HARDWOOD BLACK LIQUOR TREATED ACCORDING TO THE

PROCESS

Field of invention

The present invention relates to a process for treating a hardwood black liquor in order to prevent undesired fouling. The present invention also relates to a hardwood black liquor treated according to the invention.

Background

In the papermaking industry, wood chips are treated in a digester system to separate the cellulose fibers and to remove desired amounts of lignin etc, which binds the fibers together in the natural state of wood, for the production of paper pulp. Digestion of wood chips using an alkaline liquor is a common practice in the industry. In this process, commonly wood chips and an alkaline digesting liquor are introduced to a digester. After the digestions process the produced pulp and the digestion black liquor are separated to be further treated separately. The digestion black liquor at this point is withdrawn from the digester and then further treated in order to recover usable

components and/or to make use of the energy of the liquor, preferably by burning it in a recovery boiler.

The black liquor is evaporated to a higher dry matter content in a multi- stage evaporation plant in order to increase the efficiency of subsequent recovery steps. A common problem in evaporation plants is fouling of the heat transfer surfaces which decreases the efficiency of the heat transfer. This leads to loss in productivity and higher operation costs. In order for the evaporation plants to run satisfactory the heat surfaces needs to be cleaned from time to time.

Calcium carbonate has been shown to be a key component of fouling on surfaces in the evaporation plant. Fouling tendency of calcium in cooking and/or black liquor has been shown to decrease dramatically after the liquor has been heated at or near typical full cooking temperatures. This action is, at times, referred to in the art as calcium deactivation by heat treatment. An example of such calcium deactivation is described in EP 0313730 A1 which describes heating of cooking liquor high in calcium at or near full cooking temperature, holding it at this temperature in a vessel for a period of time, typically longer than ten minutes, and returning the heat treated liquor, with "deactivated" calcium, to a digester system. Because foul forms on the surfaces of this "sacrificial" vessel, generally at least two vessels are needed in order to maintain continuous operation of calcium deactivation, with at least one vessel being online and one vessel being cleaned of fouls. This technology is probably effective, but requires addition capital and operating costs, and therefore is not widely practiced in the industry. Another example of treatment of a black liquor in order to inhibit fouling is described in US 6,090,240 which describes heat treatment of a black liquor at a temperature of 1 10-145°C for a period of 1 -20 minutes in order to reduce the amount of calcium in the liquor by precipitating the calcium of the liquor. In this patent black liquor from digestion of softwood is treated.

Further, treatment of a black liquor is described in Frederick et. al., "Preventing Calcium Carbonate Scaling in Black Liquor Evaporators", Part 2 of 2, Southern Pulp and Paper Manufacturer/Sep. 1979, pp.22-29. The treatment is performed at 151 °C.

The market and use of hardwood pulp is increasing due to its superior cost efficiency and availability in many countries. As a consequence the production of hardwood black liquors are also increasing and thus also the need for recovery of the components and energy of these liquors in an efficient way. One problem with the recovery of hardwood black liquors is the high calcium content in some hardwood species resulting in high calcium content in the black liquor resulting in increased fouling in the evaporation plant. Furthermore, the usual heat treatment at temperatures of 1 10-145°C, as described in prior art, does not reduce the amount of fouling when hardwood black liquors are treated. Moreover, digestion of hardwood pulp is normally done at lower cooking temperatures compared to digestion of softwood pulp. As a result of the lower cooking temperature the amount of dissolved calcium in the black liquor remains high. During digestion of softwood pulp most of the calcium will deactivate and the amount of soluble calcium in the black liquor is thus reduced. Cooking temperatures during digestion of hardwood pulp is kept low since higher cooking temperatures, e.g. temperatures above 155°C, will decrease the yield during cooking.

There is thus a need for an improved process for treatment of hardwood black liquors in order to reduce the problems with fouling of calcium compounds, such as calcium carbonate. Summary of the invention

It is an object of the present invention to provide a process for reducing the fouling problems during evaporation of hardwood black liquors.

Another object is of the present invention is to provide a black liquor with reduced fouling tendency.

Yet another object of the present invention is to provide a black liquor with reduced viscosity. These objects, as well as other objects and advantages, are achieved by the process according to claim 1 . The present invention relates to a process for treating a hardwood black liquor from the manufacturing of hardwood pulp in order to precipitate calcium ions present in the liquor wherein the process comprises the steps of; providing a hardwood black liquor from the manufacturing of hardwood pulp and treating at least one part of said liquor at a temperature of above 160°C wherein the calcium ions present in the black liquor will precipitate. In order to inhibit fouling of calcium components, such as calcium carbonate, on unwanted surfaces, such as heating transfer surfaces in evaporation vessels, when recovering hardwood black liquors it has been found that treatment at temperatures exceeding 160°C will precipitate the calcium ions in the liquor in an easy and very efficient way, making it possible to control where the precipitation occurs and thus prevent undesired fouling e.g. on heating surfaces in evaporation vessels.

The hardwood used for the manufacturing of pulp is preferably eucalyptus, i.e. the black liquor is thus an eucalyptus black liquor. It has been found that eucalyptus black liquor comprises a large amount of free calcium ions, mainly due to the low cooking temperature but some eucalyptus species also comprises an increased amount of calcium. Consequently, the problem with undesired fouling is increased. By treating an eucalyptus black liquor at temperatures exceeding 160°C it was found that the calcium ions precipitated and it is thus possible to control the location for the precipitation and thus prevent or even eliminate fouling on undesired surfaces. The hardwood is further preferably a plantation based wood.

The temperature during the treatment is preferably between 160- 190°C, more preferably above 165°C and also then preferred between 165- 185°C, and even more preferred between 170-180°C. Too low temperatures will not precipitate the calcium in a satisfactory way and too high

temperatures demand a lot of energy and it is thus not cost efficient. The treatment preferably lasts for less than 60 minutes, preferably below 40 minutes. The time needed depends on the black liquor being treated and on the temperature used.

The temperature is preferably increased by direct contact with a heating medium, preferably steam or vapor. Suitable steam or vapor may be low-pressure fresh steam, medium-pressure fresh steam, high-pressure fresh steam or secondary vapor from the first evaporation stage. The treatment is preferably done in a retention vessel, for example a separate vessel, a bottom part of an evaporator vessel or a separate section in an evaporation vessel. With this arrangement there will be no risk of fouling of heat surfaces in the evaporation vessel. The heating medium may be added prior to the retention vessel, for example to a separate condenser located before the retention vessel or injected to a pipe located before the retention vessel. It is also possible to add the heating medium directly into the retention vessel. It is preferred to treat 20-100% of the provided black liquor at a temperature above 160°C. It may not be necessary to treat the entire black liquor stream at increased temperatures according to the invention. In some cases it might be sufficient to treat a smaller part of the black liquor and still be able to reduce the fouling tendencies.

The dry content of the black liquor to be treated is preferably between 30-45%.

The present invention further relates to a black hardwood liquor treated according to the process described above. By treating a hardwood black liquor at temperatures exceeding 160°C the calcium ions in the liquor will precipitate and form calcium carbonate crystals. It has also been shown that the viscosity of the treated hardwood black liquor is reduced at a given dry content which facilitates further treatment and handling.

Detailed description of the invention

During recovery of black liquor from the manufacturing of hardwood pulp, e.g. digestion of hardwood chips, there is a problem with undesired fouling on the surfaces of the recovery vessels, e.g. heat transfer surfaces of evaporation vessels. When the hardwood black liquor was treated according to prior art, e.g. at an increased temperature of below 145°C, no precipitation of calcium ions occurred, i.e. the calcium was still in dissolved form. It has now been found that it is possible to precipitate the calcium ions of hardwood black liquor by treating the liquor at higher temperatures, i.e. at temperatures above 160°C.

The hardwood pulp is preferably hardwood kraft pulp. However, other pulps, such as pulps from sulfur free pulping processes may also be used.

It may not be necessary to heat treat the entire stream of black liquor from the pulping process of hardwood pulp. It may be sufficient to only treat one part of the black liquor and still be able to reduce the fouling tendencies. It is preferred to treat 20-100%, more preferably between 30-90% of the black liquor withdrawn from the pulping process. In some cases it is necessary to treat the entire stream, i.e. 100% of the black liquor, at temperatures exceeding 160°C. It has also been shown that the hardwood black liquor being treated according to the process of the invention will have decreased viscosity at a given dry content compared to hardwood black liquors not being treated at temperatures exceeding 160°C. This is advantageous since any subsequent treatments of the liquor are facilitated, e.g. pumping of the liquor etc.

Treatment of liquids with high viscosity is always more difficult so there is always a desire to reduce the viscosity.

The hardwood used for the manufacturing of hardwood pulp is preferably eucalyptus. It has been shown that eucalyptus black liquors comprise large amounts of calcium ions. It was found that temperatures exceeding 160°C is necessary in order for the free calcium ions to precipitate. However, black liquor from digestion of other hardwood species such as birch, aspen, acacia or poplar can also be treated according to the process of the invention.

The temperature during the treatment is increased to above 160°C. It is preferred that the temperature is between 160-190°C, more preferred above 165°C and thus also preferred between 165-185°C, more preferred between 170-180°C. Temperatures below or at 150°C have not been found to be able to precipitate any free calcium ions in a hardwood liquor in a satisfactory way. Too high temperature, e.g. temperatures above 190°C, is also not favorable since it requires a lot of energy to heat the liquor too much and it is thus not cost efficient. It is preferred that steam or vapor or a mixture of these is used in order to increase the temperature of the liquor. Depending on the temperature of the liquor before the treatment and on the temperature needed in order to precipitate the calcium ions of the liquor to be treated steam with different pressure can be used. If it is sufficient to use low pressure steam it is of course the most energy efficient choice, however, since the temperatures needed are relatively high it most likely necessary to use medium or high pressure steam. Suitable steam or vapor may be low-pressure fresh steam, medium-pressure fresh steam, high-pressure fresh steam or secondary vapor from the first evaporation stage

It is preferred to reduce the temperature of the black liquor after the treatment by flashing in one or several steps. Flashing will generate steam that can be re-used and thereby minimize overall energy consumption.

Another option to reduce the temperature of the black liquor after treatment is by cooling. Cooling will generate warm water which could be of interest for instance for a mill producing district heating.

It is preferred to keep the treatment time as short as possible. It has been shown that a time of less than 60 minutes is sufficient in order to precipitate the calcium ions in the form of calcium compounds, such as calcium carbonate, of the liquor. It is preferred that the treatment time is below 40 minutes, preferably between 5-30 minutes. The treatment may also last for less than about two hours. It is preferred that the treatment is done in a retention vessel for example a separate vessel, a bottom part of an evaporator vessel, a separate section in an evaporation vessel or in a separate condenser located before the retention vessel. The retention vessel does not comprise any heat transferring surfaces that the precipitated calcium can cause fouling on. The treatment is typically made before the second or the third effect in a seven stage counter-current evaporation plant. The purpose of the present invention is to make sure that the precipitation of calcium does not take place on e.g. the heat transferring surfaces of an evaporation vessel, i.e. to control the precipitation in order to prevent unwanted fouling.

The calcium will predominantly precipitate in the form of calcium carbonate. Once the calcium ions have precipitated they are no longer causing any problems with fouling and the precipitated calcium will continue together with the liquor to further treatment in the evaporation plant

The dry content of the hardwood black liquor to be treated is chosen so that it is not too high since calcium carbonate will precipitate at higher dry solid contents. If the dry content is increased to above the limit for

precipitation of calcium carbonate it is not possible to control where the precipitation occurs. Treatment of hardwood black liquor with too low dry content is not economically beneficial since it requires more energy to heat the liquor to the required temperatures. It may be preferred that the hardwood black liquor to be treated according to the invention has a dry content of 30- 45%.

It can be expected that the deactivation process as set out above (and also reflected by claim 1 ) may be different depending on the different raw material used in the pulping process since the raw material may be different in chemical composition. The required treatment temperature of the black liquor must though exceed the design cooking temperature but not be too high to negatively affect the energy balance of the mill. Suitable liquor treating temperature range according to this invention is as set out above 160-190°C, and preferably 165-185°C. In the tests of the example part below, eucalyptus was used as raw material but it is expected that similar results would be achieved with other hardwoods e.g. aspen, poplar, acacia and birch. Preferred features of each aspect of the invention are as for each of the other aspects mutatis mutandis. The prior art documents mentioned herein are incorporated to the fullest extent permitted by law. The invention is further described in the following examples which do not limit the scope of the invention in any way. Embodiments of the present invention are described as mentioned in more detail with the aid of examples of embodiments the only purpose of which is to illustrate the invention and are in no way intended to limit its extent.

Example 1

Black liquor from eucalyptus kraft cooking process was evaporated to 34.5 % dry solids content and treated at temperatures of 140°C, 170°C and 180°C for a period of 30 minutes.

Table 1 below shows the results of measurement of soluble calcium in the black liquor after heat treatment according to the invention.

The calcium content was measured with inductively coupled plasma optical emission spectrometry (ICP-AES).

Table 1 : Comparison of soluble calcium in untreated black liquor as reference and soluble calcium in black liquors treated for 30 minutes at different temperatures.

Soluble calcium content

(mg/kg dry solids)

Untreated black liquor (ref) 960

Black liquor treated 30 minutes at 140°C 940

Black liquor treated 30 minutes at 170°C 800

Black liquor treated 30 minutes at 180°C 460 A can be seen from Table 1 , the amount of soluble calcium is strongly reduced after a treatment at 180°C for period of 30 minutes. It can also be seen from Table 1 that the amount of soluble calcium is unaffected after treatment at 140°C for period of 30 minutes and that treatment at 170°C is able to reduce the amount of soluble calcium.

Example 2

Black liquor from eucalyptus kraft cooking process was evaporated to 34.5 % dry solids content and treated at 180°C for periods of 5, 10, 20 and 30 minutes.

Table 2 below shows the results of measurement of soluble calcium in the black liquor after heat treatment according to the invention. The calcium content was measured with inductively coupled plasma optical emission spectrometry (ICP-AES).

Table 2: Comparison of soluble calcium in untreated black liquor as reference and soluble calcium in black liquors treated at 180°C for different periods of time.

As can be seen from Table 2, the amount of soluble calcium is reduced after a treatment at 180°C. The effective treatment time is leveling off after 20 minutes, i.e. it is not necessary to treat this eucalyptus black liquor for more than 20 minutes.

Example 3

The black liquors in example 3 were based on Eucalyptus wherein the results are reflected in table 3 and 4. Table 3 shows the effect of heat treatment at 150°C. Table 4 shows that treatment at 180°C during 2 hours gave a drastic effect of dissolved calcium. Table 3:

Comparison of soluble calcium in untreated black liquor as reference and soluble calcium in black liquors treated at 150°C for different periods of time.

Table 4:

Comparison of soluble calcium in untreated black liquor as reference and soluble calcium in black liquors treated 2 hours at 180°C.

Soluble calcium

content

(mg/kg dry solids)

Untreated black liquor (ref 3) 790

Black liquor treated 2 hours at 180°C 50 In view of the above detailed description of the present invention, other modifications and variations will become apparent to those skilled in the art. However, it should be apparent that such other modifications and variations may be effected without departing from the spirit and scope of the invention.