WO1998017857A1

WO1998017857A1 - Removal of metals from treatment fluids in a cellulose pulp mill

Info

Publication number: WO1998017857A1
Application number: PCT/SE1997/001749
Authority: WO
Inventors: Arne Elofson; Arne Nordgren
Original assignee: Hampshire Chemical Ab
Priority date: 1996-10-21
Filing date: 1997-10-17
Publication date: 1998-04-30
Also published as: SE9603859L; AU4797697A; SE507483C2; SE9603859D0

Abstract

A procedure in the manufacture of lignocellulose to remove non-process elements in the form of incrust forming alkaline earth metals and other metals and transition metals is described, the extraction with a combination of an organophilic chelating agent and an organic solvent being carried out in the production of chemical and mechanical pulps, and pulps from recycled fibres.

Description

REM_OVAL _OF MET_AL_S FR_OM TREATMENT FLUIDS IN A CELLULOSE PULP MILL

The present invention relates to a procedure in the manufacture of lignocellulose for removing non-process metal ions by extraction with a combination of an organophilic chelating agent and an organic solvent.

STATE OF THE ART

Chlorine based bleaching

Within the pulp industry traditionally for decades elementary chlorine and chlorine based compounds such as sodium hypo- chlorite and chlorine dioxide have been used for bleaching - delignification of chemical pulps. However, this causes discharge to the recipients of organochlorine compounds, which are defined and quantified as adsorbable organic halogens (AOX) . Among said bleaching chemicals chlorine dioxide gives the least AOX under the same premise and the allowed levels of AOX in discharges from bleach plants gradually have been reduced to such an extent and are now at such a low level that the pulp mills in at least Scandinavia and the rest of Europe have changed to only chlorine dioxide in ECF (Elementally Chlorine Free) bleaching-delignification. However, also the use of chlorine dioxide has been questioned by the pressure from different consumer organizations which require paper products completely bleached without chlorine based chemicals and, furthermore, the restrictions against discharges of organochlorine compounds have been so severe in certain regions that the requirements as to low AOX levels in the discharges hardly could be fulfilled even if only chlorine dioxide was used.

Chlorine free bleaching i.e. ECF (Elementally Chlorine Free) and TCF (Totally Chlorine Free) bleaching

The increasing environmental hesitations against discharges containing these organic chlorine compounds, AOX, accordingly during the latest decade have accelerated a rapid development of chlorine free process steps including oxygen (0) , ozone (Z) and hydrogen peroxide (P) as a complement to chlorine dioxide (D) in ECF bleaching-delignification of sulphate pulps in for instance the sequence [ODQPD] , where Q is a step for treatment with chelating agents but also to a certain extent has become a complete replacement for the chlorine dioxide in TCF bleaching-delignification, for reducering of AOX to nearly zero in for instance the sequence [OQPPP] . The Q step has been introduced since the chlorine free alternatives require a very good control of metal ions. By introduction of also new cooking methods, for instance so-called "Modified Continuous Cooking" (MCC) , "Iso Thermal Cooking" (ITC) and "Superbatch Cooking", and applying sequential bleaching with oxygen, ozone and hydrogen peroxide in for instance the sequence [OZQ(EOP)P] in TCF bleaching/deligni- fication brightness levels comparable with oxygen and chlorine dioxide [ODD] in ECF bleaching/delignification have been obtainable, i.e. about 90% ISO without any essential impairment of the CED (cupriethylene diamine) viscosity of the pulp, usually according to TAPPI or SCAN, by means of which the degree of degradation of the cellulose in the different process steps can be determined.

A key factor for attaining this in TCF production is that the levels of transition metal ions in the pulp in the above- mentioned chlorine free bleaching steps are low, preferably near zero level in at least the peroxide step (P) since especially manganese gives serious disturbances in bleaching with peroxide by catalysing certain side reactions wherein free radicals are involved, which attack the hydrocarbon chains . This means that one has to remove manganese before the P step, usually by the addition of chelating agents in a so-called Q step before one or several P steps in the bleach plant. An often used method is the so-called ignox method (EKA Nobel) which is described in SE-A-8902058. By means of our new method, which is described in the Swedish patent application No. 9404003-7 (US 5 571 378) , the so-called Hampox-Q process (Hampshire Chemical AB) , additional advantages as regards brightness and viscosity have been obtained in regular production of sulphate pulp. Furthermore, the formation of incrusts in vital parts of the apparatus thereby is reduced, which will be described later.

What is common for all Q steps is that complexes of manganese washed out from the pulp must be removed from the white water system in order to prevent that too high levels are built up in the bleach plant. Thus, this must be more or less open and includes the addition of water free from manganese complex to the wash filter in the Q step since it is the wash water which sets the limit as to how low manganese level can be achieved in the pulp into the bleaching step. Thus, the white water must be discharged from the bleach plant and replaced with water free from manganese complex, usually fresh water and condensed water. Discharge of the complexes to the recipient of course gives rise to environmental problems since the chelating agents are restrictively biodegradable and are regarded as being able to release heavy metals from the bottom sediment. Accordingly, by changing to TCF technique and thereby solving the problems with discharge of AOX to the recipient, the discharge of chelating agents to the recipient introduces a new problem, though not so big, it is still of a great concern.

Thus, it should be desirable to get rid of the manganese complexes in an environmentally suitable way. One way, the introduce of which has started, is to destruct the complexes in the chemical recovery system of the mill by system closing with EFM (Effluent Free Mill) production as a goal. This means that the effluent from the white water system of the bleach plant instead of being passed directly to external purification and then discharged into the recipient either is directly integrated counter-current with the brown side and then later indirectly reaches the chemical recovery or that it is integrated directly with the chemical recovery by transfer to the weak liquor to evaporator and the soda recovery unit together with the filtrates from the brown side.

However, closing of the system by transfer of the white water of the bleach plant containing the complexes of transition metals directly to the so-called brown side of the fibre line or directly to the chemical recovery process for destruction gives rise to incrust problems since the white water in addition to transition metals also carries high levels of other so-called non-process metal ions, commonly named non- process elements (NPE) , especially Ca, Mg, Al, Ba and K, which have been desorbed from the pulp by the acidic pH value in the Q step of the bleach plant and in optional acidic Z steps, and which together with different anions, such as sulphate, phosphate, oxalate, silicate and carbonate, can form sparingly soluble salts and form problematic incrusts in the form of barium sulphate and calcium oxalate in the bleach plant, calcium carbonate in digesters, while in the chemical recovery sodium aluminum silicate can be formed in black liquor evaporators and calcium carbonate in the surrounding equipment of the green liquor preparation, such as in pumps and pipelines. The pH lowering in the Q step must be carried out in order that the manganese ions shall be able to form complexes effectively but at the same time it causes an undesirable release of fibre adsorbed calcium and magnesium ions. By means of the previously mentioned Hampox-Q process, which uses a higher pH level and thus releases less calcium and magnesium ions, the situation can be improved but incrust problems cannot be excluded.

To pass the filtrate from the bleach plant to the weak liquor in the above described manner and then let them pass to the soda recovery unit in the same manner as the filtrate from the brown side thus results in increased incrust formation in the evaporators since the acidic filtrates from the Q steps and the optional Z steps in the bleach plant contain high levels of NPE in the form of ions such as Ca, Mg, Al, K and Ba and different anions, such as silicate, phosphate, oxalate, carbonate and sulphate, which can form sparingly soluble salts and be included in the incrusts. The incrust formation can be reduced in the evaporators by passing the filtrates not to the weak liquor, but to the wash liquor, which goes to the melt dissolver and then to the green liquor clarification where the transition metal ions can be expelled but then incompletely since the ligand instead of a complete degradation such as in the soda recovery unit is subjected to a gradually splitting with a certain degree of retained complex forming ability in the green liquor preparation (melt solution, green liquor clarification) , white liquor preparation (causticizing, white liquor clarification) , impregnation, cooking, discharging etc., by means of which optionally some non-process metal ions can be returned and built up in the system to a balance level which depends on the ratio in rapidity between addition and degradation of the complexes in the soda recovery unit and other parts of the system.

In summary, if in TCF-EFM production one wishes to avoid accumulation of high levels of non-process metal ions in the bleach plant (in order to prevent disturbances from manganese ions in the bleaching process and reduce calcium and magnesium incrusts in washing filters and other equipment) one can choose one of the following alternatives according to the present state of the art .

(a) From the bleach plant discharge part of the collected wash filtrates in the white water system of the bleach plant to the recipient, whereby chelating agents in the white water are discharged as well, or to avoid discharge of chelating agents :

(b) transfer of the wash filtrates to the brown side by integration of the bleach plant and thereby destruction of the metal complexes in the soda recovery unit, which gives rise to incrust problems or

(c) transfer of the wash filtrates directly to the melt dissolver in the chemical recovery process which also gives rise to incrust problems.

However, in all the alternatives there are problems. In case (a) environmental problems owing to chelating agents in the recipient, in cases (b) and (c) severe incrust problems on the brown side and in the chemical recovery system. Thus, it is desirable that the white water is free from, or has such a low level of chelating agents which are controversial from an environmental point of view that the white water can be passed directly to the external purification of the mill and be discharged in the recipient.

In the literature different methods are described concerning internal purification and recirculation of spent liquors from the bleach plant in order to manage the NPE problem and thus the incrust problem but all have in common that they are not cost effective for different reasons. One method is the use of ion exchanger and another membrane filtration but both have in common that vital components easily are contaminated and require large maintainance costs and must be frequently replaced. Thus, said methods have not obtained any practical importance .

The solution of the problem

The present invention relates to a procedure in the manufacture of lignocellulose to remove non-process elements (NPE) in the form of : a) incrust forming alkaline earth metals, preferentially calcium, magnesium and barium, b) other incrust forming metals, preferentially aluminum, c) transition metals, preferentially iron, manganese and copper, wherein extraction is carried out with a combination of an organophilic chelating agent and an organic solvent, which extraction is carried out in at least one position in at least one stage in a chemical process such as in the kraft process, the sulphite process or in the manufacture of semi- chemical, chemi-mechanical, mechanical and recycled pulps.

The organophilic chelating agent, i.e. a chelating agent with one or more lipophilic groups, is selected from organophilic derivatives of aminocarboxylic acids, hydroxy alkyl amino- carboxylic acids, hydroxy carboxylic acids, aminophosphonic acids, phosphonic acids, hydroxy benzyl aminocarboxylic acids, hydroxy alkyl benzyl aminocarboxylic acids, hydroxy sulpho benzyl aminocarboxylic acids, hydroxy carboxy benzyl aminocarboxylic acids or mixtures thereof.

Examples of suitable chelating agents are an organophilic derivative of DTPA, EDTA, DTPMPA, monomeric or oligomeric forms of N,N-bis (2-hydroxy-5-sulphobenzyl) glycine, N,N-bis- (2-hydroxy-5-alkylbenzyl) glycine, N,N-bis (2-hydroxy-5- carboxybenzyl) glycine or mixtures thereof.

The total amount of chelating agents which is added is calculated according to the amount of metal ions in the pulp which shall be subjected to complex forming and e.g. it can be from 1 to 10 kgs/ton absolute dry pulp but since the chelating agents are recovered and recirculated after "make up" with new chelating agents this is not the same as the consumption but the consumption can be of the order of one tenth of the dosage.

The amount of organic solvent can be determined within wide limits the lower limit being determined by the solubility of the organophilic chelating agent and of the complex and, furthermore, by the type of extraction equipment, and for instance can be up to the order five to ten times the amount of pulp which is subjected to extraction in the extraction zone in integrated extraction in some washing apparatus, for instance a diffuser, while in differentiated extraction of the white water flow in an extractor alongside of the pulp flow can be set lower, how much depends on the type of extractor. The upper limit should for practical and economical reasons of course not be set higher than what is necessary for an effective extraction.

The organic solvent can belong to the group of terpenes, for instance terpentine from the mill's own terpentine recovery or be some other suitable solvent which has restricted solubility or is sparingly soluble in water.

In the enclosed drawings

Figure 1 shows integrated extraction in a diffuser with an organic solvent and an organophilic chelating agent in the pulp flow, and

Figure 2 shows differentiated extraction with an organic solvent and an organophilic chelating agent with the use of an extractor on the white water system.

According to the present invention NPE are removed in an integrated extraction, preferably on the brown side, by means of an organophilic chelating agent dissolved in an organic solvent, whereby fibre adsorbed and water dissolved NPE are transferred to the organic phase in the form of a metal ion complex. The stability of complexes of the category transition metal ions is maintained by maintainance of reductive conditions by the addition of a suitable reducing agent as is described in our copending patent application relating to "Process in preparing cellulose pulp" in treatment with water soluble chelating agents. The extraction can be carried out for instance in an diffuser such as is shown in Figure 1.

Suitable reducing agents is one or a combination of reducing agents selected from hydrogen sulphide, sulphide, sulphur dioxide, hydrogen sulphite, sulphite, boron hydride, dithio- nite, tetrathionite, dithionate, tetrathionate, thiosulphate, hypophosphite, orthophosphite etc. where the the counterion to the anions in said examples can belong to the groups alkali or alkaline earth metals, usually sodium, magnesium and calcium, an especially suitable reducing agent is for instance sodium dithionite.

The organic phase with the dissolved complexes passes together with the wash liquor out from the diffuser to the white water system from which the organic phase with the dissolved complex is separated from the liquor phase in a separator. The organic phase with the organophilic metal ion complex is extracted with diluted acid, for instance sulphuric acid, the metal ion in the complex being released from the ligand (chelating agent) and passes over into the acid phase while the ligand remains dissolved in the organic phase. The organic phase with the ligand (chelating agent) is recirculated to the diffuser after "make up" with organophilic chelating agent and the procedure is repeated in a circle process. The acid phase is neutralized with for instance sodium hydroxide, and any remaining organic solvent in the neutralized acid phase is separated in the turpentine decanting before it goes to the external purification.

According to a further embodiment according to the invention the extraction can be carried out differentiated in an extractor on the white water system, i.e. alongside of the pulp flow, such as is illustrated in Figure 2, and then the white water after having been freed from NPE can be recirculated to the extractor. This second embodiment of the invention means a process which is simpler from a process technical point of view since the organic solvent will not enter the main flow of the pulp. On the other hand, the extraction of organosolve soluble resinous substances and NPE will be inferior since the organic solvent and the dissolved organophilic chelating agent respectively will not come into direct contact with the fibre. Thus, this process is less suitable for fibre adsorbed Mn, which must be lowered to zero level in order not to disturb in the bleaching process. However, the process is suitable for removal of NPE in the form of alkaline earth metals, Ca, Mg and Ba. In this context no reducing agent is necessary which, however, as can be seen from what has been stated above, is the case as regards transition metals.

According to further embodiments of the invention it is possible to combine the extraction process (organophilic chelating agents) with the process which is described in our copending patent application with the title "Process in the manufacture of cellulose pulp" which describes reductive complex formation of transition metal ions on the brown side of the craft process, in order to effectively extract also fibre adsorbed transition metal ions, especially manganese. The different embodiments can, in addition to the brown side, also be applied in the bleach plant, after the bleach plant, in chips handling and in digesters. Two phase extraction according to the invention can in addition to craft pulp also be used in the manufacture of other chemical pulps such as sulphite pulp and in the production of semi-chemical, chemi- mechanical and mechanical pulps such as GW, PGW, SPGW, TMP, RMP and CTMP and recycle fibre pulps, on the whole when there is a need to remove metal ions from a system in order to solve incrust problems or if there are other reasons for removing metal ions .

The different forms of extraction according to the invention can be carried out batchwise or continuously in counter- current. For batchwise differential extraction/separation according to the gravitation principle stainless tanks with agitators can be used, which are used alternately while continuous differential counter-current extraction, also according to the gravitation principle, consists of a tower or column with a zone for mechanical dispersing of the liquid phases and another zone where separation takes place. More advanced continuous apparatuses for differential counter- current extraction are based on the centrifugal principle. Suitable such are for instance Podbielniak, Quadronics, (Liquid Dynamics) , Luwestra (Centriwestra) or De Laval extractors .

Claims

1. Procedure in the manufacture of lignocellulose to remove non-process elements (NPE) in the form of: a) incrust forming alkaline earth metals, preferentially calcium, magnesium and barium, b) other incrust forming metals, preferentially aluminum, c) transition metals, preferentially iron, manganese and copper, c h a r a c t e r i z e d in that extraction is carried out with a combination of an organophilic chelating agent and an organic solvent, which extraction is carried out in at least one position in at least, one stage in the manufacture of chemical pulps according to for instance the craft process and the sulphite process or in the manufacture of semi- chemical, chemimechanical or mechanical pulps and recycled fibre pulps.

2. Procedure according to claim 1, c h a r a c t e r i z e d in that it is carried out in the craft process or the sulphite process in at least one position in at least one of the following stages:

(a) in chips handling before digester, (b) in digester, (c) after digester on the brown side, (d) in the bleach plant and (e) after the bleach plant, for instance in combination with paper or board machines .

3. Procedure according to claim 2 , c h a r a c t e r i z e d in that the extraction is integrated with a washing apparatus in at least one of the following stages:

(a) in chips handling before digester, (b) in digester, (c) after digester and the brown side, (d) in the bleach plant and (e) after the bleach plant.

4. Procedure according to claim 3 , c h a r a c t e r i z e d in that the filtrate from the washing apparatus is integrated with a separator for separation of the organic phase with the dissolved organophilic metal complexes, the filtrate is recirculated after removal of the incrust forming alkaline earth metals and other incrust forming metals by the extraction, the organic phase is integrated with an acidic extraction for decomposition of the complexes, the organic phase with the organophilic chelating agent is recirculated after make up with organophilic chelating agent in a circle process and the acidic aqueous phase with the released metal ions is after neutralization transferred to the recipient.

5. Procedure according to claim 4 , c h a r a c t e r i z e d in that the extraction is carried out differentiated in an extractor on the white water system, i.e. alongside the pulp stream, whereupon the white water after removal of the incrust forming alkaline earth metals and other incrust forming metals can be recirculated to the white water system.

6. Procedure according to any of the preceding claims, c h a r a c t e r i z e d in that in one stage in at least one position an organophilic chelating agent and an organic solvent are added and in at least one position a reducing agent for reduction, chelation and extraction of NPE also in the form of transition metal ions, preferentially iron, manganese and copper.

7. Procedure according to claims 2 and 6, c h a r a c t e r i z e d in that a water soluble chelating agent and a reducing agent are added on the brown side for efficient removal of transition metal ions, preferentially iron, manganese and copper.

8. Procedure according to any of the preceding claims, c h a r a c t e r i z e d in that the organophilic chelating agent, i.e. a chelating agent with one or more lipophilic groups, is selected from organophilic derivatives of aminocarboxylic acids, hydroxy alkyl aminocarboxylic acids, hydroxy carboxylic acids, aminophosphonic acids, phosphonic acids, hydroxy benzyl aminocarboxylic acids, hydroxy alkyl benzyl aminocarboxylic acids, hydroxy sulpho benzyl aminocarboxylic acids, hydroxy carboxy benzyl aminocarboxylic acids or mixtures thereof .

9. Procedure according to claim 8 , c h a r a c t e r i z e d in that the chelating agent is an organophilic derivative of DTPA, EDTA, DTPMPA, monomeric or oligomeric forms of N,N-bis (2 -hydroxy-5-sulpho benzyl) glycine, N,N-bis (2 -hydroxy-5-alkyl benzyl) glycine, N,N-bis-

(2 -hydroxy-5-carboxy benzyl) glycine or mixtures thereof.

10. Procedure according to any of the preceding claims, c h a r a c t e r i z e d in that the organic solvent belongs to the groups of terpenes or is another solvent which has limited solubility in water or is sparingly soluble in water.

11. Procedure according to claim 6 or 7, c h a r a c t e r i z e d in that one or a combination of reducing agents is selected from hydrogen sulphide, sulphide, sulphur dioxide, hydrogen sulphite, sulphite, borohydride, dithionite, tetrathionite, dithionate, tetrathionate, thio- sulphate, hypophosphite, orthophosphite etc. where the counterion to the anions in said examples can belong for instance to the groups alkali or alkaline earth metals, usually sodium, magnesium and calcium, e.g. sodium dithionite being an especially suitable reducing agent.