PRODUCTION
Simple Step Saves Dollars Modifications in Mathieson process for chlorine di oxide production boost yield from 9 0 to 97% VJNE
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some installation changes which can be made at nominal cost may mean sav ings of from $6000 to $8000 a year per ton d a y of gas produced for those who use chlorine dioxide for pulp bleaching. Developed by Olin Mathieson Chemi cal, t h e improved process is subject of a patent application n a m i n g W . W . Northgraves and Β. Η . Nicolaisen of Olin Mathieson as inventors. The new process is based on the earlier Mathieson process n o w used ex tensively in bleach plants throughout the U . S. and Canada. It differs prin cipally in t h e additional introduction of a chloride into the generator. This is said to result in an increase in the yield of chlorine dioxide based o n the chlo rate charge of from about 90 to 97% of t h e calculated yield. Additionally, the injection of chloride effects a reduc tion in the chlorine content of the gen erator gas from what is usually 2 to 5% to less than 1.5% based o n the com
bined chlorine and chlorine dioxide in the gas. The over-all reaction forming t h e basis of these processes is: 2 N a C 1 0 3 + SOo + HUS0 4 -> 2 C1Ô 2 + 2 N a H S 0 4 < Other previous processes of chlorate reduction have operated at relatively high chlorate concentrations. Competing side reactions occurring in these, as well as in the older Mathieson process, are: 2 N a C 1 0 3 + 5 SOo 4- 4 HUO - > Cl 2 -h NaHSOj + 3 H 2 S 0 4 NaClO-, + 3 SOo + 3 HoO - > NaCl -I- 3 H 2 S 0 4 Such side reactions appear to be almost completely ehminated in the n e w process. • How It O p e r a t e s . Sodium chlorate, stored a s a concentrated solution,
Laboratory development of the improved Mathieson process for chlorine dioxide generation was carried out by D . J. Jaszka, research chemist, Β. Η. Nicolaisen, head of Olin Mathieson's Industrial Chemicals division research laboratories, and T. H . Dexter, project supervisor at Niagara Falls. T h e research team is shown here with the laboratory scale chlorine dioxide generator used in the work 3042
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is diluted with water a n d metered to the reactor. Sulfuric acid is pumped to the reactor t o maintain die concen tration of t h e reaction liquor at about 4 to 4.5 molar. In addition, sodium chloride solution is p u m p e d t o the gen erator in quantities sufficient to main tain t h e chloride concentration of the reaction liquor in the range of 0.05 to 0.25 molar. Sulfur dioxide, obtained either as a liquid or from a sulfur burner, is diluted with air to approxi mately 1 0 % S 0 2 concentration and passed into the bottom of the generator. Spent generator liquor overflows con tinuously a n d m a y b e p u m p e d directly to a kraft recovery system to utilize salt cake values or neutralized with alkali. The chlorine dioxide is continuously diluted and removed with t h e air in troduced a s t h e diluent in t h e sulfur dioxide stream. A partial pressure of about 120 m m . of chlorine dioxide is suitably maintained. T h e chlorine di oxide may be absorbed in water at 40° F . , to produce a chlorine dioxide solution containing u p to about 8 grams per liter. This solution can b e safely stored and p u m p e d to a bleach plant as required. Generator gas having a chlorine di oxide partial pressure of from 100 to 150 mm. m a y b e produced a s desired. Higher partial pressure provides im proved efficiency in the water absorp tion step. The reduction of sodium chlorate b y sulfur dioxide is an exothermic reaction which, for optimum efficiency, should b e controlled a t approximately 100° F . Olin Mathieson uses a fined jacketed steel tank as a generator. Absorption water for preparing the chlorine diox ide solution is refrigerated to about 40° F . If cooling water at about 70° F . is n o t continuously available, re frigeration capacity should be suffi cient to h a n d l e both reactor cooling and absorption water cooling. Olin Mathieson says t h e savings which can b e effected b y the use of the improved process are material. For ex ample, when 10 moles of sodium chlo ride are c h a r g e d per 100 moles of so dium chlorate, die yield of chlorine di oxide based on the chlorate is increased from 9 0 % to about 97%?. Total chemi cal costs amount to 0.162 dollar per pound of chlorine dioxide compared with 0.173 dollar per pound. T h e sav ing of 0.011 dollar per p o u n d of chlo rine dioxide amounts to a b o u t 7 % of t h e chemical costs. I n operat ing a large scale plant at 10 tons per day, this saving amounts to about $70,000 per year, company says. In the new process, construction and instrumentation are very simple. O n e operator can readily control the gen erator.
