The invention relates to a process for the bleaching of a high yield pulp, wherein, before peroxide bleaching, the pulp is pretreated with a chelating agent in order to eliminate the adverse effects of any heavy metals present in the pulp.
By high yield pulps are meant above all mechanical pulps the bleaching of which differs from the bleaching of chemical pulps in that the aim is to avoid delignification. The yield of pulp is maintained at a high level. The aim in bleaching is to render colorless the color-producing components (chromophoric groups) present in lignin. In the invention, high yield pulps are considered as also including recycled fibers in the bleaching of which the aim is to remove printing ink, not lignin.
The chelating carried out in connection with the bleaching of mechanical pulps differs from the pretreatment carried out in connection with the bleaching of chemical pulps. In the latter case the complexes formed in the chelating are removed by a wash. In the case of mechanical pulps, no wash is carried out in the pretreatment; instead, the pretreated pulp is compressed to the consistency used in bleaching, in which case only a small proportion of the metal complexes are removed from the pulp.
Mechanical pulps can be divided into two principal categories, pure mechanical pulps and chemi-mechanical pulps. These pulps can further be divided into subcategories so that mechanical pulps include stone groundwood (SGW), pressure groundwood (PGW), refiner mechanical pulp (RMP), thermomechanical pulp (TMP) and others such as TRMP and PRMP. Respectively, the chemi-mechanical pulps include low-sulfonated pulps (chemithermomechanical pulp CTMP amd BCTMP), chemically modified pulps (OPCO) and high-sulfonated pulps (CMP, UHYS).
Mechanical pulps are used in the making of, for example, newsprint, magazine paper and porous paper grades (tissue papers). Certain highly bleached chemi-mechanical pulps (BCTMP grades) are also used in bleached printing papers to replace chemical pulp.
Hydrogen peroxide is used for the bleaching of mechanical pulps and of recycled fiber which contains paper fiber made from mechanical pulps. In order for the bleaching to be successful, the bleaching conditions must be rendered such that the hydrogen peroxide will not dissociate.
Heavy metals catalyze the dissociation of hydrogen peroxide and peroxy compounds. The ions the most detrimental in terms of bleaching are manganese (Mn), iron (Fe) and copper (Cu). Also other heavy metals, such as chromium ions (Cr), etc., have a detrimental effect on the consumption of peroxy compounds. Detrimental heavy metals originate in the pulp, the treatment waters and the pulp-treatment apparatus.
In the bleaching with peroxy compounds, heavy metals are bound by using agents which chelate metal ions, for example polyaminocarboxylic acids. These include in particular ethylene diamine tetra-acetic acid and its salts (EDTA) and diethylene triamine penta-acetic acid and its salts (DTPA), as well as diethylene triamine pentamethylenephosphonic acid (DTMPA) and its salts.
The bleaching of mechanical pulps and recycled fiber is commonly carried out with hydrogen peroxide at a high pH. Magnesium sulfate and waterglass are added during the bleaching step in order to stabilize the peroxide. Many hypotheses have been presented regarding the stabilizing effect of the above-mentioned compounds. The stabilizing effect of waterglass is probably based on waterglass surrounding the hydroxides, oxyhydroxides and oxides formed under alkaline conditions when iron precipitates, thus inactivating the catalytic surfaces of these solids.
EDTA and DTMPA are regarded as non-biodegradable. DTPA is poorly biodegradable.
Mechanical pulps are made in paper mills and not in separate plants as are chemical pulps. Thus the effluents formed during bleaching end up in the other effluents of the paper mill. Therefore it would be highly desirable to use biodegradable chelating agents in connection with bleaching. To avoid environmental loads it would be preferable to replace poorly biodegradable chelating agents at least in part with biodegradable chelating agents which do not contain nitrogen.
The bleaching of recycled fibers can often also be carried out using reducing agents such as dithionite, and peroxide bleaching is not always necessary. Hydrogen peroxide is used more in the slushing of recycled pulp than in actual bleaching. For this reason the treatment of recycled fiber involves the same problems as regards the stability of hydrogen peroxide.
Previously the hydrogen peroxide bleaching of mechanical pulps was carried out in one step. At present the bleaching is carried out in two steps, since the aim is a higher pulp brightness and since for example BCTMP can be used in printing papers. The peroxide remaining after two-step bleaching is recycled to the first step. Thus the residual peroxide is highly significant for the economy of bleaching.
In those bleaching processes of mechanical pulps in which a high brightness is the aim it has become common to use chelating agents for a pretreatment at a pH lower than that at which the actual bleaching takes place. In the publication W.C. Fross et al., Tappi 1992 Pulping Conference, pp. 899-915, "Factors affecting mechanical pulp brightening when peroxide liquor is recycled," a DTPA treatment at a pH of 5.5-6.0 is described as an example.
Biodegradable complexing agents have been developed for detergent builders. They must at the same time have softening action on water, i.e. they must bind calcium ions and magnesium ions. One such sequestering agent is ethylenediamine disuccinic acid (EDDS). This compound has three stereoisomers.
The substance is known per se from F. Pavelcek's and J. Majer's publication "New Complexanes. XXXIV. Preparation and properties of the meso and rac forms of ethylenediamine-N,N'-disuccinic acid," Chem. Zvesti 32 (1978), pp. 37-41. FI Patent 86554 of Procter & Gamble discloses the use of this substance in detergent compositions. Application publication WO 94/03553 of the same applicants discloses the use of the substance in bleaching liquors as a stabilizer of peroxy compounds in the bleaching of pulp and in detergents. The more detailed description and examples relate only to the latter. In this case the pH of the bleaching liquor must be 0.5-6.0. The publication mentions that the compound is biodegradable. According to the patent, the S,S-isomer of EDDS is the best biodegradable.
EP patent application 556 782 discloses the use of EDDS as an iron complexer in photography chemicals. Example 9 of the publication mentions that the ferric salts of EDTA, DTPA and HEDTA are not biodegradable. In contrast, the Fe2+ salt of EDDS is biodegradable. The biodegradabilities of the compounds were tested in the example by a generally approved testing method (301C Amendment MITI Test (I), OECD Chemical Substance Testing Guidelines, May 1981).
Another known biodegradable complexing agent is 2,2'-iminodisuccinic acid (ISA). The use of this compound in alkaline detergents is disclosed in EP patent application 509 382. The patent application mentions the use of 2,2'-iminodisuccinic acid as a stabilizer of peroxide compounds, in particular in alkaline detergent compositions which contain hydrogen peroxide and its derivatives. In the examples of the application, only perborate is used. Since perborate releases hydrogen peroxide only slowly, no far-reaching conclusions can be drawn from the examples of the patent regarding the stabilization of hydrogen peroxide in similar detergent compositions.
DE patent application 4 216 363 discloses the use of ISA as a stabilizer of tensides. There is no mention of the use of a peroxide compound. EP patent application 513 948 mentions the use of the substance in detergents which are intended for hard surfaces and contain an organic solvent boiling at approx. 90 °C. There is no mention of the use of a peroxide compound.
DE patent application 4 340 043 discloses the use of ISA as a bleaching agent in the bleaching of groundwood. According to the publication, the purpose of ISA is the stabilization of hydrogen peroxide, and the examples show that at a pH of 10 it is a better stabilizer than DTPA. There is no demonstration in the publication of the action of ISA on heavy metals, nor of any bleaching results.
It is an object of the present invention is to eliminate the adverse effects of heavy metals in the bleaching of high yield pulps. The aim is to provide for use a biodegradable chelating agent which yields a good bleaching result. The characteristics of the invention are given in the accompanying claims.
In the process according to the invention, compounds according to Formula I, known per se, are used
n is 1-3,
m is 0-3,
p is 1-3,
R1, R2, R3 and R4 are H, Na, K, Ca or Mg, and
R5 and R6 are H, CH2OH, CH2CH2OH or
CH2O(CH2CH2O)1-10CH2CH2OH.
DTPA, which is commonly used in the bleaching of pulp, chelates heavy metals best at a pH of approx. 5.
It has now been observed, surprisingly, that compounds according to Formula I can be used advantageously, at a higher pH than can DTPA, as chelating agents in a pretreatment step preceding the bleaching of pulp. The pretreatment of pulp can be carried out at a pH of 4-8, preferably at a pH of 5.0-7.5 and most preferably at a pH of 6.5-7.5. The peroxide bleaching can be carried out at a pH of 6-12, preferably at a pH of 7-11 and most preferably at a pH of 8-10.5. Since the chelating can be carried out at a higher pH value, the consumption of alkali in the subsequent step is not as high as when the chelating is carried out at a lower pH. This is a clear advantage over, for example, DTPA.
The process according to the invention can be used in the pretreatment of all mechanical pulps, such as stone groundwood, pressure groundwood, refiner mechanical pulp, thermomechanical pulp, chemi-mechanical refiner pulp and chemi-mechanical pulp.
The process is also suitable for the bleaching of recycled fiber. The process can be used both in single-step and in two-step peroxide bleaching of mechanical pulp. The treatment is also suitable for mechanical pulp bleaching in which dithionite or formadine sulfinic acid bleaching is used instead of peroxide in some step. In the case of mechanical pulps which are sulfonated with sulfite or bisulfite, the compounds according to Formula I may be added already to the impregnation liquor, whereby the efficacy of the bleaching can be improved.
The treatment may be performed on pulps obtained from various fiber raw materials, such as softwood or hardwood.
The pH control of the acid chelating step can be carried out using conventional mineral acids, such as sulfuric acid, sulfur dioxide or an aqueous solution thereof, carbon dioxide, or organic acids such as formic acid and acetic acid.
In the process according to the invention, an especially suitable chelating agent according to Formula I is ethylenediamine-N,N'-disuccinic acid, its various isomers or its alkali metal salts, such as sodium and potassium salts, or its earth-alkali metal salts, such as calcium and magnesium salts. It is also possible to use ethylenediamine-N,N'-disuccinic acid together with calcium sulfate and/or magnesium sulfate. Another particularly suitable chelating agent is 2,2'-iminodisuccinic acid, its various isomers and its alkali metal salts, such as sodium and potassium salts, or its earth-alkali metal salts, such as calcium and magnesium salts. It is also possible to use 2,2'-iminodisuccinic acid together with calcium sulfate and/or magnesium sulfate.
Usable chelating agents also include N-(1,2-dicarboxyethyl)-N-(2-hydroxyethyl) aspartamic acid, its various isomers and its alkali metal salts and earth-alkali metal salts. The acid may also be used together with potassium sulfate or magnesium sulfate.
The chelating agent may be added in an amount of 0.1-5 kg, preferably 0.5-2 kg, per metric ton of dry pulp.
It has also been observed that EDDS and ISA can be used together with hydroxycarboxylic acids without the bleaching result being worsened. It is advantageous to use EDDS or ISA together with chelating agents which do not contain nitrogen. These include hydroxycarboxylic acids having the general formula II
R1CnHm(OH)p(COOH)qR2 (II)
where
n is 1-8,
m is 0-2n,
p is 0-n,
q is 0-2,
R1 is COOH, and
R2 is H, CH2OH or COOH.
The detrimental nitrogen load in bleaching effluents can thus be reduced. Conventional carboxylic acids, hydroxycarboxylic acids, polyhydroxycarboxylic acids and hydroxypolycarboxylic acids according to Formula II, such as citric acid, tartaric acid, lactic acid, pimelic acid, glutamic acid, glucoheptonic acid, ascorbic acid, glycolic acid, glutaric acid, adipic acid, succinic acid or malonic acid, can be used as replacement chelating agents.
It is quite surprising that hydroxy acids can be used as chelating agents in bleaching. The said substances are quite poor binders of heavy metals, but bind well calcium and magnesium. Citric acid has been used as a replacement for phosphates in phosphate-free detergents and cleansing agents, in which the substances are required to bind calcium and magnesium. Especially the binding of magnesium should be disadvantageous in terms of bleaching.
The invention is illustrated below with examples, which do not, however, limit the invention to relate only to the examples presented here.
To investigate the transfer of heavy metals, washing experiments were first performed on a chemical pulp. The removal of the metals in itself does not yet determine the success of the actual bleaching, but only gives guidelines.
The best pH range for metal removal can be sought by means of washing experiments. With the help of washing experiments carried out on the same pulp at the same pH it is possible to compare the efficacies in the chelating of metal ions by different chelating agents.
To investigate the chelating of heavy metals and earth alkali metals, an oxygen-delignified chemical pulp was washed with aqueous solutions containing EDDS. The metal contents of the washing solution were analyzed after the wash. Thereby the transfer of iron (Fe), manganese (Mn), calcium (Ca) and magnesium (Mg) into the washing waters was investigated. The transfer of iron and manganese into the washing solutions is advantageous for bleaching. In contrast, the transfer of calcium and magnesium into the washing solutions is disadvantageous for bleaching. In the reference tests the pulp was washed with DTPA or EDTA solutions. The chelating agent concentrations and the pH during the wash are indicated in Table 1.
In Table 1, Na5DTPA stands for the pentasodium salt of DTPA, Na4EDTA stands for the tetrasodium salt of EDTA, and H4EDDS stands for the acid form of EDDS. However, the pH used will determine how the chelating agents are dissociated, i.e. in which form they actually appear in the treatment. The H4EDDS (reaction mixture) mentioned in the table refers to experiments in which the chelating agent used was an unpurified reaction product directly from the process for the preparation of EDDS.
It was observed in the experiments that, when EDDS was used at a rate of 1.5 kg/metric ton of pulp, the chelating of iron and manganese was nearly as complete as when EDTA or DTPA was used at a rate of 2.0 kg/tp in the same experimental conditions. At a lower pH the chelating of manganese was not as complete as when the reference substances were used, but at a pH of 6.5 and in more alkaline conditions manganese was chelated as well as or better than when the reference substances were used. EDDS chelated calcium and magnesium less than did the reference substances, which is advantageous in terms of bleaching.
No significant differences were observed in the experiments in the chelating properties among the S,S isomer of EDDS, a mixture R,R and R,S isomers of EDDS, and a chelating agent containing all isomers of EDDS.
ISA chelated iron and manganese well at a pH of 5.8. It is to be noted that ISA removed very little magnesium from the pulp at a pH of 5.8. This is advantageous for bleaching. Chelating in an alkaline solution at a pH of 8.9 was also very successful. It is to be noted that at this pH iron has already precipitated. This explains the small iron contents in the filtrate. Chelating with a mixture of ISA and the sodium salt of citric acid was also successful, considering that the chelating was carried out at a pH of 7.7.
Table 2 shows the results of washing experiments similar to those described in Example 1, when EDDS was diluted with certain hydroxy acids.
In the chelating step, DTPA is usually dosed into a softwood pulp at a rate of approx. 2 kg/tp. In the second series of experiments (Table 2), the effect of the DTPA dose on the chelating of metals was first investigated. Chelating was clearly less when the dose of EDDS was reduced from a rate of 2.0 kg/tp to a rate of 1.0 kg/tp or 0.5 kg/tp. In the previous series of experiments (Table 1) it was observed that when Na4EDDS was used at a rate of 1.5 kg/tp, the chelating was as complete as when Na5DTPA was used at a rate of 2.0 kg/tp.
Practical experience and the above results of experiments indicate that DTPA must be used in the chelating step at a rate of approx. 2.0 kg/tp and EDDS at a rate of approx. 1.5 kg/tp in order for the heavy metals to be chelated sufficiently completely for bleaching.
When sodium salts of citric acid or gluconic acid were used as a chelating agent alongside EDDS, it was possible to reduce the dose of EDDS significantly. Even though the dose of EDDS had been lowered to a rate of 1.0 kg/tp, the chelating of metals in these experiments was as complete as when EDDS was used at a rate of 1.5 kg/tp. By the use of, for example, the above-mentioned salts of hydroxycarboxylic acids together with EDDS as the chelating agent, the nitrogen load in the effluents from the chelating step can be reduced significantly, while the chelating of the metals is still sufficiently complete for bleaching.
It can be observed from Table 2 that a water wash is without effect as regards the chelating of metals. Likewise, citric acid used alone does not remove heavy metals. Citric acid chelates only earth-alkali metals, which is not desirable for bleaching. This indicates that a good chelating result is achieved through the joint effect of EDDS and, for example, citric acid.
It is to be noted that a conclusion regarding the bleaching result cannot be drawn directly from washing experiments such as described above. For this reason the effect of corresponding chelating steps on alkaline hydrogen peroxide bleaching was investigated.
Furthermore, it is to be noted that in the bleaching of mechanical pulps the pulps are not washed; any excess water is removed by compression in order to obtain an appropriate consistency for bleaching.
A thermomechanical pulp was treated with a chelating agent and was bleached with hydrogen peroxide in the conditions described in Table 3. In each experiment the chelating agent was added in the pretreatment step at a rate of 1.5 kg/metric ton of pulp (kg/tp). Furthermore, in the bleaching step each chelating agent was added at a rate of 1.5 kg/tp.
The results show that the consumption of hydrogen peroxide was clearly greatest in the experiment in which no chelating agents were used. The brightness of the pulp was also lower. In the experiments in which EDDS or ISA was used as the chelating agent, the brightness of the end product was as good as in the experiment in which DTPA was used as the chelating agent.
A refiner mechanical pulp was treated with chelating agents and was bleached with hydrogen peroxide in the conditions described in Table 4. In each experiment the chelating agent was added at a rate of 2.0 kg/metric ton of pulp in the pretreatment step. No chelating agents were used in the bleaching step.
After peroxide bleaching, no significant differences in pulp brightness were found among the pulps treated with different chelating agents. The bleaching results for the pulps treated with EDDS and with ISA were as good as for the pulp treated with DTPA. The consumption of hydrogen peroxide was lower in the bleaching of EDDS-pretreated pulp than in the bleaching of DTPA-pretreated pulp.
Refiner mechanical pulp was treated with chelating agents and was bleached with hydrogen peroxide in the conditions described in Table 5. In each experiment the chelating agent was added at a rate of 2.0 kg/metric ton of pulp in the pretreatment step. Furthermore, in the bleaching step each chelating agent was added at a rate of 2.0 kg/tp. The bleaching result is described relatively well by the residual peroxide content. Therefore the brightness of the pulp was not determined in all of the experiments.
No significant differences in the residual peroxy level were found in the bleaching of pulps treated with different chelating agents.
To an alkaline hydrogen peroxide solution which contained chelating agents and had an H2O2 content of 3 g/l, iron and manganese ions were added so that the Fe content in the solution was 2 mg/l and its Mn content was 4 mg/l. The solutions were kept at 50 °C, and the dissociation of hydrogen peroxide was followed by titrating H2O2 with an ammonium cerium sulfate solution. The results are shown in Table 6.
It was observed that an alkaline hydrogen peroxide solution which contained DTPA was clearly the most stable. In solutions which contained ISA and EDDS the hydrogen peroxide dissociated clearly more rapidly. It was shown in the previous examples that the brightness results for a mechanical pulp treated with EDDS or ISA were comparable to the results for those treated with DTPA in spite of the poorer ability of EDDS and ISA to stabilize an alkaline hydrogen peroxide solution. This experiment shows that a conclusion regarding the functioning of chelating agents in the pretreatment of bleaching, or in actual bleaching, cannot be drawn directly from their efficacy in stabilizing alkaline hydrogen peroxide solutions.