In the sulphate pulping process, the fatty and resin acids derived from wood resin saponify to sodium soaps. These further form micelles, and unsaponiflable substances of resin, i. e. neutrals, dissolve to some extent in the micelles. When black liquor is cooled, the soap particles join one another and rise, as lighter than the liquor, to the surface of the black liquor tank forming crude soap. The soap is decanted and converted to crude tall oil by means of sulphuric acid and heat.

Soap is recovered from the black liquor tanks of the washing department and the evaporator. Effective separation of soap is important not only to increase tall oil yield but also to reduce the drawbacks caused by the residual soap in the process.

It has been found that pulp quality, the energy economy of the process, and the trouble-free flow of operations are endangered if separation of soap is not taken care of properly.

Separation of soap from black liquor is affected by the composition of extractives, the rising velocity of soap in the liquor, and the flows inside the liquor. The rising velocity of soap depends on the size and composition of soap particles and on the dry solids content and temperature of the liquor. All soap contained in the black liquor cannot be recovered because part of it is present as dissolved in the black liquor and part of it forms so small colloidal soap particles that they are not separated to the surface of the liquor. Soap rises to the surface when the solubility of extractives decreases, which occurs, among other things, when the temperature of the black liquor decreases and when its salt content or dry solids content

increases. From the viewpoint of separation of soap, an optimal dry solids content has been 25-28 %.

Generally, separation of soap is slow and control of the separation process is difficult. To enhance separation of soap, the size of separation tanks has been increased and the point of separation of soap in the process has been transferred to a lower temperature and to a higher dry solids content. In addition, attempts have been made to keep the flow inside the separation tank low and efforts have been made to avoid downwards directed turbulences.

Attempts have also been made to artificially enhance separation of soap by adding different organic compounds or mixtures to the black liquor and/or by using complicated separation methods. Some known arrangements are described in US patents 3, 880, 704, 3, 890, 295, 4, 022, 653 and 4, 142, 967. The use of artificial additives in separation of soap becomes expensive and makes the process complicated, which is why the known methods have not gained widespread popularity.

An object of the invention is to provide a novel arrangement for enhancing separation of soap. The aim is a method by which the fatty and resin acid molecules dissolved in the black liquor as well as the micelles formed by them can be converted quickly and effectively into a liquid crystalline phase. In the form of a liquid crystalline phase that is lighter than water, these soap particles can be caused to rise to the surface of the black liquor.

With a view to achieving the objects described above and those coming out later, the method according to the invention is characterized in what is stated in the characterizing part of claim 1.

In accordance with the invention, separation of soap is enhanced by adding a neutral substance colloid stabilized with fatty and resin acids to the black liquor,

which neutral substance colloid can be prepared, for example, from separated soap. The neutral substance can also be separated through pulp mill processes other than soap separation. Usable colloidal neutral substance can be produced in suitable process conditions, in which case it need not be prepared separately.

In this description, the concepts of fatty acid and resin acid are used in the sense which also includes the sodium soaps of fatty and resin acids.

Experimentally it has been found that even a relatively small amount of a neutral substance colloid is sufficient to convert the mixed micelles dissolved in the black liquor into a liquid crystalline form. The amount of the neutral substance colloid needed is advantageously of the order of 0.01-0. 04 % of the amount of the dissolved fatty and resin acids contained in the black liquor. The change of phase is a result of an increase in the size of the colloidal particles and of the neutral substance which is contained in them and which converts the colloid into a water insoluble liquid crystalline phase. The fatty and resin acid molecules move in the solution relatively freely from one micelle to another. When the amount of the neutral substance exceeds the critical concentration, the micelles of fatty and resin acids precipitate as colloidal liquid crystalline phase particles, which gradually join one another, whereby their size increases. A suitable composition and a suitable amount of the colloid lead to quick generation of a separating liquid crystalline phase, whereby separation of soap is quickened.

By means of the invention, soap can be separated at a higher temperature than before and/or at a lower dry solids content than before. Soap can also be caused to be separated in a desired process stage by suitable selection of the location of feeding the neutral substance colloid. The size of the soap separation apparatus can be reduced since a shorter residence time than before is sufficient for separation. Alternatively, it is possible to reduce the amount of residual soap by increasing the recovery of soap, so that there will be less disturbances caused by soap in the evaporator.

In the following, the invention is illustrated using examples and figures which are based on results obtained in laboratory tests.

Figure 1 is a graphic representation of transmission of light in black liquor to which no neutral substance colloid has been added.

Figure 2 is a graphic representation of transmission of light in black liquor to which 0.02 % of a neutral substance colloid has been added.

Figure 3 is a graphic representation of transmission of light in black liquor to which 0.04 % of a neutral substance colloid has been added.

Figure 4 is a graphic representation of transmission of light in black liquor to which 0.20 % of a neutral substance colloid has been added.

Figure 5 is a microscopic image of model substance soap separated in accordance with the method.

Separation of soap from black liquor to which various amounts of a neutral substance colloid had been added was investigated in a laboratory scale test series.

In the investigation, a TurbiScan 2000 MA device was used, by which it is possible to measure light transmission from a mixture in a test tube in a layer by layer fashion.

As model substance black liquor was used a mixture that contained 1 % of the sodium salt of fatty and resin acids in a ratio of 1: 1, and typical salts of black liquor and, in some cases, also lignin. Sodium oleate (NaOl) was used as the model substance for fatty acids and sodium abietate (NaAb) was used as the model substance for resin acids. Sodium sulphate was used as the model substance for salt and sodium hydroxide as the model substance for alkali.

For preparation of the model substance black liquor, NaOl and NaAb were dissolved in a 0.1 M NaOH solution and the obtained solution was heated to 80 °C. A salt solution or a salt containing lignin solution was also heated to 80 °C.

The hot solutions were mixed in a test tube by a vortex mixer, after which the test tube was transferred to a water bath which was at 60 °C. After that, a desired amount of the neutral substance colloid was added to the mixture and transmission measurements were started immediately after mixing had occurred. During the time between measurements, the test tube was kept in the water bath at 60 °C.

In the test series, the neutral substance colloid was used in an amount that represented 0.01 %, 0.02 %, 0.04 %, 0.10 %, 0.20 % and 0.40 % of the amount of soap contained in the model substance black liquor. In addition, a blank test was made in which no neutral substance was added. The neutral substance colloid was prepared by mixing 0.5 g of a 10 % sodium oleate solution and 0.5 g of a 10 % sodium abietate solution with each other and by adding 0.4 g of a 100 % neutral substance to the obtained solution. The neutral substance did not dissolve but formed ; when mixed, a relatively stable neutral substance colloid. The final ratio of the neutral substance to the fatty acid/resin acid solution, used as an emulsifying agent, in the neutral substance colloid was 4: 1, i. e. the proportion of the neutral substance was 80 %, the proportion of the fatty acid (NaOl) was 10 % and the proportion of the resin acid (NaAb) was 10 % of the total soap amount contained in the colloid.

The neutral substance was prepared from tall oil soap as follows. 10 g of soap was dissolved in 100 ml of petroleum ether. 100 ml of a mixture of ethanol and water (50 % ethanol), to which 1 M KOH solution had been added, was added to the mixture. The mixture was shaken and the ethanol-water phase was recovered. The recovered ethanol-water phase was washed four times with a small amount of ether. The ether layers were recovered and combined. The ether phases were

washed three times with an alkaline ethanol-water mixture and further three times with distilled water. The ether was evaporated off in a fume cupboard.

A TurbiScan 2000 MA device was used for measuring the amount of light passed through the sample in the test tube at different levels of the test tube. The measurement results are shown in Figs. 1-4 as an amount of light passed through the sample in the test tube, i. e. as transmission expressed in percent of the transmission of a reference sample (silicone oil). The horizontal axis represents the distance between the measurement point and the bottom of the test tube. The measurement was repeated at certain intervals, thereby obtaining an idea of the behaviour of the mixture as a function of time. The first measurement was performed immediately after the neutral substance colloid had been mixed with the model substance black liquor, and the last measurement was performed when more than 18 hours had passed from the addition.

Fig. 1 shows blank test results obtained by measuring the transmission of light in a model substance black liquor to which no neutral substance colloid had been added. The soap was fully dissolved all the time and light transmission was 92 % of the transmission of the reference sample. No changes occurred in light transmission during the more than 18 hours which passed between the first and the last measurement time.

When neutral substance colloid was added to the model substance black liquor in an amount that represented 0.01 % of the amount of soap contained in the model substance black liquor, the black liquor remained a clear solution, and the transmissions measured substantially corresponded to those shown in Fig. 1.

Apparently, the total amount of soap even after the addition was so small that the soap dissolved completely.

When neutral substance colloid was added to the model substance black liquor in an amount that represented 0.02 % of the amount of soap contained in the model

substance black liquor, the sample turned cloudy at first but soon became clearer, so that two clear phases were distinguishable from it. Fig. 2 shows the results of the transmission measurement immediately after the addition as well as on four subsequent occasions of measurement. The times of measurement are listed on the right-hand side of the figure. The lowermost curve represents the transmission measured at the moment at the starting moment 0: 00 and the uppermost curve represents the transmission measured at the moment 18: 15. The contents of the test tube already separated in 10 minutes into two phases, which were separated from each other by a distinct boundary surface. During the next 18 hours, the light transmission of both phases increased and the boundary surface became stronger.

The figure also gives indications that the soap layer separated to the surface of the test tube at the end phase is further divided into two different layers.

In the example of Fig. 3, neutral substance colloid was added in an amount that represented 0.04 % of the amount of soap contained in the model substance black liquor. The effect of the neutral substance colloid was similar to that in Fig. 2, but the sample now became clearer more slowly. Transmission dropped at first close to zero and was still low after 13 minutes from the colloid addition. Gradually, two clear phases were discernible in the test tube.

When neutral substance colloid was added 0.10 % of the amount of soap contained in the model substance black liquor, the effect was of the same type as that in the case of Fig. 3.

In the example of Fig. 4, neutral substance colloid was added in an amount that represented 0.20 % of the amount of soap contained in the black liquor. The neutral substance colloid immediately turned the sample cloudy and transmission did not increase significantly during the 18 hours after the addition.

When neutral substance colloid was added 0.40 % of the amount of soap contained in the model substance black liquor, the effect was of the same type as that in the case of Fig. 4.

Fig. 5 is a microscopic image of soap separated to the surface of the model substance black liquor. The figure shows a clear phase which is soap in a liquid crystalline form.

In comparative tests, corresponding amounts (0.01-0. 40 %) of a mixture of sodium abietate and sodium oleate, which did not contain any neutral substance, were added to the model substance black liquor. The tests showed that the mere addition of saponified fatty and resin acids did not cause the dissolved soap to separate from the black liquor.

The effect of the neutral substance colloid was also investigated by means of mill black liquor. The results were similar to those of the model substance black liquor. In this case, even a small added amount of the neutral substance colloid, about 0.01 % of the amount of soap dissolved in the black liquor, was sufficient to enhance separation of soap. This gives indications that the need for the addition of the neutral substance colloid is smaller in the case of the mill black liquor than in the case of the model substance black liquor.

The test results showed that the addition of the neutral substance colloid, emulsified using fatty and resin acids, to the black liquor causes soap micelles and dissolved soap to be separated more effectively than before as a separate phase to the surface of the solution. The results also showed that it is possible to optimize the amount of the neutral substance colloid to achieve the desired end result. Too small an amount of the neutral substance colloid dissolves totally in the black liquor. Too large an amount of the neutral substance remains in the black liquor as a colloid. It has been found experimentally that an optimal amount of the neutral substance colloid is of the order of 0.01-0. 04 % of the amount of soap dissolved

in the black liquor. In different conditions, the optimum amount can, of course, be different, wherefore it is not desired to limit the invention to the above-mentioned limit values.

Advantageously, the neutral substance colloid contains neutral substance (MN) separated from soap, fatty acid (MF), and resin acid (MR) in a ratio of MN : MF : MR, where MN is in a range of 7-9, MF is in a range of 0.5-1. 5, and MR is in a range of 0. 5-1. 5. One composition found to be effective contained neutral substance separated from soap, fatty acid and resin acid in a ratio of 8: 1: 1.

Besides separation of soap, the method in accordance with the invention can also be applied to separation of other surface active substances from water.

The neutral substance colloid need not be a colloid separated from soap or another process product of a pulp mill provided that it contains active compounds that are the same as or equivalent to those of the neutral substance derived from soap.

The claims are presented in the following and the various details of the invention may vary within the inventive idea defined by said claims and differ from the disclosure given above by way of example only.