Method of separating impurities from lime and lime sludge

Details Method of separating impurities from lime and lime sludge Method of separating impurities from lime and lime sludge

1998-01-08

2000-06-13

Nguyen, Dean T.

Paper making and fiber liberation

Processes of chemical liberation, recovery or purification...

With regeneration, reclamation, reuse, recycling or...

162 301, 423637, 423640, D21C 1100

Patent

active

060745215

DESCRIPTION:

BRIEF SUMMARY
BACKGROUND AND SUMMARY OF THE INVENTION

The invention relates to a method of separating impurities in connection with the chemical recovery system of a pulp mill, in particular from the calcium-containing substances used therein, which are mainly lime sludge and lime, and from its liquor circulation.
The term impurities is used in the present patent application to denote various detrimental substances such as silicon, phosphor, vanadium, sulphur, etc., which may also be called non-process elements, building up in the chemical circulation of a pulp manufacture process and originating from various raw material sources. Sources of impurities are mainly wood raw material (phosphor), make-up lime (silicon) and combustion oil (vanadium, sulphur) if oil is used as fuel in the lime sludge reburning kiln. Some raw materials used in the manufacture of cellulose pulp also may contain in their cells lots of silica (SiO.sub.2). The most usual of these are annual plants such as bamboo, sugar cane, rice and wheat. It has also been discovered that some tropical wood species contain such amounts of silicon that are detrimental for pulp production processes.
When pulp is manufactured using the sulphate method the cooking liquor used, or black liquor, is evaporated, combusted and the residue obtained, the so-called soda smelt, is dissolved in water. The green liquor thus formed is causticized to produce cooking liquor. During the causticizing process, the sodium carbonate contained in the green liquor is allowed to react with burnt lime (CaO) as follows. First, lime slaking takes place:
Subsequently, the causticizing reaction itself takes place:
The mixture thus produced contains sodium hydroxide (white liquor) and calcium carbonate (lime sludge) is separated and the white liquor is recirculated to pulp digestion. White liquor and lime sludge may be separated either by settling or filtering. White liquor filters are usually conventional candle filters or drum or disc filters operating with pressure or vacuum. Lime sludge is washed in order to remove alkali therefrom. Lime sludge is thickened in a lime sludge filter for combustion in a calcinating device such as lime sludge reburning kiln in which the lime sludge is regenerated to calcium oxide to be returned to the causticizing. This circulation process of lime is called the lime cycle.
In the chemical recovery system of a pulp mill the material flow described above is partly cleaned as part of the soda smelt does not dissolve in the soda dissolving stage and the so-called green liquor sludge formed is removed. Undesirable substances may thus be removed from the process in this way.
The problem in the process is that certain impurities are not easily removed because they are not sparingly soluble enough and thus to a large extent remain in the solution. For example phosphor, silicon and vanadium compounds as well as many other compound are this kind of substances. These compounds tend to precipitate during causticizing in the lime sludge and thus build up in the lime cycle. For example silicates precipitate as calcium hydrosilicates to the lime used in the causticizing process. In this way, impurities are accumulated with time in the lime sludge which means that the lime sludge is contaminated. Phosphor tends to concentrate in the fine dust of the lime sludge reburning kiln and it has been desirable to try to remove it in this form. Fine dust has sometimes been used as phosphor-containing soil conditioner.
Contamination of lime sludge results in various problems, for example deposit rings and balls accumulate in the lime sludge reburning kiln, filtration of white liquor becomes difficult and burnt lime passivates, i.e. it is bound in non-reactive compounds. For this reason, at least a part of the lime sludge must be replaced from time to time, i.e. a part of the lime sludge must be removed continuously and the lime cycle must thus be kept open to some extent.
A further problem is that the lime sludge discharged must either be transported to a landfill area or used to neutra

REFERENCES:
patent: 4302281 (1981-11-01), Ryham
patent: 5607548 (1997-03-01), Pettersson
patent: 5607549 (1997-03-01), Koskiniemi

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