FI78748B - Process and device for reclaiming chemicals from a slurry yielded by the burning of pulp by-product containing sodium carbonate - Google Patents

Abstract

The invention pertains to a process and device for reducing a slurry yielded by the burning of pulp waste containing sodium carbonate with the use of carbon, in which a suspension (2) of the slurry and carbon is heated and simultaneously stirred to reduce the sodium carbonate to sodium oxide as well as, in some cases, sodium hydroxide. Per the invention, a substantial part of the heat the reduction reaction requires is generated by conducting high-voltage AC current through the slurry (3).<IMAGE>

Description

The present invention relates to a process for recovering chemicals contained in a sodium carbonate-containing melt from the incineration of pulp sludge. In particular, this invention relates to a process for reducing the above-mentioned melt with carbon by heating and at the same time stirring a mixture of sodium carbonate-containing melt and carbon in the reaction zone and removing the molten product containing sodium oxide therefrom.

Black liquor concentrated to a sufficient dry matter content is incinerated under strictly controlled combustion conditions to recover the heat and black liquor chemicals in such a form that they can be reconstituted in subsequent processing steps. During combustion, 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.

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However, the calcium carbonate precipitated during 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 the direct reduction of sodium carbonate from a melt obtained from the combustion of black liquor to sodium hydroxide, by means of which a very concentrated sodium hydroxide solution is obtained without causticization and associated lime 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 iron (3) oxide precipitate, which iron (3) oxide can be reused to reduce sodium carbonate to sodium hydroxide.

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

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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 of 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 the gases. o are subjected to flash cooling below 950 ° C to give 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 4,78748, requires a lot of energy, and it has proved difficult to adjust the composition of the obtained 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 eliminate the above-mentioned drawbacks and to provide a process for the reduction of sodium carbonate contained in the melt obtained from the recovery boiler directly in the molten phase to sodium oxide at a much lower temperature. The process according to the invention does not require any expensive and cumbersome plasma generator or rapid cooling of the gases to prevent the regeneration of sodium carbonate.

SU patent publication 48264 discloses a process for recovering alkali metal oxides from their carbonates with carbon and hydrogen, thereby obtaining carbon monoxide and hydrogen, which in this case is an inert gas. In this process, carbon and hydrogen or gases containing them 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.

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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 melt from a recovery boiler directly in the molten phase to sodium oxide and optionally hydroxide by heating and stirring the sodium carbonate-containing melt and carbon mixture. melt. In the process and apparatus 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 in addition, 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 heat-economically advantageous process which operates at surprisingly low temperatures.

The present invention therefore relates to a process for reducing sodium carbonate-containing melt from the incineration of pulp-waste liquor on carbon by heating while stirring a mixture of sodium carbonate-containing melt and carbon in the reaction zone and removing the sodium oxide-containing and possibly hydroxide-containing melt. According to the invention, the melt in the reaction zone is heated by passing a high-energy alternating current through the melt.

According to the invention, powdered carbon is used as the reducing agent, which is fed to the reaction zone either separately or as a suspension with a melt containing sodium carbonate to be reduced. The carbon used as the reducing agent is preferably obtained by burning the black liquor from the incomplete so that substantial amounts of carbon remain, whereby a melt from the pyrolysis of the black liquor is fed to the reaction zone.

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 sodium carbonate-containing melt and carbon under the surface of the melt in the reaction zone. Alternatively or in addition, a separate mixer may be used.

In order to introduce heat into the melt in the reaction zone, a high-energy alternating current is passed through it, e.g. with electrodes embedded in the melt. The melt in the reaction zone is thus maintained at a temperature of 1100-1600 K, preferably 1200-1400 K.

Water vapor can 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.

The sodium carbonate-containing melt from the soda boiler reacts with the carbon suspended in it or formed by pyrolysis in the reduction reaction zone to form sodium oxide and carbon monoxide gas according to the following reaction equation (A) 7 78748

Na CO + C + Q = Na O + 2CO 1 "(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, this is not possible because the carbon dioxide generated in the combustion reacts immediately with the sodium oxide back to sodium carbonate. According to the present invention, this problem is solved by keeping the melt in the reduction reaction zone hot by passing a high-energy alternating current through it with electrodes immersed in the melt. In this way, at least part of the heat Q required by the reaction (A) can be satisfied by an electric current.

The carbon monoxide gas generated in the reaction thus does not have to be burned above the melt, but is recovered for use elsewhere, e.g. as a raw material in chemical reactions or as a fuel for heat generation.

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 zone but in melt communication with the reduction reaction zone. Reaction (B) is exothermic, so that part of the hot melt returned by reaction (B) back to the reduction reaction zone can satisfy part of the heat demand 0 of the reduction reaction (A) by the reaction heat of reaction (B) of 8,78748 Q, i.e. Q = Q + O.

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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.

The invention will be described in more detail below with reference to the accompanying drawing, which shows a cross-sectional vertical view of a reactor for carrying out the process according to the invention.

In the drawing, the reactor is generally indicated by the reference numeral 1. In the wall of the reactor 1 there is an inlet line 2 for feeding melt and carbon suspension containing sodium carbonate and sodium sulfide from black liquor combustion to the reactor 1 and electrodes 3 immersed in the melt to conduct high energy alternating current. The melt and carbon are fed from the pipeline 2 to the reactor at such a rate that the melt is vortexed to produce a stirring effect. In addition, the reactor 1 can be provided with a separate stirrer 8 mounted on its bottom. The bottom of the reactor 1 also has an outlet line 5 for removing the molten product containing mainly sodium hydroxide and sodium sulphide from the reactor. In addition, the upper part of the reactor 1 has an outlet line 9 for removing CO gases from the reactor 1.

According to the invention, the reactor 1 is divided into two melt-communicating parts 6 and 7. The division is carried out by a partition wall 10 extending from top to bottom at a distance from the bottom 11 of the reactor 1 so that the sodium oxide-containing melt formed in the reduction reaction zone A can flow from part 6 from below to part 7, to which water vapor 4 is fed to form sodium hydroxide in hydroxylation zone B in part 7. In hydrolysis zone B, heat is also generated which is utilized in reduction reaction zone A by recovering part of the hot sodium hydroxide melt shown in part 7. The mixing and recycling of the melt is carried out with a stirrer fitted to part 7 and the rest of the sodium hydroxide melt is removed to part 7 from the bottom of the bottom 11 of the reactor 1.

The electrodes 3 are arranged to extend through the closed wall 12 of the closed part 6 into the molten and CO gas 9 is removed from the closed gas space of the part 6 through the upper wall 12 of the part 6.

The shape of the reactor 1 part 6 can vary within wide limits, but is preferably cylindrical, the inlet line 2 preferably being connected tangentially to the cylindrical side wall of the part 6 to provide a cyclone-type mixing effect in the reduction reaction zone A. The vortex effect from the center of the surface of the melt tapering downwardly. The electrodes 3 are then preferably shaped so that their flow resistance is as small as possible.

Claims (6)

10,78748 A process for reducing carbon-containing molten sodium carbonate melt from the incineration of pulp sludge by heating, heating and stirring a mixture of sodium carbonate-containing melt and carbon in the reaction zone and removing at least a portion of the molten product . Process according to Claim 1, characterized in that a suspension of the sodium carbonate-containing melt to be reduced and the powdered carbon, preferably the melt from the pyrolysis of black liquor, is fed to the reaction zone (A). Process according to Claim 1 or 2, characterized in that the sodium carbonate-containing melt and the carbon are sprayed (2) below the surface of the melt in the reaction zone (A) in order to produce a mixing effect. Method according to one of the preceding claims, characterized in that the melt is kept at a temperature of 1100-1600 K, preferably 1200-1400 K, by means of an alternating current. Process according to one of the preceding claims, characterized in that at least part of the molten product is fed (4) water vapor to convert sodium oxide to sodium hydroxide and generate heat, after which at least part of this hot sodium hydroxide melt is immediately returned to reaction zone (A) to provide additional heat and sodium oxide activity. A). An apparatus for reducing sodium carbonate-containing melt from the combustion of pulp sludge with carbon, which apparatus consists of a reactor (1) having means for feeding reduced sodium carbonate-containing melt and finely divided carbon (2) to the reactor (1) and keeping the melt hot (3) (8) and carbon monoxide gas (9) and reduced melt (5) from the reactor, characterized in that the reactor (1) is divided by a partition wall (10) into two parts (6, 7) which coincide with each other at their lower part, whereby the members (2, 9) for supplying the sodium carbonate-containing melt and fine carbon and for removing carbon monoxide gas, the alternating current electrodes (3) acting as heating means are arranged at the top of the first part (6) and means (4) for supplying water vapor from the first part (6) are arranged at the top of the second part (7). to the sodium oxide-containing melt to return at least a portion of this to the first portion (6) of the reaction a as heated sodium hydroxide melt and that the means (8) for mixing the melt are adapted to provide a circulating flow between the two parts (6, 7). i2 78748