JPH05214689A - Recovery of discharged material of chlorine dioxide of pulp factory - Google Patents

Abstract

PURPOSE: To reduce potential air pollution by eliminating chlorine dioxide from emission gas from pulp mills and reduce the total discharge of chemicals into effluent aqueous solutions discharged from such pulp mills as well. CONSTITUTION: The subject recovery comprises the following process: chlorine dioxide in the emission gas from pulp mills is scrubbed with an alkaline aqueous solution containing hydrogen peroxide to reduce the chlorine dioxide to sodium chlorite; the solution of which is then incorporated in a chlorination process or chlorine dioxide/chlorination process to convert the sodium chlorite to chlorine dioxide in situ.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention relates to a method for recovering chlorine dioxide from effluent gas from a pulp mill.

[0002]

Chlorine and chlorine dioxide are the main chlorine-based chemicals used to bleach chemical pulp.
The use of chlorine is subject to environmental pressure in an attempt to reduce chlorinated organics in the effluent solution, air (chloroform) and pulp. The replacement of some of the chlorine with chlorine dioxide in the chlorination process has been shown to reduce the release of these chlorinated compounds into the environment. However, chlorine dioxide (ClO ₂ ) is considered a toxic and dangerous chemical. Chlorine dioxide emissions result from various bleaching processes as well as from the generator itself. State and federal regulations require scrubbing of these offgases. Moreover, the use of chlorine dioxide instead of chlorine requires more generator capacity and reduces the efficiency of ClO ₂ bleaching. this is,
It tends to reduce the amount of ClO ₂ emissions in the bleach plant emissions.

ClO ₂ of the pulp mill's offgas is usually removed by scrubbing with white liquor containing alkali, preferably sulfides and other reduced sulfur compounds. Chlorine dioxide is reduced by alkaline hydrogen peroxide to form sodium chlorite,
It is well known. British Patent No. 2058602 and Japanese Publication No. 55-98965 both disclose that chlorine dioxide released from a factory that bleaches textiles with sodium chlorite is absorbed by alkaline hydrogen peroxide and flows out containing sodium chlorite. It is disclosed that the scrubber solution can be reused as additional sodium chlorite. However, pulp mills usually do not use sodium chlorite, and the sodium chlorite generated from this scrubber must be further processed before it is disposed of in the environment.

One proposed solution was to use "white water" with scrubbers (C
ampbell, P.A. R. , "Bleach Plan
tVent Gas Scrubber at Lea
f River Forest Products C
orp. ", TAPPl 1987 Pulping
Conference (1987) pp. 737-73
8). This method neutralizes chlorine dioxide, but it does not convert all of the scrubbed chlorine dioxide from gas to useful products, and it consumes both alkali and sodium sulfide from white water, The cost will be excessive.

[0005]

SUMMARY OF THE INVENTION The present invention (a) scrubs a gaseous pulp mill effluent containing chlorine dioxide with an aqueous alkaline hydrogen peroxide scrubbing solution, thereby converting at least a portion of the chlorine dioxide into an aqueous chlorite solution. Then
And (b) a pulp comprising blending at least part of the aqueous chlorite solution in a pulp mill chlorination process, chlorine dioxide process or chlorine dioxide / chlorination process, thereby generating chlorine dioxide therein. By providing a method of recovering gaseous chlorine dioxide from factory emissions, the disadvantages of conventional methods are overcome. The reaction of alkaline hydrogen peroxide with chlorine dioxide to form sodium chlorite is
It is well known. However, Sittig, M .; , Pulp
and Manufacture, Energy Co
conversation and Pollution
Prevention, Noyes Data Cor
p., Park Ridge, N .; J. (1977)
p. According to 407, until the present invention it was believed that there was a need to eliminate the sodium chlorite product by factory waste.

Not only does the process of the present invention remove chlorine dioxide from the evolved gas to reduce potential atmospheric pollution, but the process of the present invention also eliminates the total release of chemicals into the effluent aqueous solution from a pulp mill. Lower. This is because chlorine dioxide chemical bleach is recovered and recycled to the pulp mill as a solution of sodium chlorite (“chlorite”). Chlorite reacts with the process chlorine or acid and is converted to ClO ₂ as a useful bleaching chemical. This results in operational and investment savings as well as environmental improvements. Preferably, chlorite is introduced into the process containing chlorine. The reaction of chlorite with chlorine yields only 0.8 moles of Cl ₂ per mole of chlorite when reacted with acid, compared to 1 mole per mole of chlorite. Generates moles of ClO ₂ .

A particularly desirable source of alkaline aqueous hydrogen peroxide solution is the effluent from a pulp mill process having either a peroxygen process ("P") or a peroxygen assisted oxygen extraction process ("EOP"). Can be easily obtained as A particularly desirable source of alkali alone is available from either the oxygen extraction step ("EO") or the extraction step ("E"). All of these solutions can be recycled in the scrubber. Hydrogen peroxide and / or sodium hydroxide or sodium carbonate can be added as needed to provide a solution of sodium chlorite in any preferred concentration. Those skilled in the art will appreciate that the effluent from the P, EOP, EO and E processes often minimizes the overall consumption of hydrogen peroxide and / or the feedstocks of the present invention, such as hydrogen peroxide and sodium hydroxide, in significant concentrations. It will be readily appreciated that it contains sufficient alkalinity. Therefore, the present invention not only eliminates waste streams, but also converts waste chemicals to the valuable chlorine dioxide. Moreover, the method of the invention produces chlorine dioxide in situ in the process of the factory, so that without the need for additional investment,
Expand the plant's total ClO ₂ capacity.

The ClO ₂ off-gas recovery according to the invention gives rise to two substantial advantages over the conventional method of scrubbing with alkaline white liquor containing reduced sulfur compounds such as sulphides. 1. Hazardous or toxic chlorine dioxide emissions are eliminated without increasing the addition of sulfur to environmental treatment equipment. 2. Chlorine dioxide is generated during the bleaching process, resulting in operational cost savings (due to ClO ₂ effects) and a potential reduction in investment in generator capacity (due to increased capacity for ClO ₂ ).

[0009]

The following example is provided to illustrate one embodiment of the invention and is not intended to be limiting. The examples use representative data for a factory that produces Southern Pinecraft at 30 kappa. In practice, it is clear that the optimum concentration and temperature depend on the overall factory operating conditions and not on this chlorine dioxide emission control / recovery process.

Pulp is a conventional five-step bleaching plant,
Bleached with (CD) (EO) DED. kappa factor (total available chlorine charge / number of kappa) is
0.22 and the amount of ClO ₂ substitution is 15%.
This is a total available chlorine charge of 6.6% in the C process, 5.61% Cl ₂ and 0.38% ClO.
₂ (0.99% Av.Cl.). The amounts of ClO _{2 in} the steps D1 and D2 are 1 and 0.2%, respectively.
Is. Therefore, for a 100 TPD pulp mill, the amount of ClO ₂ required would be 15.8 TPD. Sitt
ig, M.I. , Pulp and Manufactur
e, Energy Conservation and
Pollution Prevention, Noy
es Data Corp. , Park Ridge,
N. J. (1977) pp. 406-407 shows that ClO ₂ a pound is per tonne of pulp release. The chlorine dioxide emissions from the first D and second D steps are 0.6 1b / T and 0.4 1b / T from a ClO ₂ generator using 20 ° C. scrubbing water.
Using the method of the present invention will result in 1 pound of ClO ₂ per ton of scrubbed pulp. Sodium chlorite can be recovered in the CD process or D process. If recovered in the CD process, this is C
The substitution of 10 ₂ would be increased from 15% to 17.1%. If recovered in the D step, this is the required C
It will reduce the 10 ₂ charge from 1% to 0.95%. For a 1000TPD plant, this would increase chlorine dioxide availability by 0.5%, resulting in the need for smaller generators.

 ─────────────────────────────────────────────────── ─── Continued Front Page (72) Inventor William Gastob Strunk 32168 New Smilena Beach Sea Street, Florida, USA 115

Claims (3)

[Claims] 1. (a) Scrubbing a gaseous pulp mill effluent containing chlorine dioxide with an aqueous alkaline hydrogen peroxide scrubbing solution, thereby converting at least a portion of the chlorine dioxide into an aqueous chlorite solution, and ( b) A pulp characterized in that at least part of the aqueous chlorite solution is blended in a chlorination process, a chlorine dioxide process or a chlorine dioxide / chlorination process of a pulp mill, whereby chlorine dioxide is generated therein. A method of recovering gaseous chlorine dioxide from factory emissions. 2. The method of claim 1 wherein the effluent from the alkaline hydrogen peroxide bleaching step is used as at least a portion of the alkaline hydrogen peroxide scrubbing aqueous solution. 3. The process of claim 1 wherein the effluent from the oxygen extraction step assisted by peroxygen is used as at least a portion of the alkaline aqueous hydrogen peroxide scrubbing solution.