Chemistry of inorganic compounds – Halogen or compound thereof – Chlorine dioxide
Patent
1991-05-22
1992-09-08
Straub, Gary P.
Chemistry of inorganic compounds
Halogen or compound thereof
Chlorine dioxide
423478, 25218721, C01B 1102
Patent
active
051456605
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
Chlorine dioxide is presently to a large extent used as a bleaching agent for production of fully bleached chemical pulps. In the 1940s it was discovered that chlorine dioxide had excellent ability to remove lighting from cellulosic fibres without noticeably effecting the cellulosic fibre itself. During this period chlorine and sodium hypochlorite were used as main bleaching and delignification agents. The drawbacks with these chemicals were that during the bleaching and delignification step they also attacked and ruptured the cellulose molecule resulting in a weak fibre and ultimately weak paper products.
A combination of chlorine and chlorine dioxide later turned out to give optimum pulp properties regarding strength, brightness and cleanliness.
Within the bleaching technology the following expressions are used: C=chlorine (Cl.sub.2), D=chlorine dioxide (ClO.sub.2), E=extraction with sodium hydroxide (NaOH), O=oxygen (O.sub.2).
Heretofore, chlorine dioxide was usually used at the end of the bleaching sequence. A typical bleaching sequence would be: C E D E D. During the 1960s and 1970s about 5-15% of the chlorine in the first bleaching stage started to be substituted by chlorine dioxide calculated as available chlorine.
The relationship between chlorine dioxide and chlorine within the bleaching field is expressed as follows: 1 kg of chlorine=1 kg of available chlorine. 1 kg of chlorine dioxide=2.63 kg as available chlorine.
The primary reason for adding a small amount of chlorine dioxide in the first bleaching stage was that the fibre strength could be maintained.
During the 1970s, but particularly during the 1980s, the use of chlorinating bleaching agents has more and more been questioned. The primary reason is that when chlorine gas is used as a bleaching agent it produces large amounts of chlorinated organic compounds.
Major experimental work has been done and will continue in order to decrease and perhaps eliminate the use of chlorine gas for bleaching of chemical pulps. Partially for this reason, oxygen delignification has been developed.
Chlorine dioxide also produces chlorinated organic compounds when used as a bleaching agent, but are only a fraction compared to when chlorine gas is used. These compounds are also considered less toxic. For this reason chlorine dioxide is now a candidate for total substitution of chlorine in the first bleaching stage.
As chlorine dioxide is a unique delignification and bleaching agent for chemical pulps with bleaching sequences like O D (E+O) D E D, both chemicals offer maximum utilization with these stages. When going from chlorine to chlorine dioxide bleaching the pulp mill will often have to increase its installed chlorine dioxide capacity substantially. This could lead to a large investment if the old reactor system has to be replaced.
In the beginning chlorine dioxide was produced by reduction of sodium chlorate with sulphur dioxide in sulphuric acid medium according to the so called Holst-procedure. This is a batch process where sodium chlorate solution and sulphuric acid is added to a reactor. A mixture of sulphur dioxide in air was then blown through spargers in the bottom of a reactor. The chlorine dioxide formed was removed from the reaction medium by the sparged air stream and then absorbed in water in a specially designed tower. Later the so called Mathieson Process was developed. Here the reaction components, sodium chlorate, sulphuric acid and sulphur dioxide in air were continuously added to the reactor as shown in FIG. 1. The liquor content from the primary reactor passes through a secondary and sometimes a tertiary reactor where the concentration of sodium chlorate is depleted as much as possible in order to minimize losses. The liquor coming out of the tertiary reactor is called spent acid.
According to other procedures, methanol (the Solvay Process), and chloride (the R-2 Process, R-3 Process, SVP Process, Lurgi Process etc) are being used as reducing agents instead of sulphur dioxide.
When producing chlorine dioxide
REFERENCES:
patent: 3347628 (1967-10-01), Sepall et al.
patent: 4081520 (1978-03-01), Swindells et al.
Eka Nobel AB
Lund Valerie
Straub Gary P.
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