Chemistry of inorganic compounds – Halogen or compound thereof – Chlorine dioxide
Reexamination Certificate
2000-11-14
2002-05-14
Nguyen, Ngoc-Yen (Department: 1754)
Chemistry of inorganic compounds
Halogen or compound thereof
Chlorine dioxide
C252S186280, C252S187210
Reexamination Certificate
active
06387344
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a composition containing alkali metal chlorate, hydrogen peroxide and at least one of a protective colloid, a radical scavenger or a phosphonic acid based complexing agent, and a process for producing chlorine dioxide using said composition as a feed.
BACKGROUND OF THE INVENTION
Chlorine dioxide is primarily used in pulp bleaching, but there is a growing interest of using it also in other applications such as water purification, fat bleaching or removal of organic materials from industrial wastes. Since chlorine dioxide is not storage stable it must be produced on-site.
Production of chlorine dioxide in large scale is usually performed by reacting alkali metal chlorate or chloric acid with a reducing agent and recovering chlorine dioxide gas. Such processes are described in, for example, U.S. Pat. Nos. 5,091,166, 5,091,167 and 5,366,714, and EP patent 612,686.
Production of chlorine dioxide in small scale, such as for water purification applications, can also be done from alkali metal chlorate and a reducing agent but requires somewhat different processes, such as those described in U.S. Pat. Nos. 5,376,350 and 5,895,638.
The above small scale processes include feeding alkali metal chlorate, hydrogen peroxide and a mineral acid to a reactor, in which chlorate ions are reduced to form chlorine dioxide. In these processes it has now been found favourable to use a premixed solution of alkali metal chlorate and hydrogen peroxide as a feed. However, such solutions are not storage stable, particularly due to decomposition of hydrogen peroxide, but there is also a risk for a reaction between the hydrogen peroxide and the chlorate to form chlorine dioxide. The decomposition of hydrogen peroxide is particularly rapid in the presence of ferrous- and/or chromium ions, which may be introduced as in impurity in alkali metal chlorate or be released from storage containers of steel.
SUMMARY OF THE INVENTION
It is an object of the present invention is to provide a storage stable aqueous mixture of alkali metal chlorate and hydrogen peroxide that can be safely transported.
It is another object of the invention to provide a process for producing chlorine dioxide, particularly in small scale, using such a mixture as a feed.
It has surprisingly been found possible to meet these objects by a new composition being an aqueous solution comprising from about 1 to about 6.5 moles/liter, preferably from about 3 to about 6 moles/liter of alkali metal chlorate, from about 1 to about 7 moles/liter, preferably from about 3 to about 5 mols/liter of hydrogen peroxide and at least one of a protective colloid, a radical scavenger or a phosphonic acid based complexing agent, wherein the pH of the aqueous solution is from about 1 to about 4, preferably from about 1.5 to about 3.5, most preferably from about 2 to about 3. Preferably, at least one phosphonic acid based complexing agents is present, preferably in an amount from about 0.1 to about 5 mmoles/liter, most preferably from about 0.5 to about 3 mmoles/liter. If a protective colloid is present, its concentration is preferably from about 0.001 to about 0.5 moles/liter, most preferably from about 0.02 to about 0.05 moles/liter. If a radical scavenger is present, its concentration is preferably from about 0.01 to about 1 mol/liter, most preferably from about 0.02 to about 0.2 moles/liter. The water content in the composition is suitably from about 20 to about 70 wt %, preferably from about 30 to about 60 wt %, most preferably from about 40 to about 55 wt %.
It has also surprisingly been found that the performance of a small scale process for producing chlorine dioxide is improved is such a composition is used as a feed. Thus, the invention also relates to a preferably continuous process for producing chlorine dioxide comprising the steps of:
(a) feeding an aqueous solution comprising alkali metal chlorate, hydrogen peroxide and at least one of a protective colloid, a radical scavenger or a phosphonic acid based complexing agent as specified above and a mineral acid, or a mixture thereof, to a reactor to form an aqueous reaction mixture;
(b) reacting chlorate ions with hydrogen peroxide in said reaction mixture to form chlorine dioxide; and
(c) recovering a product containing chlorine dioxide.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
As high pH favours decomposition of hydrogen peroxide, while low pH favours formation of chlorine dioxide, it has unexpectedly been found that both can be avoided by selecting the above pH range. The pH is affected, inter alia, by the amount of hydrogen peroxide and by the protective colloid, radical scavenger or complexing agent used. If necessary, the pH of the aqueous solution can be adjusted to a suitable level by adding small amounts of any acid or alkaline substance compatible with hydrogen peroxide and chlorate, such as Na
4
P
2
O
7
, or H
3
PO
4
.
Any phosphonic acid based complexing agent can be used. Particularly preferred compositions comprise at least one phosphonic acid based complexing agent selected from the group consisting of 1-hydroxyethylidene-1,1-diphosphonic acid, 1-aminoethane-1,1-diphosphonic acid, 1-aminotri (methylenephosphonic acid), ethylene diamine tetra (methylenephosphonic acid), hexamethylene diamine tetra (methylenephosphonic acid), diethylenetriamine penta (methylenephosphonic acid), diethylenetriamine hexa (methylenephosphonic acid), and 1-aminoalkane-1,1-diphosphonic acids such as morpholinomethane diphosphonic acid, N,N-dimethyl aminodimethyl diphosphonic acid, aminomethyl diphosphonic acid, or salts thereof, preferably sodium salts.
Useful protective colloids include tin compounds, such as alkali metal stannate, particularly sodium stannate (Na
2
(Sn(OH)
6
). Useful radical scavengers include pyridine carboxylic acids, such as 2,6-pyridine dicarboxylic acid. It is to be understood that the composition of the invention can include mixtures of two or more of at least one protective colloids, at least one radical scavenger and at least one phosphonic acid based complexing agent.
In the aqueous solution of the new composition the molar ratio H
2
O
2
to ClO
3
−
suitably is from about 0.2:1 to about 2:1, preferably from about 0.5:1 to about 1.5:1, most preferably from about from about 0.5:1 to about 1:1. Using a composition of this ratio for producing chlorine dioxide has been found to give high conversion of the chlorate.
In order to inhibit corrosion, the composition further preferably contains a nitrate salt, preferably alkali metal nitrate such as sodium nitrate, in a preferred amount from about 1 to about 10 mmoles/liter, and a most preferred amount from about 4 to about 7 mmoles/liter.
It is also preferred that the amount of chloride ions is as low as possible, preferably below about 0.5 mmoles/liter, most preferably below about 0.1 mmoles/liter, particularly below about 0.03 mmoles/liter. Too much chloride increases the risk for corrosion, but may also cause formation of chlorine when the composition is used for chlorine dioxide production. As chloride normally is present as an impurity in alkali metal chlorate, it is advisable to use chlorate without extra added chloride, normally containing less than about 0.5, suitably less than about 0.05, preferably less than about 0.02, most preferably less than about 0.01 wt % of alkali metal chloride calculated as NaCl in NaClO
3
.
The composition normally contains as impurities ions of chromium and iron, particularly Cr
3+
and Fe
2+
. The presence of these ions increases the decomposition of the hydrogen peroxide, and it is desired to keep their content as low as possible. However, they are inevitably released during storage of the composition in steel containers, and may also be introduced as impurities in the alkali metal chlorate. The content of Cr
3+
is normally from about 0.5 to about 3 mg/liter, particularly from about 1 to about 2 mg/liter, while the content of Fe
2+
normally is from about 0.05 to about 5 mg/liter, particularly from about 1 to about 2 mg
Andresen Cecilia Bojrup
Jadesjö Gunilla
Tenney Joel
Nguyen Ngoc-Yen
Serbin David J.
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