Paper making and fiber liberation – Processes of chemical liberation – recovery or purification... – With regeneration – reclamation – reuse – recycling or...
Patent
1997-04-11
1999-06-15
Chin, Peter
Paper making and fiber liberation
Processes of chemical liberation, recovery or purification...
With regeneration, reclamation, reuse, recycling or...
162 3011, D21C 1100
Patent
active
059118547
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND TO THE INVENTION
In the cellulose industry, paper pulp is produced by cooking wood chips with cooking liquors. After the cooking, the requirement is to recover the chemicals and energy in the spent liquors by means of combustion. This takes place following evaporation in recovery boilers.
As a consequence of more stringent environmental restrictions, an ever greater amount of the material which was previously discharged from pulp mills is now recirculated. This results in increased contents of non-process elements such as chloride and potassium.
In addition to this, it is becoming ever more important to keep the contents of chloride and potassium in the chemical cycle at a low level in order to ensure trouble-free operation in association with high steam values.
For these reasons, there is a growing need for effective methods of removing chloride and potassium from the chemical cycles employed in the cellulose industry.
STATE OF THE ART
A suitable way of removing the undesirable substances from the chemical cycle is to purify the electric precipitator dust from the recovery boilers before it is returned to the black liquor, since this ash is enriched in chloride and potassium. The methods which are currently employed for expelling chloride and potassium are therefore based on removing chloride and potassium from the electric precipitator ash by means of leaching, filtering and washing the filter cake.
U.S. Pat. No. 3.833.462 discloses a process for purifying electric precipitator ash of sodium chloride by means of leaching, filtering and washing the filter cake, with hot water mixed with recirculated filtrate being used for the leaching. In this instance, the leaching is carried out using a special vessel, after adjusting the pH to produce a weakly acidic solution in order to convert carbonate to sulfate. After the leaching, purified ash is separated off.
An ash of this nature which is to be purified typically has a composition which is relatively low in chloride and potassium. This is due to the fact that low contents are necessary subsequently in order to avoid disturbances at high steam values. An ash which is to be leached can typically have the following composition:
______________________________________ Component % by weight
______________________________________
Na.sub.2 SO.sub.4
86
Na.sub.2 CO.sub.3
5
NaCl 2
KCl 7
______________________________________
potassium.
The quantity which is removed together with the filtrate, which is conducted to the discharge point, corresponds to the net quantity which is supplied to the chemical cycle.
It has been found in leaching and evaporation experiments that it is relatively easy to achieve a solution which is saturated with respect to sodium sulfate and sodium carbonate. However, it is not possible to achieve a saturated solution when electric precipitator ash is being purified continuously in a mill in which it is desired to maintain a very low content of chloride and potassium using a conventional process. When the content of the undesirable substances in the ash is low, the selectivity for expelling these substances becomes very poor.
As a result of the poor selectivity which is achieved when leaching ash containing low contents of chloride and potassium, the aim is to leach using a solution in which the concentrations of all the components are as high as possible. However, the upper limit is set by the pumpability of the slurry which is formed during the leaching.
The difficulty in handling the slurry also results in preference being given to continuously operating apparatus. A batchwise mode of operation leads to a risk of the slurry pump and pipe conduits becoming blocked with incrustations.
However, a continuous mode of operation suffers from some disadvantages. The greatest disadvantage is probably the fact that it is impossible to prevent some of the ash passing through the leaching vessel so rapidly that its content of chloride and potassium does not have time to dissolve out. Another disadvantage is that it is difficult t
REFERENCES:
patent: 3833462 (1974-09-01), Moy et al.
patent: 4929355 (1990-05-01), Ragnegard et al.
patent: 5352332 (1994-10-01), Maples et al.
Enzykopadie der technischen Chemie, pp. 793-796, 801-802, 1929.
Ullmanns Encyklopadie der technischen Chemie, pp. 156-158, 178-181, 1972.
Chin Peter
Fasth Rolf
Kvaerner Pulping AB
Nguyen Dean T.
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