A METHOD FOR REDUCING THE LEVEL OF INTERFERENCE CHEMICALS IN THE WATER CIRCULATION OF WOOD-BASED FIBRE SUSPENSION PROCESSES

Abstract: The invention relates to a method for reducing the amount of interference substances in the water circulations of processes involving wood-based fibre suspensions by binding the disturbance substances to the fibres by means of cationic starch. The amount of the said starch added to the fibre suspension is approximately 0.04-0.5 % of the fibre dry weight.

Description: A method for reducing the level of interference chemicals in the water circulation of wood-based fibre suspension processes Several changes have taken place during the past few years in the water circulations of processes dealing with wood-based fibre suspensions, especially in the case of water circulations connected to papermaking: Environmental and social pressures call for reductions in effluents to the water system. This leads to shortened water circulations and their partial closing off. The need to save expensive fibre raw material calls for increasing use of recycled fibre. The use of recycled fibre has increased significantly and there are countries where the legislation makes it compulsory to use such material. For the same reason the use of groundwood and other mechanical pulps is on the rise. As a means of cutting costs, increasingly higher propor¬ tions of the more expensive filler and coater pigments in the manufacturing of paper and board are being replaced by CaC0 3 . This requires a shift over from acidic processes to neutral processes. Due to all the above reasons, increasing amounts of what may be called interference chemicals are becoming concen¬ trated in the process water circulations. The interfer- ence materials are chemically a highly heterogenous group of compounds. Typical examples of these are wood extracts, neutral or anionically charged, substances in dissolved or colloidal form arising during cellulose pro¬ duction or originating from the wood. In addition, coating binder residues, synthetic polymers, anionic emulgators, etc., end up in the water circulation of the paper machine in the course of manufacturing of coated papers. De- nked recycled material can never equal virgin fibre raw material in terms of purity as it always contains de- inking chemical and ink residues. The aforementioned interference substances impair i.a. the runnability of the paper machine by increasing the frequency of web breaks, by blocking up the wires, by contaminating the press rolls and by reducing paper qual¬ ity in terms of increasing number of holes. The interfer¬ ence substances reduce retention by reducing the ef¬ ficiency of the available retention system as impurities bind to the retention substance and thereby bind part of the functional groups of the said retention substance. Broadly defined, all chemicals used in the above proces¬ ses become interference substances once they become con¬ centrated in the water circulation. In its more emphasis- ed form, this problem occurs expressly in paper machines. Traditionally, paper manufacturers have used alum to com¬ bat the effect of interference substances. Alum effici¬ ently binds interference substances to the fibres and thereby inactivates them. The effect of alum is based on the fact that within the pH range of 4.2 - 5.0 alum forms an abundance of positively charged cations which bind and neutralise mainly negatively charged interference sub¬ stances. In neutral paper manufacturing processes alum is ineffective and this is why other solutions must be re¬ sorted to. Another method is to use talc. Talc binds impurities to its surface and in favourable conditions the interference substances leave the system along with the paper web. However, talc does not work very well if the concentra¬ tion of impurities is high. Dispersing agents are used with the purpose of dispersing the interference substances into the circulation water. A shortening of the water circulations leads to situations in which the concentrations of the interference sub- stances rise to high levels in the circulation water, which in turn reduces retention and on the other hand makes it possible for fresh accumulations to occur, thereby reducing product quality. As alum is a cationic product, there is a trend in neutral process papermaking to replace it with short- chained cationic polymers; the best known among these are the poly-DADMAC products (i.e. poly-diallyl-dimethyl- ammoniumchlorid ). These polymers typically possess a high cationic charge (meqv/g) and their molecular weight is relatively low when compared to synthetic products traditionally used as retention polymers. Corresponding other cationic binding chemicals include various polyami- ne polymers, etc. It is believed that the high cationic short-chained polymers function by binding the interfer¬ ence substances to the fibre material and consequently the interference substances leave the system continuously along with the fibres. Products of this type are added to the system at an early enough stage to prevent binder substance particles, for instance, from forming harmful agglomerations (white pitch). There are several ways to measure interference substances in the circulating waters of paper machines and the like but all are more or less indirect. One of these methods is based on determining the charge state (the interfer¬ ence substances are generally negatively charged); i.e. the higher the negative charge, the higher the concentra¬ tion of interference substances. When added to the sys- tern, the highly cationic, short-chained polymers reduce the negative charge (= the amount of negatively charged colloids of interference substances). This change in charge state also depicts the change in the concentration of interference substances. In laboratory conditions, the efficiency of cationic polymers can be measured by determining the residual tur¬ bidity of the fibre material (mechanical stock, reject stock, etc.) filtered through the wire or filter paper before and after chemical treatment. It is also possible to measure the chemical oxygen demand (COD) or the con- ductivity of the system. The efficient dosage of the said substances depends very much on the system; the dirtier the system, the higher the required dosage. Generally, dosages amounting to 0.05 - 0.4 % of the dry weight of the fibre gives good results. In the case of paper machines, the point of dosage is typically immediately after the stock storage tank before the other chemicals are added to the system. Starches are used in papermaking as paper strength im¬ proving component. Cationised starch in the form of stock starch or cationic surface glue binds better to the fibres and does not end up in the circulating waters of the paper machine. Thus, it reduces the COD-load of waste and circulating waters. Surprisingly it has been observed that when the charge state of starch is raised by cationising starch ethers with 2,3-epoxy-propyl-trimethylammoniumchloride to the level of 2 - 3 meqv/g, the said highly cationised starches function in the manner of alum or a pDADMAC chemical by binding anionic colloidal substances present in papermaking regardless of whether the said substances originate from a mechanical stock or coated reject. Con- trary to alum, the starches in accordance with the inven¬ tion function regardless of the pH level and can thus also be used in neutral papermaking processes. The invention is described in the light of the following comparative examples. The examples used in the comparison are polyDADMAC (Basf: Fixiermittel, Nalco: 8674, Mobil: Certrex 340), polyimine (Basf: Polymin SK), and polyaluminiumchloride (Oulu Oy: Oulupac 180). In all the laboratory tests conducted, the pulp was stirred for 2 minutes following the proportioning of che¬ micals therein after which it was filtered through a 150 μm wire screen. The filtrates were measured for their turbidity, charge state and/or chemical oxygen demand (COD). Retention tests were conducted using a Dynamic Drainage Jar tester (TAPPI, February, 1976, Vol. 59, No. 2, pp. 67-70). The retention tests consisted of first adding the interference removing agent to the stock and allowing it to mix for 1 min 30 sees, then of adding the stock starch and allowing this to mix for 30 sees, and finally of ad¬ ding the retention substance (Allied Colloids: Percol 47). Filtering was commenced 30 sees after the addition of the retention substance. The filtrates were measured for their COD values. Turbidity was measured in TE/F units (Dr. Lange LTP 5) and the charge state was determined by titrating using a PCD-02 charge state detector (Mϋtek) with an 0.001 N MGC (methyl glycol kitosane) solution as the titration solu¬ tion. COD was determined using a test kit (Dr. Lange). All dosages were determined as active substance per dry matter of stock. Example 1 Stock: coated reject, pH 4.6 Turbidity, charge state, COD. Two batches of cationic starch differing in degree of purity were compared to commercial polyDADMAC. The cationisation degree of both was 2.7 meqv/g. Dosage Turbidity Charge state COD % (1:10) μeqv/1 mg/1 45 1921 658 1400 585 613 540 160 483 1579 653 871 603 454 618 1506 618 776 570 291 525 Example 2 Stock: Groundwood pulp, pH 7 Retention test Points 1-3 served as comparison points for the test points 4-6 which include cationised starch (2.8 meqv/g) as the fixing agent. Stock starch : Raisamyl 125. Example 3 Stock: TMP, pH 4.6 Turbidity, COD. Two batches of cationic starch and two commercial poly- DADMACs were compared. Chemical Dosage Turbidity COD % (1:10) mg/1 0 test 154 2385 2325 1795 1595 1775 1535 1585 1765 1595 1420 2020 1610 1430 Example 4 Stock: MWC reject, pH 8 Turbidity, charge state, COD. Two cationic starches and commercial polyDADMAC. Example 5 Stock: Double coated reject, ph 7.5 Turbidity, charge state. Two cationic starches, each 2.5 meqv/g and a commercial polyDADMAC and polyaminechloride (PAC). Chemical Dosage Turbidity Charge state % (1:10) μeqv/1 183 135 120 92 149 113 78 119 96 87 127 100 98 Example 6 Stock: De-inked stock Turbidity, COD. Cationic starch (2.8 meqv/g) and a commercial polyDADMAC. Chemical Dosage Turbidity COD % (1:10) mg/1 0 test 383 438 PolyDADMAC 0.2 52 390 Mobil 0.4 49 355 0.6 85 348 Cationised 0.2 40 431 starch 0.4 33 402 0.6 30 338 Example 7 Stock: test stock mixture (50% groundwood stock + 50% coated reject), pH 8.4 Turbidity Two cationised starches and commercial polyDADMAC and polyimine. Chemical Dosage Turbidity % (1:10) Stock: groundwood stock, pH 5.3 Turbidity, COD. Two cationic starches and a commercial polyDADMAC. Example 9 Stock: Double coated reject, pH 7.5 Turbidity Three cationised starches of different eationisation deg¬ rees and a commercial polyDADMAC. Chemical Cationised starch 1 (2.8 meqv/g) Cationised starch 2 (1.8 meqv/g) Cationised starch 3 (1.0 meqv/g) PolyDADMAC Nalco Example 10 Stock: Test stock mixture (50% groundwood stock + 50% coated reject), pH 8.4 Turbidity Three starches of different eationisation degrees. Based on the above test results it can be observed that the results of using a cationic starch polymer in the reduction of interference substances in water circulation fibre suspensions are of the same order as those when using polymers traditionally used for the purpose.

Claims: Patent claims: 1. A method for reducing the amount interference substan¬ ces in the water circulations of processes involving wood-based fibre suspensions by binding the interference substances to the fibres, characterised in that to the fibre suspension to be treated in the process, cationised starch in an amount of approximately 0.04 - 0.5 % of the fibre dry matter is added. 2. A method as claimed in claim 1, characterised in that a starch ether cationised with 2,3-epoxy-propyl-tri- methylammoniumchloride is used. 3. A method as claimed in claims 1 or 2, characterised in that cationised starch with a charge state of 1.5-3.5 meqv/g is used. 4. A method as claimed in any of the claims 1-3, charac¬ terised in that the cationised starch is added to the fibre suspension before the web formation stage.

Inventors: VIHERVAARA TAPIO, VIHERVAARA, TAPIO
Applicants: RAISION TEHTAAT OY AB
Application year: 1992
Main classification: D21H17
IPC codes: D21H17