Chemistry of inorganic compounds – Halogen or compound thereof – Ternary compound
Reexamination Certificate
1999-06-08
2001-06-26
Nguyen, Ngoc-Yen (Department: 1754)
Chemistry of inorganic compounds
Halogen or compound thereof
Ternary compound
C423S478000
Reexamination Certificate
active
06251357
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a process for the preparation of chlorite compositions with a very low carbonate content by the reduction of chlorine dioxide generated from chlorate in a subatmospheric type chlorine dioxide generator.
BACKGROUND TO THE INVENTION
Alkali metal chlorites are well known precursors of chlorine dioxide with a wide range of applications, mainly in water treatment, pulp bleaching and textile bleaching. Chlorites are prepared typically by the reaction of chlorine dioxide, a reducing agent and an alkali. An exhaustive discussion of various preparative methods for chlorite synthesis can be found in the basic textbook entitled: “Chlorine Dioxide. Chemistry and Environmental Impact of Oxychlorine Compounds” by W. J. Masschelein, 1979, pp. 130 to 145.
Various improvements to the basic concept of reacting chlorine dioxide with the reducing agent and alkali to form chlorite are disclosed in the U.S. Patents discussed below.
U.S. Pat. Nos. 2,092,944 and 2,092,945 (Vincent) disclose the preparation of water soluble chlorites by reacting chlorine dioxide with an alkaline solution containing sulfur or a carbonaceous reducing agent.
U.S. Pat. No. 2,194,194 (Cunningham) discloses the use of metallic reducing agents for the preparation of chlorites.
U.S. Pat. No. 2,332,180 (Soule) discloses the use of hydrogen peroxide and alkali metal bicarbonate in chlorite synthesis. The same reducing agent is disclosed in the U.S. Pat. No. 2,616,783 (Wagner), related to the preparation of solid chlorite.
U.S. Pat. No. 3,101,248 (Hirschberg et al) discloses a process for chlorite synthesis involving the use of various alkali metal and alkaline earth metal amalgams as reducing agents.
U.S. Pat. No. 3,450,493 (Du Bellay et al) discloses a method for the manufacture of alkali metal chlorites, employing a continuous monitoring of redox potential and pH for correct process control.
U.S. Pat. No. 3,828,097 (Callerame) discloses a process for the preparation of chlorous acid, involving the use of nitrite in a column containing a cation exchange resin.
U.S. Pat. No. 4,087,515 (Miller) discloses the use of alkali metal amalgams as reducing agents whereby the process is carried out under an atmosphere of nitrogen gas to prevent an excessive build-up of chlorine dioxide.
U.S. Pat. No. 5,597,544 (Barber et al) and U.S. Pat. No. 5,639,559 (Mason et al) disclose a gas phase reaction between chlorine dioxide and reducing agent whereby the resulting chlorous acid is subsequently reacted with aqueous solution of the base, such as hydroxide, carbonate or bicarbonate to form chlorite in high yield.
A major drawback of all of the above described processes is a high content of certain impurities, particularly carbonates and bicarbonates, in the final product. According to the published literature (see, for example, previously cited Masschelein, p. 131, lines 10 and 11) a typical, commercial 80% sodium chlorite product generally contains about 5% sodium carbonate.
Such a high level of carbonates is detrimental at the point of use of alkali metal chlorite, in particular when chlorite is converted to chlorine dioxide to be used for water disinfection or pulp bleaching. The presence of carbonates causes the formation of scale in the equipment employed for chlorine dioxide generation, resulting in higher operating costs and troublesome maintenance. While there are known methods for the purification of sodium chlorite from the carbonate impurity, they are very costly and often they create more problems than they solve. For example, a carbonate removal method based on the precipitation of lead carbonate (see Masschelein, p. 138) may result in the contamination of chlorite with highly poisonous lead compounds, rendering the product unsuitable for water treatment applications.
There is a need, therefore, to develop an economical process enabling the manufacture of alkali metal chlorite with a very low carbonate content, thus eliminating the costly purification step of the final product.
SUMMARY OF THE INVENTION
Accordingly, the present invention is directed towards alleviating the problems and disadvantages of the prior art by providing an economical process for the manufacture of alkali metal chlorite with a very low carbonate content which does not require purification of the final product.
Surprisingly, it has been found that by combining a chlorine dioxide generation system operating at subatmospheric pressures with the chlorite formation reactor involving the use of hydrogen peroxide as a reducing agent, it is possible to obtain an alkali metal chlorite with a carbonate level significantly lower than that reported in the prior art for the conventionally produced chlorite product.
Such a combination of a subatmospheric chlorine dioxide generator and a chlorite formation reactor yields a chlorite product with the carbonate content being significantly below about 1 wt. % (based on an about 37 wt. % sodium chlorite solution) and below about 2 wt. % (based on the solid about 80% sodium chlorite). The 37 wt. % solution of sodium chlorite manufactured according to the process of the present invention contains preferably less than about 0.5 wt. % sodium carbonate (as Na
2
CO
3
) and most preferably less than about 0.3 wt. % Na
2
CO
3
, while the solid 80% sodium chlorite contains preferably less than about 1 wt. % Na
2
CO
3
and most preferably less than about 0.6 wt. % Na
2
CO
3
.
Without being bound by any particular theory, it is believed that the enhanced purity of the product resulting from the process of the present invention can be attributed to a specific mode of chlorine dioxide generation whereby the effect of subatmospheric pressure in the chlorine dioxide generator is reflected in a lower content of carbon dioxide in the chlorine dioxide gas/water vapour mixture leaving the generator. Such a mixture, upon being reacted with hydrogen peroxide and alkali in the chlorite formation reactor, yields, in turn, a chlorite product with a lower carbonate content.
Accordingly, in one aspect of the present invention, there is provided a method of producing an alkali metal chlorite with a low carbonate level, which comprises:
effecting the generation chlorine dioxide by reducing chlorate ions to chlorine dioxide in an aqueous reaction medium at its boiling point under a subatmospheric pressure in a first reaction zone,
remove a gaseous admixture containing chlorine dioxide from said first reaction zone,
feeding chlorine dioxide to a second reaction zone,
reacting the chlorine dioxide with an aqueous alkali metal hydroxide solution and hydrogen peroxide as a reducing agent in said second reaction zone, and
removing an aqueous solution of alkali metal chlorite having a low carbonate ion concentration from said second reaction zone.
The chlorite formation reactor is preferably operated under vacuum, which may further improve the chlorite product purity.
REFERENCES:
patent: 2092944 (1937-09-01), Vincent
patent: 2092945 (1937-09-01), Vincent
patent: 2194494 (1940-03-01), Cunningham
patent: 2323180 (1943-10-01), Soule
patent: 2616783 (1952-11-01), Wagner
patent: 2833624 (1958-05-01), Sprauer
patent: 3101248 (1963-08-01), Hirschberg et al.
patent: 3450493 (1969-06-01), Bellay et al.
patent: 3760065 (1973-09-01), Rapson
patent: 3828097 (1974-08-01), Callerame
patent: 3997462 (1976-12-01), Denaeyer et al.
patent: 4081520 (1978-03-01), Swindells et al.
patent: 4086329 (1978-04-01), Cowley et al.
patent: 4087515 (1978-05-01), Miller
patent: 4421730 (1983-12-01), Isa et al.
patent: 4465658 (1984-08-01), Fredette
patent: 4473540 (1984-09-01), Fredette
patent: 4683039 (1987-07-01), Twardowski et al.
patent: 5091166 (1992-02-01), Engström et al.
patent: 5091167 (1992-02-01), Engström et al.
patent: 5116595 (1992-05-01), Scribner et al.
patent: 5205995 (1993-04-01), Scribner et al.
patent: 5366714 (1994-11-01), Bigauskas
patent: 5593653 (1997-01-01), Scribner et al.
patent: 5597544 (1997-01-01), Barber et al.
patent: 5639559 (1997-06-01), Mason et al.
patent: 2189289 (1996-10-01), None
patent: 55-09
Cowley Gerald
Dick Peter David
Nguyen Ngoc-Yen
Sim & McBurney
Sterling Canada Inc.
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