FI125988B - Procedure for washing pulp - Google Patents

Description

FIELD OF THE INVENTION This invention relates to an improved process for washing a pulp such as cellulosic pulp.

Background of the Invention

The pulp production involves pulping the wood into a pulp. The quenching can be chemical or mechanical, or a combination thereof, and produces pulp and solution. A chemical cooking process, e.g., a sulphate soup, produces a pulp and a cooking solution containing cooking chemicals and wood-related materials, particularly lignin, in dissolved form. In mechanical pulping processes, such as various semi-chemical processes, the refining of wood in a refining mill is a key activity. In semi-chemical processes, such as chemimechanical (CMP) or wood-pulp (CTMP) processes, the wood to be refined is impregnated with suitable chemicals, e.g. NaOH and Na 2 SO 3.

After cooking or refining, the mass must be washed in several steps. The mass gradually moves from one degree to the next, while the wash solution flows upstream. After the first washing step, the washing solution comprising wood-related solids, any cooking chemicals or impregnating chemicals are removed as a waste solution. Cooking and impregnating chemicals are removed from the waste solution and wood-related solids are incinerated.

There are several different types of washing equipment in the prior art. In recent years, pressure washers have become commonplace. Thus, the washing process is performed by repeatedly diluting and pressing the pulp. One type of press washer is a screw press in which the press screw rotates in the cylindrical housing of the torn perforated plates through which the discharge solution is discharged.

Pure water is typically used as a washing solution in the final washing step. The solution used in this step becomes the washing solution of the previous step. This continues backwards through all the washing steps. In the preparation of bleached pulp, the spent solution is usually derived from bleaching in one of the preceding washing steps.

One problem primarily associated with the production of unbleached pulp is that excess pressure may develop at the inlets of the presses of some washing steps. At high inlet pressures, the pressure exerted on the screen surface becomes too high and, furthermore, fine fibers can be forced out through the screen together with the waste solution, resulting in undesirable fiber loss. When the inlet pressure exceeds a critical value, e.g., above 50 kPa, production must be slowed down or stopped, leading to a reduction in production and a reduction in the production capacity of the process.

Summary of the Invention

One object of the present invention is to increase the production capacity of the pulping process.

Another object of the present invention is to avoid too high an inlet pressure of the press washers.

A third object of the present invention is to improve the compressibility of the pulp without substantially affecting the degree of pulverization.

It has now surprisingly been shown that adding bicarbonate to the pulp or to the washing solution, between or before at least one washing step, eliminates the problems of increased pressures of the presses. Due to the addition of bicarbonate, the need to slow down or interrupt the process is eliminated and, as a result, the production capacity of the process increases. Further, it has been found that the compressibility of the pulp is improved by the addition of bicarbonate without substantially affecting the pulverization rate.

The inventors of the present invention have shown that the inlet pressure of the washing presses decreases when bicarbonate is added to the washing solution or pulp. As a result, production capacity can be increased. The production may be increased by at least 10% by the process of this invention.

Bicarbonate is a cheap and environmentally friendly substance commonly used in pulp mills. Thus, the washing process can be improved in a simple way without the need to purchase expensive chemicals.

Further, the bicarbonate can be added to the process without the need for a major change in the factory production equipment. The bicarbonate can be added to the pulp or to the washing solution simply by adding one additional tube to the position most suitable.

The present invention relates to a process for washing fibrous pulp, wherein the pulp is washed with the wash solution in one or more successive steps of adding the bicarbonate to the pulp and / or the wash solution. At least one washing step involves washing the pulp in a screw press because the problem of too high pressure is particularly associated with screw presses. In addition, the inlet pressure level of the at least one screw press is monitored and the amount of bicarbonate added is controlled according to the measured inlet pressure.

In one preferred embodiment of the invention, the bicarbonate is added to the wash solution at or before at least one wash step. Addition of bicarbonate to the washing solution makes mixing the bicarbonate with the pulp efficient.

In another preferred embodiment of the invention, the bicarbonate is added to the washing solution used in the final washing step, after which said washing solution is passed to the preceding washing step. Adding bicarbonate to the wash solution as late as possible in the wash process, i.e. one of the last washing steps in the wash solution, causes a rapid increase in the bicarbonate concentration in the pulp since the flow of the wash solution is generally upstream of the pulp flow. However, the addition of bicarbonate to the washing solution prior to the final washing step can in some cases lead to the loss of bicarbonate since in some processes some of the washing solution is not led to the previous washing step but to other pulp or paper process.

In yet another preferred embodiment of the invention, the bicarbonate is added to the pulp during or before one washing step, preferably immediately before the first washing step.

All pulps, e.g. CTMP, CMP, sulphate or recycled pulp can be washed by the process of the invention. The method of the invention is particularly effective in washing unbleached pulp. Thus, in one preferred embodiment, the pulp treated by the process of the invention is unbleached.

The bicarbonate is preferably added to the pulp or the washing solution in the form of a bicarbonate salt, such as an alkali metal salt, most preferably sodium bicarbonate. Sodium bicarbonate is a cheap and environmentally friendly compound commonly used in pulp mills.

Preferably, the bicarbonate is added to the wash solution in an amount that maintains the inlet pressure of the at least one screw press below a predetermined value, e.g., 50 kPa. By keeping the pressure below a predetermined critical value, problems associated with excessively high inlet pressure of the screw presses can be avoided.

Suitably, the sodium carbonate is added to the wash solution in an amount of 0.1 to 10 kg / tonne, preferably 2-8 kg / tonne, most preferably 4-6 kg / tonne, e.g. 5 kg / tonne.

Brief Description of the Drawings

Fig. 1 is a schematic view of a plant washing pulp in two successive steps.

Figure 2 is a schematic view of an example plant of the present invention.

Figure 3 shows how the inlet pressure of a single jet press varies with the biocarbonate level of the wash solution.

Figure 4 shows how the pulverization rate varies with the increased bicarbonate level in the wash solution.

Detailed Description of the Invention

The principle of operation is schematically illustrated in Figure 1. For simplicity, only two washing steps are shown. However, one of ordinary skill in the art will recognize that the process of the invention can be used in a number of washing steps arranged sequentially and / or in parallel. Further, it will be apparent to those skilled in the art that additional screening steps and / or reject refinement may be performed between successive wash steps.

Referring to Figure 1, which illustrates an embodiment of the invention, reference numerals 1-2 refer to two successive washing steps. The washable mass, eg refined or boiled mass, is led to the first washing step (1). The pulp and wash solution are led upstream through the washing steps. Pure water is usually used as a washing solution in the final washing step (2). The spent solution from the final washing step (2) is collected in the collecting tank (3) and is further used as the washing solution in the first washing step (1). The last solution used is recovered from the first washing step (3).

The bicarbonate may be added to the pulp or wash solution at one or more sites (a-g) where (f) or (g) are most preferred as these sites involve efficient mixing and rapid increase in bicarbonate content in the pulp.

The bicarbonate may be added to the pulp at one or more of the following points: (a) before adding the washing solution, before or immediately before the first washing step (1), (b) in the first washing step (1), after adding the washing solution, (c) between two washing steps; in the second washing step, after adding the washing solution.

The bicarbonate may be added to the wash solution at one or more of the following: (e) before it is added to the final wash step (2), (f) after one wash step, before it is added to the preceding wash step, (g) after one wash step when it is in the hopper (3).

Figure 2 shows a washing arrangement of the second embodiment in which the pulp is washed in three successive stages (1, 2 and 3). The washable mass is directed to the first washing step, where it is washed with a washing solution, e.g. water. After the first washing step (1), the pulp is diluted to the desired pulp concentration in the first pulp collecting tank (6) using a second washing step washing solution. The pulp is then screened (7) to remove impurities such as sticks, bark or sand from the pulp. The reject obtained from the screening (7) is conveyed to the reject treatment (9), where the sticks and the like are further processed and returned to the pulp container (6) and further directed to the screen (7). The accept of the screening (7) is directed to the second pulp collecting container (8), after which it is washed in a washing step comprising two parallel screw presses (2a and 2b). Finally, the pulp is washed in the final third washing step (3) and then transported to a board or paper machine.

Pure water is usually used as a washing solution in the final washing step (3). The washing solution used in the final washing step is directed to the washing solution collecting tank (4) and then used as the washing solution in the adjacent second washing steps (2a and 2b). The washing solution of the washing steps (2a and 2b) is directed to the first pulp collecting container (6) where it is used as a dilution solution.

The bicarbonate may be added to the pulp or to the washing solution as it is introduced into, between or before any of the washing steps shown in Figure 2. Most preferably, the bicarbonate is added to the washing solution in a collecting tank (4) which is directed to the adjacent washing steps (2a and 2b). Addition of bicarbonate to the washing solution in the collecting tank ensures a rapid increase in the bicarbonate content and effective mixing of the bicarbonate with the pulp.

Example

In industrial full-scale experiments using a scrubbing arrangement similar to the system depicted in Figure 2, the CTMP pulp was washed in three countercurrent steps comprising screw presses.

To rapidly increase the bicarbonate level, the experiment was started by adding about 1000 liters of a 5% NaHCO 3 solution to the collecting vessel (4) and the pulp collecting vessel (8), respectively. Thereafter, 3.5 kg / tonne of NaHCO3 was added to the wash solution in the collection tank (4), which was passed to the preceding wash step (2a, 2b).

During the experiment, the bicarbonate level of the wash solution in wash steps 2a and 2b was determined by residual carbonate analyzes and Total Inorganic Carbonate (TIC) analyzes. Further, the inlet pressure of the screw presses (2a) was continuously measured.

A slight increase in pH was observed during the addition of bicarbonate. The initial pH of the washing solution of wash step 3 was less than 7. During the addition of bicarbonate, the pH was raised to about 7.6-7.8.

It was found that the inlet pressure of the screw presses (2a) decreased as the bicarbonate content of the washing solution in the washing step (2a and 2b) increased. When NaHCO 3 addition was stopped, the inlet pressure of the presses began to increase.

Table 1 shows the measured inlet pressure at the press (2a) at different carbonate levels of the washing solution of the washing steps (2a and 2b). Figure 3 shows the correlation between the inlet pressure measured by the press (2a) and the measured carbonate level of the washing solutions of the washing steps (2a and 2b). As can be seen in both Table 1 and the figure, the inlet pressure decreases as the bicarbonate level in the wash solution increases.

table 1

The production rate was maximized before the experiment. At a production rate of about 35 t / h, the inlet pressure of the screw press (3) increased above the critical value (50 kPa). With the addition of bicarbonate, the inlet pressure decreased and as a result, the production rate could always be increased to 38.5 t / h. At a production rate of 38.5 t / h, feed rate of chips for pre-impregnation and washing of chips limited the production rate. Thus, the bottleneck shifted from pulp washing to chips feed.

Thus, the addition of bicarbonate to at least one washing stage allowed an increase in the production capacity of at least 3.5 t / h, i.e. the production capacity could be increased by at least 10%.

Figure 4 shows the measured pulverization rate of samples taken after the pulp type (8) at different bicarbonate levels. As can be seen from the figure, the addition of bicarbonate does not affect the pulverization rate. Thus, the compressibility of the pulp and the capacity of the process can be improved without substantially affecting the pulverization rate.

Claims (13)

A method for washing a pulp, wherein the pulp is washed with the washing solution in one or more successive steps, characterized in that the bicarbonate is added to the pulp and / or the washing solution, the at least based. Process according to Claim 1, characterized in that the bicarbonate is added to the washing solution during or before at least one washing step. Process according to Claim 1, characterized in that the bicarbonate is added to the pulp during or before at least one washing step. Process according to Claim 1, characterized in that the bicarbonate is added to the pulp immediately before the first washing step. Method according to Claim 1, characterized in that the flow of the washing solution is upstream of the mass flow. A process according to claim 2, characterized in that the bicarbonate is added to the used washing solution from the final washing step, whereupon said washing solution is directed to the preceding wash. Method according to one of the preceding claims, characterized in that the pulp is unbleached. Process according to any one of the preceding claims, characterized in that the bicarbonate is in the form of a salt. Process according to Claim 8, characterized in that the bicarbonate is in the form of an alkali metal salt. Process according to claim 9, characterized in that the bicarbonate salt is sodium bicarbonate. Process according to Claim 10, characterized in that the sodium bicarbonate is added to the wash solution in an amount of 0.1 to 10 kg / tonne, preferably 1-5 kg / tonne, most preferably 2-4 kg / tonne. Method according to one of the preceding claims, characterized in that sodium bicarbonate is added to the wash solution in an amount sufficient to maintain the inlet pressure of at least one screw press below 50 kPa. A method according to any one of the preceding claims, characterized in that the inlet pressure level of the at least one screw press is monitored and the amount of added bicarbonate is controlled based on the measured inlet pressure.