Paper making and fiber liberation – Processes of chemical liberation – recovery or purification... – With testing – sampling or analyzing
Patent
1994-09-29
1996-12-10
Czaja, Donald E.
Paper making and fiber liberation
Processes of chemical liberation, recovery or purification...
With testing, sampling or analyzing
162 61, 162 62, 162238, 436120, 436123, D21C 712
Patent
active
055826849
DESCRIPTION:
BRIEF SUMMARY
The present invention relates generally to a method of determining the sulfide concentration and optionally also the polysulfide concentration of waste liquors and smelt solutions obtained in the manufacture of sulfate pulp. The invention particularly relates to the application of this method in determining the degree of reduction in green liquor and in smelt that derives from liquor combustion processes.
When burning sulfate waste liquor in soda recovery units with the intention of recovering the liquor, one of the main objects of the process is to reduce sulfur to sulfide. The degree of reduction indicates how much of the sulfur present in the waste liquor or in the smelt exists in the form of sulfide. This constitutes an important measurement of the quality of the waste liquor, since it is the sulfide form of sulfur that is active in the pulp cooking process, whereas sulfur in any other form can be considered to be a useless ballast material.
The white liquor used in pulp cooking processes is produced by causticizing green liquor and isolating the calcium carbonate (mesa) formed. The green liquor is produced by dissolving the smelt from the furnace hearth in a weak liquor obtained when washing pulp after the cooking process. It is thus the sulfide content of the smelt which is significant to the sulfide content of the white liquor, and therewith its quality. Consequently, it is important to be able to determine the degree of reduction in the smelt and therewith to effect this determination quickly and to obtain accurate and well-reproducible measuring results.
Hitherto, samples of the green liquor are taken manually several times over a calendar day, when determining the reduction degree of the smelt. These samples are then analyzed to determine the concentrations of different sulfur-containing compounds with the aid of a number of titration processes, from which the reduction degree can then be determined. This method is complicated and slow. In recent times, automatic titration apparatus have been developed which facilitate the task of determining these concentrations.
Automatic titrating apparatus, however, are encumbered with a number of drawbacks. For instance, such apparatus are relatively slow and it is scarcely possible to increase the measuring frequency above a few times per hour. Handling of liquids is difficult to effect during the titration processes, and requires a high degree of precision. Furthermore, the requisite determination of sulfate concentrations requires the use of conditioning chemicals, which is not entirely acceptable from the aspect of health.
The aforesaid drawbacks are eliminated by the present invention. According to the invention, the concentrations of sulfide and optionally also polysulfide in liquors and solutions of smelt deriving from the sulfate process are determined by measuring the light absorption of the liquor or the solution in the ultraviolet range (UV-absorption assaying). According to one preferred embodiment of the invention, the degree of reduction in sulfate liquor is determined by measuring the liquor concentrations of sulfide, sulfate, thiosulfate and optionally sulfite and polysulfide and/or carbonate and calculating the reduction degree from the determined concentrations, wherein the concentrations of sulfate, thiosulfate and optionally sulfite and/or carbonate are determined by measuring the light absorption in the infrared range (IR-absorption assaying), and the concentration of sulfide and optionally polysulfide is/are determined by measuring the light absorption in the ultraviolet range (UV-absorption assaying).
The smelt produced in the soda recovery unit or boiler includes three forms of sulfur, namely sulfide, polysulfide and sulfate. The polysulfide concentration is normally negligible. When dissolving the smelt to form green liquor, a minor part of the sulfide is oxidized, to form primarily thiosulfate. The sulfate, on the other hand, is not affected. A minor quantity of sulfite may also be present. The degree of reduction R can now be defin
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Holmqvist Richard
Jonsson Torbjorn
Czaja Donald E.
Nguyen Dean T.
STFI
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