CA1069672A - Method and apparatus for preparing aqueous solutions of pure sodium hydrosulfite and composition of matter for such solutions - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
Apparatus and method are disclosed for continuously handling sod-ium hydrosulfite for use in textile dyeing including means for receiving the pure dry chemical in semi-bulk returnable containers, for dissolving the same in a caustic solution and for storing the resulting solution in low inventory automatic make-up tanks. The novel resulting solution is stabilized against decomposition and may be metered to specified locations in the textile mill.

Description

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There are currently serious economic and safety fac~or problems encountered in procedures for handling the bleaching agent sodium hydrosul-fite in certain mills and in related applications. The problem arises espec-ially among textile manufacturers who desire to feed all components automa-tically to dyeing operations, The use of liquid feeds is essential for com-puter controlled, fully automated operation. Sodium hydrosulfite is today delivered to a textile mill, for example, in relatively small quantities of liquid, say approximately 4,000 to 6,000 pounds. To maintain the liquid sodium hydrosulfite in such a state, relatively large quantities of caustic soda are now mixed into this solution, say, approximately one weight percent concentration. Even so, the stability and storage life of the solution is on the order of days only since there is significant sodium hydrosulfite re-duction. The concentration of solution at use time is often significantly dif~erent from that at the time of mixing or delivery. Close coordination of delivery and stock depletion with mill shut-downs, vacation times, etc., naturally pose significant problems.
This invention seeks to provide a solution to the problems exper ienced with prior art apparatus and methods for handling sodium hydrosulfite.
The pure dry chemical may be stored at the site in large quantities for up to three months. As the aqueous solution is required, the chemical may be metered into a mixing tank with a metered quantity of water and caustic soda solution to provide an essentially pure solution which may be metered out as required for on site use. Due to the purity of the dry chemical, resultant solution contains substantially less stabilizing caustic soda, relative to the amount of water, than found in previously available commercial composi-tions, yet because of the continuous manufacture of the solution, decomposi-tion of the sodiumhydrosulfite is minimized.
m us this invention provides a method of preparing and handling an aqueous solution of sodium hydrosulfite comprising the steps of:
A) providing a mixing volume;

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1~69672 B) controllably injecting into a first stream of water approximatelyone percent by weight of caustic soda for each from 84 to 94 parts by weight of water in said stream to form a second, mixed stream;
C) admitting said second stream to said volume;
D) simultaneously with Step C) controllably admitting to said volume from S to 15 parts by weight of sodium hydrosulfite in powder form;
E) simultaneously and continuously with Steps C) and D) mixing the parts to form a solution; and F) delivering said solution in a third stream to use points.
In preferred embodiments of this method the following steps are also included:
G) generating a first signal proportional to the flow rate of said first stream of water; and H) controlling the volume of said caustic soda injection and said sod-ium hydrosulfite admission in response to said signal to produce said pro-portions in said third stream;
I) generating a second signal proportional to the density of said solu-tion in said third stream; and J) further controlling the admission of sodium hydrosulfite in response to the variation of said second signal from a predetermined norm.
The Figure illustrates a schematic of the inventive apparatus ussd to produce the sodium hydrosulfite solution.
The system consists of semi-bulk returnable shipping containers and unloading stands, a dry chemical feeder, caustic pump, fluid controls, a densimeter for monitoring and trimming solution strength, a dissolver tank and a vertical storage tank from which the solution is drawn to process, A typical arrangement is shown in the Figure.
Referring to the Figure, sodium hydrosulfite is delivered to a mill site location in dry or powder form in any bulk quantities desired.
For example, 3,500 lb. quantities have been delivered with satisfactory - . , . ~

storage and handling results Sodium hydrosulfite 1 thus delivered is contained at on-site location in commercially available, returnable con-tainers 2 As indicated, the sodium hydrosulfite can be stored in con- -tainer 2 in its dry state for as long as three months. To protect the sodium hydrosulfite in container 2 from air, moisture and other reducing -~
elements, a sliding door 3 is disposed at the bot~om of the container and a sealed barrel closure 4 at the top. The door 3 may be open as indicated by the arrow and shut at the termination of use to seal the container

2 airtight.
Dry sodium hydrosulfite 1 delivered to the mill site in container 2 is disposed above motorized conveyor feeder 5.- A series of containers 2 can be, of course, placed in a row with a conveyor belt, not shown, thereunder, the b01t being controllably operable to deliver the dry sodium hydrosulfite to the feeder 5. From feeder 5, the dry powder is directed to the dissolver tank 6 which then mixes the sodium hydrosulfite with caus-tic solution by stirring with mixer 7. The dissolver tank is covered except for the openings to admit caustic soda and water, via line 10, sodium hydrosulfite via feeder 5 and sample solution via line 37. This minimizes contact with the atmosphere and resultant reduction of the sol-ution. The mixing operation may be conducted satisfactorily in a temp-erature range of from 32 F. to 110F.
Liquid cau5tic soda is moved by pump 8 through pipe 9 directly ~ -into a steady stream of water flowing in pipe 10. The water flows through meter ll and valve 12 to mix with the caustic soda at point 13. A level control 14 automatically reads the level in dissolver tanX 6 and controls the solution input to the desired level by coaction with valve 12. Mbter 15 shows the liquid level.
Meter 11 sends a pulse signal to pre-amplifier 16 and via scaler 17 to the blend ratio controller 18. A signal is also directed from pre_ amplifier 16 via processing circuitry 19 to recorder Z0. Blend ratio

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~069672 controller 18 produces two outputs, one to the controller 21 for caustic soda pump 8 and one to the controller 22 for conveyor feeder 5. Controller 21 may comprise a driver unit 23 and a relay 24, as shown. Controller 22 may comprise a trim unit 25, pulse to current converter 26 and rectifier 27. Thus the appropriate amounts of caustic soda and sodium hydrosulfite are metered into the dissolver tank 6 in proportion to the amount of water passing meter 11. Concentrations to within + 2.5 weight percent can be maintained in this system. When valve 12 shuts in response to a high level signal from level control 14, no output is produced by blend ratio controller 18, and the feeder 5 stops.
The sodium hydrosulfite in solution may then be delivered via pump 28 and pipe 29 directly to the point of application if desired. How-ever, it is preferred to use a covered and vented storage tank 30 wherein the stabilized sodium hydrosulfite solution is held prior to delivery to the in-plant locations through pipe 31. Tank 30 may be refrigerated if extended storage is anticipated prior to use. A storage tank level con-trol 32 can be placed in connection with the storage tank 30 which by pre-selected control of valve 33 may control the volume of solution in the tank.
Meter 35 shows the level in the storage tank. Obviously, if valve 33 is restricted or switched into a non-flow state, the level in dissolver tank 6 will increase to a point at which level control 14 will adjust valve 12.
In connection with solution flow pipe 29, there is provided a densimeter 36 for sensing the density of the solution. A sample line 37 recycles the sample flow back to the dissolver tank 6. The output from the densimeter is directed via processing circuitry 38 to recorder 20.
The output from circuitry 38 is also directed via density alarm 39 to density controller 40. If the density of the mixture leaving tank 6 is at variance with the setpoint of controller 40, a signal is transmitted to trim unit 25 which may be set to adjust the speed of feeder 5 within, say, plus of minus 10% of the setting of blend ratio controller lg. When . ' ' ' ,, ~0~67Z :

the density is beyond this range, alarm 39 may be set to actuate and to close valve 33 via solenoid 34, as indicated. At such a time, the opera-tor could perform any necessary trouble shooting operations to return the system to automatic operation, or operate the feeder manually until the problem is rectified.
In practice, the following commercially available components have been found acceptable for use in this system, though one skilled in the art will appreciate that many variations are possible within the scope of this invention:
Element in Figure Available Co~ponent 11 Foxboro M/81SFSC3, 1/2 inch 12, 33 Foxboro M/V4A Needle 14, 32 Foxboro M/13PA-MS315 W/lAS-F
15, 35 Foxboro M/43A-A4 W/PC3-15 16 Foxboro A2020LA W/A2021BZ
17 Foxboro M/99M-lOOSP
18 Foxboro M/99M-212 19 Foxboro M/FR316C-5-2, M/FR316C-5 -with large capacitor output net-work Foxboro M/6420 HF-O
23 Foxboro M/99M-731 24 Foxboro A93746 Foxboro M/99M-720H
26 Foxboro M/FR316C-5-2 38 Foxboro M/66BT
39 Foxboro M63U-BT-O~EA
Foxboro M/62H-4E-O

This listing is merely representative and components not listed may be selected as needed by one of ordinary skill in the art.

Once a container 2 is placed on the stand and its slide gate 3 - : . .

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removed, feeder 5 will proportion the hydrosulfite into the caustic-water solution, both chemical feed rates being automatically paced with the inlet water flow, as discussed. The solution will then be pumped to storage tank 30 in which a constant level will be maintained. By operating with essentially constant level in the dissolver tank and the storage tank and minimizing flow of air over the surface of the liquids in these tanks, the solution and the small volume of air essentially trapped under the tank covers interact to form a primarily nitrogen atmosphere over the sol-ution, thus minimizing further reduction of the solution during continued system operation. From the storage tank, the solution will be withdrawn either by gravity or will be pumped through liquid meters to the applica-tion points, Under normal conditions tank 6 is sized so that at the maximum feed rate approximately fifteen minutes are required as a mini~um for complete solution of sodium hydrosulfite in caustic to form a solution containing 1% caustic and 15% sodium hydrosulfite by weight, Tank 30 is sized to provide one to two hour's reserve supply for a particular plant in the event that the feed system is shut down for minor repairs.
Feeder and fluid controls will all meet the specified accuracy over a lO/l span, i.e., if the normal draw is lO0 pounds per hour, the system will supply solutions containing 20 to 200 pounds of dry product without adjustment. Manual resets can be used to move the control range upward on demand, Recorded signals will be the solution density, and the solution flow-rate. Alarms also may be installed on the caustic pump output and holding tan~ 30 level.
In solutions having a concentration of sodium hydrosulfite varying from between 0.2% to 15% by weight, it is necessary to use only approxi-mately 1% by weight caustic soda. This low caustic soda ingredient is made possible since the sodium hydrosulfite solution is available for use almost immediately after mixing, Thus, no time exposure to air a~d .

other reducing elements occurs. The concentration of solution at the use poir,t and in the original mixture is almost identical. Prior known commer-cial solutions had a maximum of 11% by weight of the sodium hydrosulfite because refrigeration was required and included from 0.9% to 1.1% caustic soda. Concentration of the prior art solutions at the use point is unre-liable.
A typical solution which has been found to be highly sa~isfactory in the practice of the invention is:
Na2S204 10.0~ (by weight) NaOH 1.0%
Inerts 0.8%
Water 88.2%
With a concentration of 0.2% to 15% by weight of sodium hydro-sulfite and approximately 1.0% by weight sodium hydroxide, water from 82.8% to 98.78% by weight and inert from 0.016% to 1.2% by weight may be used with satisfactory results.
This solution, which has a saturation temperature of less than 34F " has a stability factor of less than 2% decomposition in 48 hours at 90F; with the initial p~l factor of 11.7 and the initial specific gra- -vity of 1.0925. Because pure sodium hydrosulfite may be used in this system, the solution has approximately 5% less impurities than known commer-cial solutions, which do not use the pure chemical due to the instability of the solution and associated problems of shipment and storage. The solution may be used with pH factor in the range of 8.0 to 12, with accep-table results Among the advantages achieved through the practice of the invention are the following:
A, The complete system is competitive from a materials han-dling viewpoint with liquid sodium hydrosulfite, and offers some additional benefits. The inventory and delivery frequency problem resulting from the transportation of small quantities (4000-6000 pounds) liquid sodium - . ,, . . . .

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hydrosulfite is eliminated, as the mill now may take truckloads in 35,000 pound quantities Invbntory of the dry chemical is stable and can be kept three months Shutdowns for maintenance and/or for vacation periods do not require tight coordination of inventory depletion and quitting time;
and start-ups are not dependent upon delivery of a new load of hydrosul-fite on the day preceding the beginning of operation. Further, this system may be operated without the use of either a nitrogen blanket or refrigera-tion as required in the prior art systems. Finally, the on-site prepared hydrosulfite solution is essentially pure sodium hydrosulfite and caustic soda. Because the solution is pure it can be delivered to the point of application in the mill with a loss no greater than one percent.
B, Economically, the container 2 contains 3500 pounds net weight, or the equivalent of 14 drums (250-100 pound net) of sodium hydro-sulfite. The container may be designed to be handled with a fork~lift truck and, hence, labor for materials handling during unloading and in-plant movement will be reduced to a minimum, Larger containers of up to 5000 pounds net may also be used.
C. The cost of weighing, hand-carrying, and dumping small charges of hydrosulfite can be eliminated.
D. Losses of hydrosulfite through decomposition can be redu-ced to a negligible value. In many textile mills, batch mixtures of sod-ium hydrosulfite and caustic soda are prepared Although initially these solutions are quite stable, during the time between preparation and use, losses of 5 percent may occur as a result of air contamination. This figure will vary in different mills and primarily is a function of ~he amount and time the solutions are in contact with air. Assuming the 5 percent loss is typical of a batch system and one percent in a contin-uous system, savings of 4 percent should be available.
E. Since the solution will be delivered directly to contin-uous textile mill ranges, the a~ount of reduction will be const~nt and ~0696 ~2 end-to-end color matching is better than that obtained with solutions pre-pared batchwise.
Having described an invention in such a manner as to enable one skilled in the art to make and use it, we claim: ~ :

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Claims (5)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A method of preparing and handling an aqueous solution of sodium hydrosulfite comprising the steps of:
A) providing a mixing volume;
B) controllably injecting into a first stream of water approxi-mately one percent by weight of caustic soda for each from 84 to 94 parts by weight of water in said stream to form a second, mixed stream;
C) admitting said second stream to said volume;
D) simultaneously with Step C) controllably admitting to said volume from 5 to 15 parts by weight of sodium hydrosulfite in powder form;
E) simultaneously and continuously with Steps C) and D) mixing the parts to form a solution; and F) delivering said solution in a third stream to use points.

2. The method of Claim 1 wherein the said steps are performed in an ambient temperature of from 32°F. to 110°F.

3. The method of Claim I including the steps of covering the mixing volume and maintaining an essentially constant solution level therein.

4. The method of Claim 1, further including the steps of:
G) generating a first signal proportional to the flow rate of said first stream of water; and H) controlling the volume of said caustic soda injection and said sodium hydrosulfite admission in response to said signal to produce said proportions in said third stream.

5. The method of Claim 4 including the steps of:
(I) generating a second signal proportional to the density of said solution in said third stream; and (J) further controlling the admission of sodium hydrosulfite in response to the variation of said second signal from a predetermined norm.