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Efficient and closed chemical recovery is a great benefit in the kraft process. It provides efficient recirculation of cooking chemicals inside the process while using only marginal amounts of makeup chemicals. Black liquor is separated from the pulp during the pulp washing. This weak black liquor contains too much water for direct use as fuel in the recovery boiler.
Reliable valves and services for chemical pulping
The main purpose of an evaporation plant is to increase the dry solids content of the black liquor by evaporating water until reaching a concentration that allows it to be burned in the recovery boiler. The recausticizing stage completes the chemical recovery cycle, converting green liquor to white liquor.
Handling of black liquor and lime mud
Black liquor gets more viscous and corrosive after each effect due to an increase in solids content. This can cause scaling and corrosion problems in valves. Valves must be able to perform in both control and shut-off applications.
Careful valve selection is recommended for the last part of the evaporation plant and in pipes feeding the subsequent processes. When used in shut-off services, the valves must provide tight closure. There is also a large variety of valve sizes due to the steam used in the process, which increases the cost of the valves in the process significantly.
Recausticizing is a very demanding process because of high erosion and possible valve scaling.
Right material selections
For black liquor with high solids content, ball and segment valves are preferred. For control services, the primary recommendation is the segment valve as well as the ball valve. For lower solids content, a butterfly valve is recommended. Butterfly valves are also preferred in large pipe size steam applications. When solids content rises above 75%, Metso recommends using 4A duplex as the valve material.
Raw white liquor and white liquor dregs require the use of ball valves with rugged metal seats and a scraping and locking mechanism, like the Metso ball valve. Lime mud is a very erosive flow medium. For such applications, our ceramic ball valve offers the best resistance to erosion.
Long life time and reliable operation
- Ball valves do not interrupt the flow as much as butterfly valves; therefore, they are preferred for use in viscous black liquor applications with high solids content
- For control services, more accurate flow control can be obtained by selecting a segment valve
- Butterfly valves are a cost-efficient way to handle low solids content, less viscous black liquor and large pipe size steam applications
- Due to the erosiveness of black liquor in high solids contents, normal stainless steel CF8M is usually not sufficient; with 4A duplex material, the valve can prevent corrosion and erosion
- Due to the erosiveness of lime mud, normal stainless steel CF8M is usually not sufficient; with ceramic material, the valve can prevent corrosion and erosion