesan combustion.

SE-A-392 927 discloses a process for recovering sodium hydroxide from waste liquor obtained by cooking or bleaching a cellulosic substance. In this method, iron (3) oxide is mixed with the concentrated lye, after which the mixture is burned to form a melt which is poured into hot water to form a sodium hydroxide solution and an iron (3) oxide precipitate, which iron (3) oxide can be reused to reduce sodium carbonate to sodium hydroxide.

Il 3 81139 This method allows sodium hydroxide to be recovered directly from the waste liquor, but has the disadvantage that it is only suitable for lyes which do not contain substantial amounts of sulfur compounds. Therefore, this method cannot be used for waste liquors obtained from sulphate and sulphite pulp soups.

Various methods are also known for gasifying black liquor or a melt obtained from its combustion in a recovery boiler under a plasma generator under reducing conditions to decompose sodium carbonate directly to monoatomic sodium and carbon monoxide, which when cooling

SE-A-448 007 discloses a process of the above type in which the pulp and the waste liquor are first subjected to pyrolysis at low temperature, after which the mixture containing sodium carbonate and carbon thus obtained is fed to a gasification reactor to give sodium hydroxide, sodium sulphide and some sodium carbonate. includes e.g. hydrogen and carbon monoxide, which can be burned to recover heat.

In the method according to SE publication 447 400, black liquor is fed to a recovery boiler, but part of the black liquor is first co-directed by a plasma generator and only then fed to the recovery boiler.

In the process according to FI patent application 853 789, black liquor, steam and carbon or oxygen are decomposed by a plasma generator and fed to a reaction zone where a temperature of at least 1100 ° C is maintained, after which the melt containing mainly sodium sulphide is separated from gases, ... o for rapid cooling to a temperature below 950 C 4 81139 to obtain an aqueous solution containing mainly sodium hydroxide and a gas containing hydrogen and carbon monoxide.

In the above methods, the decomposition of sodium carbonate is performed in the gas phase using a high-energy plasma generator. However, the use of such a plasma generator is cumbersome, requires a lot of energy, and it has proved difficult to adjust the composition of the resulting gas phase and cool it so that sodium carbonate is not regenerated. There are large amounts of reducing components in the gas phase that tend to react with sodium and its compounds to form sodium carbonate before the gases have been cooled from a high gasification temperature to such a low temperature that these reactions no longer occur substantially.

The object of the present invention is to obviate the above-mentioned drawbacks and to provide a process for the reduction of sodium carbonate contained in the product obtained from the pyrolysis of black liquor directly in the molten phase to sodium hydroxide at considerably lower temperatures. The process of the invention does not require any expensive and cumbersome plasma generator or rapid cooling of the gases to prevent sodium carbonate regeneration.

SU patent publication 48264 discloses a process for recovering alkali metal oxides from their carbonates with carbon and hydrogen, thereby obtaining carbon monoxide and in this case inert hydrogen. In this process, carbon and hydrogen are fed to an alkali carbonate melt to reduce alkali carbonates to oxides. However, no technically and economically viable method is disclosed in the patent publication. Namely, it has been found in the experiments carried out that the alkali metal oxide formed reacts very easily back to alkali metal carbonate, in addition to which the alkali metal oxide formed is highly volatile at the high temperatures required to keep the sodium oxide molten. In addition, the reduction reaction is endothermic, requiring a lot of heat.

It is therefore an object of the present invention to provide a technically and economically feasible method and apparatus for reducing sodium carbonate in a molten phase directly to sodium oxide and hydroxide by heating a mixture of sodium carbonate-containing melt and hydrocarbons. melt. In the process of the present invention, the reaction conditions are effectively controlled so that the sodium hydroxide and oxide formed cannot react back to sodium carbonate, and the evaporation of the sodium oxide formed is effectively prevented. The object of the invention is not only to reduce sodium carbonate in high yield to sodium oxide and hydroxide but also to provide a thermally economically advantageous process which operates at surprisingly low temperatures.

The present invention therefore relates to a process for pyrolysing black waste liquor and reducing a sodium carbonate-containing melt from pyrolysis with a carbonaceous substance, while heating and stirring a mixture of the sodium carbonate-containing melt and the carbonaceous substance in the reaction zone and removing sodium oxide therefrom. According to the invention, the surface of the melt in the reaction zone is protected from oxidizing gases by a layer of carbon in the partial formation and at least part of the heat required for the reduction reaction is passed through said carbon layer by burning the carbon layer and gases rising therethrough with oxygen or oxygen-containing gas.

6 81139

According to the invention, powdered carbon is used as the reducing agent, which is fed to the reaction zone as a suspension with the sodium carbonate-containing melt to be reduced and the hydrocarbons. The hydrocarbons used as reducing agents are preferably obtained by thermally decomposing the black liquor to leave substantial amounts of hydrocarbons and carbon, thereby feeding to the reaction zone a mixture of black liquor pyrolysis with sodium sulfide, sulfur-free Na salts and oxides, predominantly sodium carbate, , carbon and carbon monoxide.

The black liquor can be pyrolyzed in a separate closed cyclone connected below the molten surface, from which gases (mainly CO) are removed in a controlled manner so that the gas pressure in the cyclone is such that the pyrolysis product (mainly sodium carbonate and sulfide, hydrocarbons and carbon) is transferred at the desired rate. .

The process of the present invention requires agitation of the melt in the reaction zone. This mixing effect can be achieved, for example, by spraying a mixture of sodium carbonate-containing substances under the molten surface in the reaction zone. Alternatively or in addition, a separate mixer may be used.

To introduce heat to the melt in the reaction zone, the overlying carbon layer formed from the carbon entrained in the reaction mixture is lighter as it rises to the surface of the melt, and preferably by blowing oxygen or oxygen-containing gas towards the carbon layer to pass the carbon and gases through it. The carbon layer then acts not only as an anti-oxidation layer for the melt but also as a fuel for introducing heat to the melt.

Il 7 81139

The melt in the reaction zone is thus maintained by heating at a temperature of 1100-1600 K, preferably 1200-1400 K. The pyrolysis in the cyclone is instead carried out at 600-1000 K, preferably 700-900 K.

Alternatively, water vapor may be fed to at least a portion of the molten product removed from the reduction reaction zone to convert sodium oxide to sodium hydroxide and generate heat, after which at least a portion of this molten product is immediately returned to the reduction reaction zone to keep the melt fed therein hot and molten. In this way, approximately half of the heat required to reduce sodium carbonate can be satisfied. In addition, the sodium hydroxide formed not only lowers the melting point of the melt but also prevents the sodium oxide and sodium formed from evaporating. Electrodes embedded in the melt can also be used for heating.

The sodium carbonate-containing pyrolysis product from the pyrolysis cyclone reacts with the carbonaceous substance formed by pyrolysis, mainly hydrocarbons, in the reduction reaction zone to give sodium oxide and carbon monoxide gas according to the following reaction equation (A)

Na CO + C + Q = Na O + 2CO 2 + (A) 2 3 2 where Q is the heat required for the reaction.

The released carbon monoxide gas rises towards the surface of the melt and exits the melt, which tends to shift the reaction equilibrium in the other direction towards the sodium oxide. This reaction is endothermic and it would be preferred that the carbon monoxide released in the reaction could be combusted to introduce heat into the melt. However, with the conventional technique, this is not possible because the carbon dioxide generated in the combustion would react immediately with the sodium oxide back to the sodium carbonate β 81139. In the process of the present invention, this problem does not exist because the melt in the reduction reaction zone is covered by a layer of pyrolysis carbon, so that the carbon monoxide released in the reaction and passed through the carbon layer can be burned above the layer with oxygen or oxygen-containing gas to react with the carbon dioxide in the combustion. . In addition, the carbon in the carbon layer burns, releasing heat to the melt.

In a preferred embodiment of the invention, at least a portion of the sodium oxide melt obtained as a result of the reduction reaction (A) is mixed with water vapor to effect the reaction (B).

Na O + H 0 = 2NaOH + Q (B) 2 2 2 This reaction (B) is preferably carried out in a separate but molten state connected to the reduction reaction zone. Reaction (B) is exothermic, so by returning part of the hot melt obtained by reaction (B) back to the reduction reaction zone, part of the heat demand Q of the reduction reaction (A) can be satisfied by the reaction heat Q of reaction (B), i.e. Q = Q + Q, where Q is 2 12 1 heat generated by burning coal and carbon monoxide.

Thus, in the reaction (B), sodium hydroxide is formed, which lowers the melting temperature of the melt, whereby the reduction reaction (A) can be carried out at a lower temperature without solidifying the melt. In addition, the resulting sodium hydroxide prevents otherwise volatile sodium oxide and sodium from leaving the melt.

In the drawing, the cyclone used as a pyrolysis reactor is generally indicated by the reference number 1. Black liquor is fed to the upper part of the cyclone 1 by means of a compartment valve 3. The black liquor is pyrolyzed in a cyclone by heating piping 4, which maintains the temperature in the cyclone

II

9,81139 is maintained at 700-900 K, for example at 800 K. In pyrolysis, gases are generated which are removed via line 18 in a controlled manner by valve 17, so that the gas pressure in the cyclone is set to the desired level. At the bottom of the cyclone 1 there is a connecting line 2 which leads to the actual reduction reactor below the molten surface therein. The pyrolysis product is fed under pressure from the pipeline 2 to the reduction reactor 12 at such a rate that a vortex is generated in the melt to produce a stirring effect. In addition, the reduction reactor 12 can be provided with a separate stirrer 8 mounted on its bottom. The opposite end of the reduction reactor 12 also has an outlet line 5 for removing molten product containing sodium oxide, sodium hydroxide and sodium sulfide from the reactor.

The gases released from the melt are burned above the melt by blowing oxygen gas towards the melt from a pipe 13. The melt is protected from undesired oxidation by a cumulative layer 14 rising on the surface of the melt, through which the released gases rise to the space above the melt in the reactor. In addition, the top of the reactor 12 has an outlet line 9 for removing flue gases and water vapor from the reactor 12.

When the reduction reactor 12 is started, coke has to be fed to the surface of the melt to protect its surface, but in the future coke from black liquor pyrolysis will rise lighter from the melt.

The flue gas line 9 can be connected to the heating pipe 4 of the cyclone 1, whereby pyrolysis can be carried out with hot gases coming from the reduction reactor.

According to the invention, the reduction reactor 12 is further divided into two closed spaces 6 and 7 in melt connection with each other. The partitioning is carried out by a partition wall 10 extending from top to bottom at a distance from the reactor bottom 11 so that in the reduction reaction zone A from below the lower edge of the partition 10 to space 7, where water vapor 16 is fed to form sodium hydroxide in hydroxylation zone B, space 7. Hydroxylation zone B also generates heat which is utilized in the reduction reaction zone 6 by returning some of the hot sodium hydroxide to The mixing and recycling of the melt is carried out with a stirrer fitted in space 7 and the remainder of the sodium hydroxide melt is removed from space 7 at its outlet 5. The hydroxylation zone, i.e. space 7, is further provided with a heating electrode 15 to provide additional heat to the melt if necessary.

Il

Claims (11)

A process for reducing with a carbonaceous substance a sodium carbonate-containing melt derived from black liquor pyrolysis, by heating and simultaneously stirring a mixture of the sodium carbonate-containing melt and the carbonaceous substance in a reaction zone and removing gases and a melting product containing sodium oxide therefrom, characterized in that in the reaction zone (A), a sodium carbonate-containing mixture of hydrocarbons and carbon dioxide derived from black liquor pyrolysis is measured below the surface of a melt protected by a carbon layer (14) and that at least part of the heating reaction heat demand is fed to the melt through said carbon layer (14) by burning the carbon layer and passing gases therein with oxygen or an oxygen-containing gas (13). Process according to claim 1, characterized in that the black liquor is pyrolyzed in a separate, advantageously communicating, closed space (1) communicating beneath the surface of the melt, from which gases (17) are removed (18) so that the gas pressure in space t (1) is fixed at such a level that the pyrolysis product is moved to the reaction zone (A) at the desired rate. 14 81139 Process according to claim 2, characterized in that the black liquor is heated in the space (1) at a temperature of 600-1000 K, preferably 700-900 K for pyrolysing thereof. Method according to any of the preceding claims, characterized in that oxygen or oxygen-containing gas is combusted onto the surface of the melt layer (14) for combustion of the coal in the layer (14) and thereby rising gases. Process according to any of the preceding claims, characterized in that the melt is poured to a temperature of 1100-1600 K, preferably 1200-1400 K. Process according to any one of the preceding claims, characterized in that in the at least part of the melt product (4) water vapor is fed for conversion of the sodium oxide to sodium hydroxide and the development of heat, whereupon at least part of this hot melt is immediately returned to the reduction reaction. A) for melting the pyrolysis product added thereto and for pouring this heat and lowering the activity of the sodium oxide in this zone. Apparatus for pyrolysing black liquor and reducing a sodium carbonate-containing melt derived therefrom, comprising a reactor with means (1) for pyrolysing the black liquor and feeding (2) of the pyrolysis product into the reactor, for pouring the melt (13) and even -tually for stirring (8) thereof and removal from the reactor of the gases (9) and the reduced melt (5), characterized in that the reactor with a partition (10) is divided into two down-to-communicating portions (6, 7). , wherein the means (2) for feeding the pyrolysis product are disposed in the lower portion of the first portion (6), while the means (9, 13) for removing the gases and heating the melt are arranged in the upper portion of the first portion (6). first part (6) and means (16) are arranged in the upper part of the second part (7) for feeding water vapor into the sodium oxide-containing melt in ice 81139 originating from the first part (6) for feeding at least one portion thereof is added to the first portion (6) of the sodium hydroxide melt heated in the reaction, and means (8) for stirring the melt are arranged to effect a flow circulation between the two portions (6, 7). 8. Device according to claim 7, characterized in that the second part (7) has one or more alternating electrodes (15) dipped in the melt. 9. Device according to claim 7, characterized in that the heating means are means (13) for supplying oxygen or oxygen-containing gas to a coal layer (14) on the surface of the melt for combustion of the carbon layer and passing gases therein before removing the flue gases (9). ) from the reactor. 10. Device according to any one of claims 7-9, characterized in that the means for pyrolysing the black liquor and feeding the pyrolysis product into the reactor is a gas-tight cyclone (1) provided with a heating element (1) which is connected at its lower end with the first part. (6) of the reactor. 11. Device according to claim 10, characterized in that the means (9) for removing gases from the reactor are joined to the heating means (4) of the cyclone (1).