The present invention relates to a process for separating off chloride from a sulphide-containing alkaline liquor which is obtained in association with the production of chemical pulp.

The closed, discharge-free pulp mill has long beckoned as the final goal to meet all environmental demands. Of course, such a plant is a physical impossibility and efforts should be directed, therefore, to attempting to limit the discharges to the greatest extent possible and to convert them into a manageable form in order to be able, in the long run, to return the residual products to their source as a part of the natural cycle.

In recent years, the introduction of modified cooking and oxygen bleaching in combination with the use of chlorine dioxide for final bleaching has decreased the pulp mill discharges of chlorinated compounds to a quite remarkable degree.

Closing mills by returning the bleaching effluents to the recovery step is another area which has recently excited increasing interest, not least because of the consider¬ able costs which are associated with chemical and bio¬ logical purification of water.

The trend towards increasing the closure of a mill in conjunction with producing paper pulp will result in increasing concentrations of alkali-soluble non-process elements, such as chlorides, in the liquor system of the mill, in turn leading to undesirable operational problems.

A novel bleaching technique, so-called TCF bleaching, in which no chlorine chemicals whatsoever are added and

which is based on delignifying with ozone and carrying out final bleaching with peroxy compounds, simplifies the return of the bleaching effluents to the recovery system of the mill.

However, the principal impetus for converting to TCF bleaching is that discharges of chlorinated organic compounds in association with pulp production are minimized. Present and future requirements for AOX discharge levels can be met.

However, it is very costly for existing mills to convert from current standard bleaching sequences, i.e. the chlorine-based CEHDED and chlorine dioxide-based 0D(EP0)DD, to TCF bleaching, and there is some doubt as to whether it is necessary and economically defensible to replace chlorine chemicals entirely in order to fulfil future environmental requirements.

Chlorine dioxide is a very effective and selective bleaching chemical for kraft pulp, and will not be abandoned entirely in favour of chlorine-free bleaching chemicals without resistance.

Chlorine dioxide bleaching is compatible with modern delignification techniques such as ITC, oxygen bleaching, enzyme treatment and ozone - methods which per se are aimed at decreasing the lignin content of the pulp prior to the chlorine dioxide stage. Moreover, the chlorine dioxide is compatible both with oxygen and peroxide-based alkaline extraction.

The development of reliable chlorine dioxide generators has been of vital importance for introducing chlorine dioxide into the pulp industry. All the commercial generators are based on reducing chlorate in a strongly acidic solution.

The standard bleaching sequence in Scandinavian kraft pulp mills is OD(EPO)DD. Strict environmental requirements down towards 0.5 kg AOX/ADMT are satisfied.

While the gradual transition from chlorine gas-based bleaching systems to complete substitution with chlorine dioxide has resulted in diminished discharges of AOX, it has also resulted in the degree of chlorination of the aromatic compounds in AOX being decreased. It is primarily monochlorophenols and di-substituted chloro- phenols which are found as the aromatic compounds from a D stage effluent, and hardly any highly chlorinated compounds are present.

However, knowledge of this is not sufficient, and the permitted content of chlorinated organic compounds, holistically summed up as AOX, in discharges will be decreased constantly.

If chlorine dioxide is to survive as a bleaching agent, the mills must therefore be closed on the effluent side and the residual products converted into a manageable form.

A closed, chlorine dioxide-based bleaching plant system requires a bleed-out mechanism for chlorides. A technical solution to this problem represents the object of the present invention. The development of systems for bleeding out chlorides from the recovery system of a kraft pulp mill has been the subject of extensive research for many years. For example, during the 70s, a mill-scale programme was carried out in order to produce the fully closed kraft pulp mill at Great Lakes Forest Products Co. in Canada. The effluents from both the chlorine stage and extraction stage of the bleaching plant were returned to the chemical recovery system, the white liquor was evaporated and the chlorides were separated off by being crystallized out. Returning

chlorides in this way results, inter alia, in extensive corrosion and inefficiency in the recovery boilers. Large volumes of white liquor have to be evaporated, resulting in a high consumption of steam and large capital investment.

US 4,093,508 discloses a process for eliminating chloride by crystallizing out sodium carbonate, with the sodium chloride remaining in the sulphide-rich mother liquor. The mother liquor is causticized in order to convert remaining carbonate into hydroxide, and the mother liquor is then concentrated by evaporation and chloride is crystallized out from the sulphide and separated off.

US 4,249,990 discloses a similar process for eliminating chlorides from white liquor by evaporative crystalliza¬ tion and leaching. The system is based on evaporation of the entire white-liquor flow, resulting in a high consumption of steam.

US 4,253,911 discloses a method for eliminating chlorides by treating green liquor with carbon dioxide, resulting in the formation of a solution which is saturated with respect to bicarbonate. The bicarbonate is precipitated out and converted by thermal decomposition into carbonate at a temperature of approximately 300°C in a downstream oven. The carbonate is dissolved and causticized. The mother liquor from the bicarbonate precipitation, with its content of chlorides, is divided into two streams; one is recirculated and one is bled off for the purpose of eliminating chlorides from the system. Precipitation of solid bicarbonate and thermal conversion in this manner is expensive and elaborate. In addition, a large amount of sodium in the form of sulphate and bicarbonate is lost in the chloride bleed-out. The high content of chloride in the internal circulation in US 4,253,911 can result in significant corrosion problems.

SE 8305751-3 discloses a method for recovering chemicals from chloride-containing green liquor by bringing the green liquor into contact with flue gases for pre- carbonizing. Hydrogen sulphide is driven off by reacting the pre-carbonized solution with bicarbonate. Soda is crystallized out and the mother liquor is causticized, after which the chloride is crystallized out by evapor¬ ation. All or part of the hydrogen sulphide which is formed is used for preparing white liquor. This method is based on the so-called Tampella process for preparing sulphite cooking liquors in which pre-carbonized solution is contacted by>a specially prepared bicarbonate solution for driving out hydrogen sulphide.

Non-process elements, such as chlorides, in cellulose mills have to be bled out in association with increasing closure of the mill. The quantity of chloride supplied to the mill varies considerably, chiefly depending on the bleaching sequence and the chloride content of the wood used as raw material. The following table indicates the approximate quantities of chloride which are supplied in conjunction with different bleaching systems.

Table

Bleaching Kappa after Quantity of chloride sequence cooking supplied to the bleaching plant/kg of

NaCl/ADMT

CEHDED 35 155

DEDED 35 36

DEDED 22 26

ODEDED 17 22

0D(E0P)DD 17 15

In addition to this, chloride is supplied in the wood. Naturally, the quantity varies but can be estimated to be from 1 kg of Cl/tonne of wood for inland mills to about 5-10 kg/tonne of wood for mills which use sea-floated wood. This means that large quantities of chloride have to be bled off from the recovery system in order to keep the concentration in the white liquor below about 10 grammes/litre.

The object of the present invention is to bring about an improved process for separating off chloride from a sulphide-containing liquor, which process makes it possible to eliminate troublesome discharges to the environment.

The process according to the invention is characterized in that it comprises the following steps: (a) the alka¬ line liquor is contacted, in a first gas/liquid contact zone, with a carbon dioxide-containing gas, the carbon dioxide partial pressure of which exceeds 0.2 atm, in order to obtain an alkaline liquid containing dissolved alkali hydrogen carbonate and chloride compounds; (b) alkaline liquid containing dissolved alkali hydrogen carbonate and chloride compounds is drawn off from the first gas/liquid contact zone and transferred to a second gas/liquid contact zone, in which a lower pressure is maintained than in the first gas/liquid contact zone, in order to obtain an alkaline liquid, which contains alkali carbonate and chloride compounds, and hydrogen sulphide, which is driven out of the liquid; (c) the hydrogen sulphide which has been formed is driven off from the second gas/liquid contact zone; (d) alkaline liquid containing alkali carbonate and chloride compounds is drawn off from the second gas/liquid contact zone and transferred to the first gas/liquid contact zone; and (e) alkaline, and in the main sulphide-free, liquid containing alkali carbonate and chloride compounds is draw off from the second gas/liquid contact zone and

transferred to a system for separating off chloride compounds.

The said Tampella process contrasts with the simplified process according to the present invention, in which the green liquor is contacted and finally carbonized in one and the same first gas/liquid contact zone by means of contact with carbon dioxide-containing gas. This can only be accomplished efficiently if the carbon dioxide partial pressure in the gas is high, preferably greater than 1 atm.

The invention will be explained in more detail below in regard to its implementation and different embodiments.

In the process according to the invention, the chloride is bled out by the effluent concentrate from the bleaching plant being partially or completely combusted, with or without the addition of supplementary fuel, in a reaction space operating at a temperature exceeding about

700°C. During the combustion, a smelt is generated which contains various alkali salts, principally sodium chloride, sodium sulphate and sodium carbonate. If the operation is carried out under reducing conditions, sulphur is obtained in the form of sodium sulphide and gaseous hydrogen sulphide which accompanies the combus¬ tion gas.

Several arrangements for partial or complete combustion can be used when implementing the present invention and suitable supplementary fuels include both black liquor and gas or oil. US 4,808,264 discloses a combustion arrangement, which is particularly useful when implement¬ ing the present invention, in which concentrated cellu- lose spent liquor is partially combusted in a reactor and alkaline compounds which are carried off with the gas are separated in a waterseal. The readily soluble alkaline

compounds, which also include sodium chloride, are recovered in the green liquor and white liquor system.

According to the present invention, sodium chloride- containing, alkaline liquor, for example green liquor, is contacted, in a first gas/liquid contact zone, by a carbon dioxide-containing gas having a carbon dioxide partial pressure which exceeds approximately 0.2 atm. Due to the relatively high carbon dioxide partial pressure, the carbonate of the liquor is converted to bicarbonate more rapidly and in greater extent than is the case with carbonizing at lower carbon dioxide partial pressure. A gas of this nature which is suitable is the gas from pressurized black liquor gasification or from gasifica- tion or combustion of other cellulose spent liquors such as effluent concentrate from the bleaching plant, concen¬ trate from oxygen delignification, or combinations of these, with or without addition of supplementary fuel. The green liquor should first have been purified and filtered before it is transferred to the contact zone. The contact between the green liquor and the carbon dioxide-containing gas should take place at a temperature exceeding approximately 70°C, preferably exceeding 100°C, and at a total pressure exceeding 2 atm, preferably exceeding 10 atm. Owing to the relatively high total pressure, the temperature can be kept high in the contact zone, thereby decreasing the risk of bicarbonate precipi¬ tating out. Expulsion of water steam in the contact zone can also be kept at a reasonable level using an increased total pressure. The contact device, or the first gas/liquid contact zone, can expediently consist of a plate column or of a column containing packing material.

The following important reactions, inter alia, take place in the first gas/liquid contact zone:

Na ₂ S + C0 ₂ → NaHS + NaHC0 ₃

Na ₂ C0 ₃ + C0 ₂ + H ₂ 0 - 2NaHC0 ₃

Na ₂ C0 ₃ + H ₂ S → NaHC0 ₃ + NaHS

COS + H ₂ 0 → H ₂ S + C0 ₂

Thus, acidic gases are absorbed in the alkali and the pH falls to below 11. Hydrolysis of carbonyl sulphide takes place, without any special catalyst, in alkaline medium at temperatures exceeding 70°C, and in this way the gas is largely freed of this poisonous sulphur compound. Sodium chloride passes through the system without reacting.

A liquid containing dissolved alkali hydrogen carbonate is drawn off from the first gas/liquid contact zone, which liquid is transferred to a second gas/liquid contact zone, for example a stripper, which operates at a lower pressure than the first gas/liquid contact zone, preferably around or below atmospheric pressure.

As a result of the fall in pressure, hydrogen sulphide is driven out of the liquid in accordance with the reaction:

NaHC0 ₃ + NaHS → Na ₂ C0 ₃ + H ₂ S

The alkaline liquid now contains alkali carbonate and alkali chloride and is transferred, after bleeding off liquid which is returned to the first gas/liquid contact zone, to a system for separating off chlorides and processing the liquid to form cooking liquor. Any remaining sulphides can be oxidized to sulphate before the processing by contacting the liquid with an oxygen- containing gas.

The alkaline liquid, which has in the main been freed of sulphide, can be processed by two principal routes. The first is based on cooling the liquid down to a tempera-

ture below that at which the bicarbonate is soluble; alternatively, the liquid is cooled, in this first variant, to such an extent that the temperature is below that at which carbonate is soluble. The solubility of sodium chloride in water does not vary appreciably with the temperature (357 grammes/litre at 0°C and 391 grammes/litre at 100°C) . As a result, the sodium chloride remains in solution even after cooling has taken place and can be readily separated off and bled out from the mother liquor.

Cooling of the liquid, which is in the main sulphide- free, can be carried out using a known cooling technique, for example by heat exchange or by evaporative cooling. The cooling is continued down to a temperature of less than 40°C and preferably to a temperature of less than approximately 20°C.

The liquid which has been freed of sulphides can be treated again with carbon dioxide, in one or more steps, in order to convert remaining carbonate to bicarbonate and thereby decrease sodium losses when bleeding off the chloride-containing mother liquor.

The second procedure, which is a preferred embodiment, is based on the liquor which is drawn off from the second gas/liquid contact zone, and which is in the main sulphide-free, being causticized in one or more steps prior to the chloride separation. In this way, alkali is converted into very readily soluble hydroxy form and the chlorides can be selectively precipitated out by evapora¬ tive crystallization, with the hydroxide remaining in the mother liquor.

The hydroxide-rich mother liquor, which has in the main been freed of chlorides, is returned to the bleaching plant or the liquor system of the pulp mill. The

chloride-containing material which has been separated off can be subjected to further processing and returned to the chlorine dioxide-generating system and/or exported.

Other methods of separating off the chlorides from the alkaline liquid which is in the main sulphide-free are also conceivable, such as selective chemical precipita¬ tion of the chlorides with, for example, silver nitrate, or an ion-exchange technique or by means of an ion- selective membrane technique.

The invention will be illustrated in more detail using two exemplary embodiments.

Example I

A kraft pulp mill having an annual production of 280,000 tonnes of bleached softwood pulp (890 ADMT/D) is to be closed on the effluent side (TEF) . The fibre line is equipped with a two-vessel hydraulic digester and a diffuser bleaching plant having the sequence (DC) (EO)HDED with 30% chlorine dioxide substitution. The kappa after cooking is 32 and the pulp is bleached to a brightness of 90 ISO. The plant does not have an oxygen stage. The chlorine dioxide is generated in an R3 process. The mill requires to be relieved on the recovery boiler side by 180 tonnes of dry substance/24 hours.

The chemical consumption in the existing bleaching plant is:

Cl ₂ 41 kg/ADMT

C10 ₂ 14 kg II

Hypo 20 kg II

NaOH 67 kg " (incl. hypo)

Oxygen 4 kg II

Following reconstruction in the bleaching plant, and installation of a Chemrec® gasifier for the thermal conversion of effluent concentrate from the bleaching plant, and systems for eliminating chlorides by crystal- lization from liquid, according to the present invention, which is causticized and in the main sulphide-free, the mill data are as follows:

Bleaching plant

Bleaching plant .sequence D(EOP)DED

Chemical consumption:

C10 ₂ 38 kg/ADMT

NaOH 47 kg "

0 ₂ 4 kg "

H ₂ 0 ₂ 4 kg "

Effluent quantity 10 m /ADMT Dry substance content in the effluent approx. 2%

Total quantity of dry substance in the bleaching plant effluent 7.5 tonnes/hour

NaCl content in the effluent approx. 16% of dry substance Chlorine in the effluent as NaCl 1215 kg/hour

The bleaching plant effluent is concentrated by evapor¬ ation to approximately 50% dry substance.

Chemrec gasifier

Fuel Black liquor/effluent concentrate from the bleaching plant

Gas purification Absorber/stripper Capacity 15 tonnes of dry substance/hour

When operating at 7.5 tonnes of bleaching plant effluent concentrate and 7.5 tonnes of black liquor per hour (both

calculated as dry substance) , the production of chemicals from the Chemrec system is as follows:

Sulphide-free alkali 4650 kg/hour (as Na ₂ C0 ₃ ) Hydrogen sulphide gas 300 kg/hour Sodium chloride 1215 kg/hour

The lean combustion gas from the Chemrec system is transferred to the lime sludge reburning kiln for oxida- tion of H ₂ , CO and hydrocarbons. The hydrogen sulphide- containing gas is transferred to the impregnation stage in the digester.house. The sulphide-free alkali is used, with or without preceding causticization, for pH adjust¬ ment and buffering in the bleaching plant. The excess is transferred to the green liquor system of the mill. Part of the sodium chloride stream is returned to the chlorine dioxide generator and the excess is exported.

In this way, troublesome discharges to the environment have been eliminated and the AOX, COD and BOD emissions are in practice zero (zero impact) after the reconstructio .

Example II

The point of departure for this example is a kraft mill having a production of 1270 ADMT/24 hours. The mill is equipped with a two-vessel hydraulic digester and a diffuser bleaching plant having the "chlorine-free" sequence OD(EOP)DED. The kappa is 25 after the cooking and 15 after the oxygen stage. The pulp is bleached to a brightness of 90 ISO.

The mill is to be closed on the effluent side and the capacity in the fibre line is to be increased by 10%.

Following reconstruction in the digester house and the bleaching plant, and installation of a Chemrec® gasifier

for thermal conversion of effluent concentrate from the bleaching plant, and of systems for eliminating chlor¬ ides, the mill data are as follows:

Capacity 1400 ADMT/24 hours

Kappa number after digester house 20

Kappa number after oxygen stage 12

Bleaching plant

Chemical consumption cιo ₂ 20 kg/ADMT

NaOH bleaching plant 42 kg " (incl. oxygen stage) o ₂ 24 kg " (incl. oxygen stage)

H ₂ 0 ₂ 4 kg "

MgS0 ₄ 4 kg "

Effluent quantity from bleaching plant 10 m /ADMT Dry substance content in effluent 1.0% Total quantity of dry substance 5.8 tonnes/hour NaCl content in effluent approx. 17% of dry substance

Chlorine in effluent as NaCl 990 kg/hour

The bleaching plant effluent is concentrated by evapor¬ ation to approximately 50% dry substance.

Chemrec gasifier Fuel Black liquor/effluent concentrate from the bleaching plant Gas purification Absorber/stripper Capacity 14 tonnes of dry substance/hour

When operating at 5.8 tonnes of bleaching plant effluent concentrate and 8 tonnes of black liquor per hour (both calculated as dry substance) , the production of chemicals from the Chemrec is as follows:

Sulphide-free alkali 6030 kg/hour (as Na C0 ₃ ) Hydrogen sulphide gas 320 kg/hour Sodium chloride 990 kg/hour

The lean combustion gas from the Chemrec system is combusted in the bark boiler of the mill. The hydrogen sulphide-containing gas is transferred to the impreg¬ nation stage in the digester house. The sulphide-free alkali is used, with or without preceding causticization, for pH adjustment and buffering in the bleaching plant. The excess is transferred to the green liquor system of the mill. Part of the sodium chloride stream is returned to the chlorine dioxide generator and the excess is exported.

In this way, troublesome discharges to the environment have been eliminated and the AOX, COD and BOD emissions are in practice zero (zero impact) after the recon- struction.