CA1247808A - Sodium hydrosulfite slurries - Google Patents

Abstract

ABSTRACT
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Aqueous sodium dithionite slurries, which are non-settling during shipment thereof and are thereafter pumpable, and a method for their manufacture are provided. The slurries contain at least 20% by weight of crystalline pure sodium dithionite and at least about 0.13% by weight of a xanthan gum. As textile bleaching compositions, the slurries preferably comprise, on a weight basis, at least about 36% of commercial tripolyphosphate-free sodium dithionite, at least about 3% of sodium hydroxide, at least about 0.25% of a chelate, and at least about 0.25% of a xanthan gum. As woodpulp bleaching compositions, the slurries preferably comprise, on a weight basis, about 28% of commercial sodium dithionite, at least about 2% of sodium carbonate, at least 2%
of sodium tripolyphosphate, and at least about 0.13% of particular xanthan gums.

Description

~z~7~S~8 SODIUM HYDROSULFITE SLURRIES
BACKGROUND OF THE INVENTION
Field of the Invention This invention relates -to aqueous slurries and particularly relates to non-settling and flowable aqueous slurries of sodium dithionite that remain in pumpable form without significant expansion, settling, or gellation.
Review of the Prior Art Sodium dithionite, commonly termed sodium hydrosulfite and, less correctly, sodium hyposulfite, is a powerful reducing agent that has long been used for bleaching, particularly for bleaching textiles and wood pulps such as ground wood and semi-chemical pulps.

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Sodium dithioni.te has usually been manufactured by significantly different processes that are alternatively based Il on zinc dust, sodium formate, sodium borohydride, or sodium ¦ bisulfite (electrolytic). Such processes are disclosed, for example, in United States Patent Numbers 2,938,771: 3,004,825;
I 3,259,457; 3,411,875; 3,718,732; 3,872,221; 3,887,695;
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3,897,544; 3,927,190; and 4,127,642.
The products of these processes are herein I respectively identified as zinc-derived, formate-derived, ~ borohydride-derived, and electr-olyticall.y-derived sodium dithionite. Because the zinc process produces zinc dithionite, which is no longer ecologically acceptable, zinc dithionite is ¦ converted to sodium dithionite by adding sodium hydroxide or ¦ sodium ca.rbonate, whereby zinc hydroxide or zinc carbonate is precip~tated and removed by filtering.

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~ 2~8~1 ¦l When anhydrous sodium dithionite crystals are dissolved under either aerobic or anaerobic conditions to make a large quantity of aqueous solution, the resulting solution cannot be stored for use over a long period of time. Due to hydrolytic decomposition at the natural pH of the sodium dithionite solution, decomposition will proceed rapidly from that point by self-propagation because the decomposition products create an acidic condition which accelerates the ¦ll decomposition.
ll Aqueous solutions of the dithionite will decompose at 11 a commercially tolerable rate, however, if stabilized by ¦l additives such as are disclosed in United States Patent Nos.
il 3,819,807 and 3,985,674. These additives include chelating ¦ agents, sodium carbonate, sodium tripolyphosphate, sodium hydroxide, and amines.
¦l Although such stabilized solutions can be protected jl from decomposition for long enough periods Eor shipment and ll .

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routine commercial use under suitable conditions, it has been a j more common practice to store the anhydrous dithionite crystals ¦ under a dry, inert gas in a sealed container. Even though the crystals are thereby chemically stable for long periods, they begin to decompose as soon as exposed to the air and moisture when the container is opened for use thereof.
Furthermore, commercially available solutions of I sodium dithionite are expensive to transport because they are !! typically at concentrations of 12-13 5~, when combined with ll suitable additives, and, additionally, generally require refrigeration during shipment and storage. Thus, the transport oE about seven times as much water as product tends to cause il the sale oE the commodity to become distance-dependent. In consequence, slurries have seemed to offer an inviting means to avoid or at least to minimize the cost of storage and difficulties associated with solution forms of sodium ,l dithionite, without decreasing the convenience that the I purchaser derlves from solutions.
I¦ However, the economical preparation, stabilization, 1! handlingr and shipping of such slurries is not simple.
j Adequate suspension without agitation, so that pumping can be j done from a tank truck after shipment, is also not easy. In ~ fact, after considering the variety of processes that are '~i i jl ~l2~;L7~

- available for manufacturing sodium dithionite, including t~e indigenous by-products, crystal structures, and the like, the complexities of the concept are readily appreciated. Moreover slurries have not been as widely investigated nor as Il commercially utilized as other forms of sodium dithionite.
: I United States Patent 3,536,445 describes a process for i~ making sodium dithionite from sodium-zinc alloy by initially producing zinc dithionite and then converting it to sodium Il dithionite by adding caustic soda. After removal of the zinc ll hydroxide by filtration, the dihydrate of sodium dithionite is ¦l "salted out" of the mother liquor with sodium chloride and alcohol to form a slurry.
United States Patent 3,804,944 gives some stability storage data for 30% slurries (18.5% formate-derived and 11.5 i¦ zinc-derived sodium dithionite) containing 1-8% caustic soda ¦ (dithionite basis). Tests showed that these slurries required ! frequent agitation to prevent caking and handling difficulties.
I¦ United States Patent 3,83~,217 shows that by reducing 20 1I the particle size of the sodium dithionite crystals and/or ,j introducing a suspending or thickening agent into a liquid 1~ containing the crystals, such as alcohollc brine, it is ! possible to form a fluent, homogeneous, pourable dispersion of , . I

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'I the solid dithionite particles which is chemically and 1 physically stable for long periods of time, provided that a 1, .
material, such as the salt in the brine and/or an alcohol, be present which suppresses the dissolution of the dithionite so that the dispersion can be stored at about 20C. The majority of the particles should be about 0.6-0.8 micron in size.
Methylcellulose, hydroxyethyl cellulose, polyvinyl alcohol, guar gum, and other common thickening, dispersing, or suspending agents can be used. The thickened dispersion lll exemplarily has a Brookfield viscosity of 9,000 cps and contains up to 34% Na2S2O4.
United States Patent 3,839,218 provides a method Eor maintaining a dispersion of crystalline zinc or alkali metal dithionite by continuous or periodic mechanical agitation so that the crystals can be stored for long periods without decomposition, the dispersing medium being aqueous or non-aqueous and containing a material which suppresses dissolution of the dithionite solids. The pH of the liquid must be at least 6.5, the viscosity of the dispersion must be below about 50,000 centipoises, and the suppressing material may be a water-soluble organic compound or a saturated brine or mixtures thereof. A thickening and suspending agent can be used. suitable agents i~clude polysaccharides, water-soluble .

~2~7 ii polymers, and proteins of moderate molecular weight. Exemplary agents include guar gum, gum tragacanth, gelatin, and starchO
United States Patent 4,283,3Q3 discloses a method for making substantially stable slurries containing 30 35% by ¦ weight of sodium dithionite by evaporating sodium dithionite solutions while maintaining the heating medium at 220-250F and the solution and slurry at 110-155F under a vacuum of at least , 25 inches Hg and by promptly cooling the resultant slurry while Il agitating it. The vacuum is preferably 26.5-27.5 inches Hg.
~~ Zinc-derived sodiation liquor is the preEerred sodium dithionite solution to which 4-5~ by weight of the sodium dithionite, NaOH, and a chelator, as a stabiliæing agent, are added.
! Although these evaporated slurries have excellent stabilization qualities, they have developed problerns with settling which has occurred over a period of 2-~ days and especially under the vibrations produced by tank car shipment.
Such settling, and subsequent hardening, has resulted in j shipments which could not be unloaded by pumping as would I normally be done.
Slurries in general are utilized as foods, coatings, paints, dyes, explosives, oilwell fluids, and the like, and often include natural or synthetic gums to form a liquid colloidal , I

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system in which the solid particles are dispersed. Such a gum-i containing fluid system, without the solid particles, is identified as a sol and is more accurately termed a hydrosol ¦1 when based on water.
Il The gums typically impart viscosity to sols in which they are incorporated and thereby function as thickeners. When shear forces are created by agitating a sol and there is no change in viscosity, the behavior of a thickener is said to be ~ Newtonian. ~1hen the viscosity of the sol in a quiescent state ~l is greater than when a shear force is applied through agitation, wherl the viscosity decreases as the applied shear I force increases and when the viscosity recovers immediately I' when the magnitude of the shear force is decreased, the - l¦ behavior of a thickener is said to be plastic. When the rate il of Elow increases faster than normal in relation to the applied shearing stress, the sol is described as pseudoplastic.
I Generally, when the sol is at rest, the molecules of a plastic ¦ thickener arrange themselves into a more or less stable form.
Il In order to break this stable molecular arrangement and cause !I the sol to yield, the application of a shear force is necessary. The shear force that is required to cause the sol to yield and flow is termed the yield point or the gel strength. Once the gel strength of a plastic sol is overcome, ; ~
'',1 the viscosity of the sol proportionately decreases as greater shear force is applied.
Numerous natural and synthetic gums are widely used for manufacturing hydrosols. Favored gums for many hydrosols are galactomannan gums such as guar gum, which is derived from the endosperm of the guar plant, Cyamopsis tetragonolobus.
Other water-soluble gums which are increasingly utilized are the xanthonomas hydrophilic colloids, commonly termed xanthan gums, which may be produced by the action of various bacterial species of the genus Xanthomonas on carbohydrates ~and like materials). The fermentation product of the reaction of the bacteria Xanthomonas _mpestris, a preferred species, on carbohydrates is commerically available as "ICelzan " made by Kelco Corporation of San Diego, California.
In a typical process for clarification of a xanthomonas fermentation broth and/or recovery of the xanthomonas hydrocolloid component, the broth is diluted with water to reduce its viscosity and, optionally the diluted broth is centrifuged or filtered to remove suspended insoluble solids.
A salt such as potassium chloride and a nonsolvent such as methanol or isopropanol are added to the broth to flocculate the gum in the potassium form, whlch gum is then recovered by centrifugation or other solid/liquid separation *Trade Mark _g_ .

techniques. Further dissolving, reprecipitating, and washing steps are usually employed. The heteropolysaccharides thus prepared by bacteria of the genus xanthomonas on carbohydrates are normally obtained as thick viscous solutions having a dull ¦ yellow c~lor.
Xanthan gum is an excellent and widely used suspending and viscosity building agent. Some oE its particular uses are in oil well fluids, paint, sprays, and cleaning fluids.
I Xanthan gum, however, has a few disadvantages. It is very 1I difficult to disperse and wet i-n water or brine so that hydration can take place. h high degree of she~r is usually j necessary in order to wet each gurn particle. Once dispersal and wetting are accomplished, the hydration of the gum, as evidenced by the development of viscosity, is quite rapid.
Xanthan gum and guar exhibit very different rheological characteristics, having different molecular configurations, and are obtained from entirely different sources.
There is clearly a need Eor a stable dithionite I hydrosol composition having such pseudoplastic properties that 1 it is readily storable, even though subject to vibrations during tank car or tank truck shipment to a textile mill or to a pulp mill, for exarnple, and readily pumpable when thereaEter delivered to a storage tank for dilution to a desired solids 7~

. 1 content and short-term storage until needed, such as for ~I bleaching textiles or woodpulp. However, our attempts to use both guar gum and xanthan gum as suspending agents for sodium dithionite crystals have demonstrated that they have 1~ surprisingly unpredictable tendencies from each other to form 1 either gels or settled slurries, even during quiescent storage.

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SUMMARY OF THE INVENTION
It is accordingly an object of this invention to provide a process for producing stable dithionite slurries, comprising hydrosols in which dithionite crystals can be suspended for an extended period during storage and shipment and which are readily pumpable when needed.
It is also an object to provide storable and pumpable dithionite slurries as new compositions which utilize xanthan gums as the suspending component thereof.
Thus, an aspect of the invention provides a storable and pumpable aqueous slurry of sodium dithionate, which comprises solid sodium dithionate crystals, a xanthan gum, a chelate, an alkali and water, wherein the xanthan gum is in such an amount that the solid sodium dithionate crystals are stably suspended in the water and at least 0.13~ by weight.
Another aspect of the invention provides a method for making the storable and pumpable aqueous sodium dithionite slurry , which comprises:
(A) preparing a dilute hydrosol containing the xanthan gum in an amount sufficient to stably suspend sodium dithionite crystals in the final product;
(B) sequentially adding aqueous solutions of the chelate and the alkali ot the dilute hydrosol, while stirring -the said dilute hydrosol, to form an alkaline hydrosol;
(C) cooling the said alkaline hydrosol to less than about 7C in an ice bath to form a cold alkaline hydrosol;
(D) adding anhydrous sodium dithionite to the said cold alkaline hydrosol at such a rate as to maintain its .... .

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- 12a - 6S284-58 temperature below 7C and in sufficient amount to provide the said slurry.
The fermentation of carbohydrates to produce bio-synthtic water-soluble xanthan gums by the action of ~anthomonas bacteria is well known. The earliest work in this field was conducted by the United States Department of Agriculture and is described in United States Patent 3,000,790.
Particularly well known is the action of Xanthomonas campestris NRLL B-1459 on a glucose subs-trate.
Xanthomonas hydrophilic colloid (i.e., xanthan gum) is produced by transferring Xanthomonas campestris bacteria to a suitable medium and conditioning i-t to growth through two steps before allowing it to grow ina Einal medium contain-ing 3% glucose. ~fter 96 hours at 30C with suitable aeration and stirring, Xanthomonas hydrophilic colloid is produced in approximately 1% concentration.

- 12a -~ hile Xanthomonas compestris is the bacteria of choice for the purpose of producing the biosynthetic Xanthomonas hydrophilic colloid, other Xanthomonas species may be employed such as X. begoniae, X. malvacearum, X. carotenase, X. incanae, X. phaseoli, X. vesicatoria, X. papaveriocola, X. translucens, X. vasculorum, and X. hedrae.
There are numerous patents and publications describing the preparation of Xanthan gum including United States Patents Nos. 3,391,060; 3,391,061; 3,427,226; 3,455l786; 3,565,763;
3,966,618; 4,094,739, 3,773,752; 4,051,317; 4,135,979, 4,296,203; 3,919,189; 3,119,812; 3,316,241; ~,282,321;
4,299,825; and EncYclopedia of Chemical TechnoloqY, 3rd Ed.
(1980 John Wiley & Sons, pages 62-64).
Various proprietary xanthan gums, having slightly different molecular structures and rheological properties made from X. Campestris or mutants thereof are available from several manufacturers including the Kelco Company under the trade marks "Kelzan" or "Keltrol"; from Pfizer Chemical Division under the trade mark "FLOCON"; from Rhone-Poulenc under the trade mark "RHODOPOL 23"; from Meer Corporation under the trade marks "MEREZON 8" and "MERETEC 30" as well as from other manufacturers.

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~24~8~3 .1 For example, a widely available polysaccharide, produced in a culture fermentation by the microorganism Xanthomonas Campestris and sold by Kelco, a division of Merck !
and Company, ~nc., under the trademar~ Kelzan as an industrial-grade xantham gum, is a dry, cream-colored powder having a moisture content of 12~, an ash content of 10~, a specific gravity of 1.6, a bulk density of 52.4 pounds per cubic foot, a nitrogen content of 1.2%, and a mesh size of 40.
As a 1% solution in distilled water, its pH is 7.0, its surface tension is 75 dynes/crn, its viscosity is 850 cps as measured with a Brookfield LVF viscosimeter at 60 rpm, and its freezing point is 0.0C.
Another solid xanthan gum which is produced by Kelco i under the trademark K9C57 has rheological propeeties of high viscosity at low concentration, pseudoplastic flow over a wide shear rate range, and a significant yield point. Such properties indicate that the xanthan gum molecules have a rigid molecular structure. At a very low shear rate ~below one I reciprocal second), this mutant xanthan gum exhibits more Newtonian flow than the Kelzan gum. Sold as a dry powder, its solids content is 85 92~. As a 1% solution in distilled water, its pH range is 6-8 and its viscosity is 630-1000 cp.
A whole xanthan broth, having a viscosity in the range of 3500-4500 cps, which is readily pumped or poured and has an , . .

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observable yield point at biopolymer concentrations above 0.1%, is available as FLOCON Biopolymer 4800 from the Pfizer Chemical Company at biopolymer concentrations of 13.8% or 3.7%. This material is described in United States Patent 4,11g,546. ;' The aqueous broth is believed to be made from a mutant strain of Xanthomonas Campestris. It is preserved with formaldehyde and -never exceeds 1.5% concentration of unreacted sugar. It is a tan gelatinous fluid in appearance. It has an apparent content of active purified carbohydrate that is higher than commercial solid xanthan.
Solutions of FLOCON 4800 are highly pseudoplastic in nature so that the viscosity, which decreases upon exposure to high shear, is fully restored when solutions return to low-shear conditions. Solution viscosities of this biopolymer are not affected by pH in the range of 5-12.
It is to be understood that any and all forms of xanthan gum are operable in the instant invention providing the slurry does not contain an alkaline tripolyphosphate such as sodium tripolyphosphate. It is only when said tripolyphosphate is present that certain modifications must be followed as will be later explained.

*Trade Mark ~`,, , ~2~78~3 l l In general, a suspendable, pourable, and purnpable dithionite slurry free of tripolyphosphates can be prepared with as little as 0.25 pound of xanthan gum per 100 pounds of ' slurry (0.25~ xanthan content) gum. This amount is equivalent Il to about 146 pounds of commercial hydrosulfite per pound of xanthan. The slurry is non-settling and completely pourable at any desired solids content within the utility limits imposed by slurry viscosities. Such slurries, for example, are readily ~ poured and pumped at viscosities of up to 8,000 cps.
~ However, by tolerating a slight settling, a soft and di.spersible settled .slurry containiny as little as 0.20%
xanthan gum is obtained. Such a partially suspended but dispersible slurry is useful for many purposes, such as for shipping over relatively short distances. This quantity is equivalent to about 182 pounds of hydrosulfite per pound of the xanthan gum, exemplified by a slurry comprising 36.4 hydrosulfite and 0.20% xanthan gum.
Preferably, the slurries comprise at least about 20%
of hydrosulEite, a chelate, and an alkali, so that the slurries I have a pH of at least about 10. The alkali is suitably sodium hydroxide. Other bases can be used such as potassium hydroxide or soda ash. Howeve~, sodium hydroxide is preferred , 1 I;
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- 17 - 6528~-58 particularly over soda ash since less is needed on an equivalent weight basis to achieve the desi:red pH.
The method of making the storable and pumpable dithionite slurry from sodium hydrosulfite comprises, (A) preparing a dilute hydrosol (from for example a l~ aqueous solution) of xanthan gum in an amount sufficient to stably suspend sodi.um dithionite crystals in the final product, such as a solution containing about 0.13 - 0.25%
by weight of xanthan gum;
(B) sequentially adding aqueous solutions of the chelate and the alkali (which is for example 50~ NaOH solution), while stirring the above solution to form an alkaline hydrosol;
(C) cooling the resulting alkaline solution to a temperature below 45F (7C) preferably slightly above 32F
(0C) in an ice bath, to form a cold alkaline hydrosol; and (D) adding the anhydrous sodium dithionite to the cold solution at such a rate as to maintain its temperature below 45F (7C) and in sufficient amount to provide the slurry, which preferably contains 28-36% of sodium dithionite (commer-cial grade) and has a pH oE at least 10.
When the slurries are to be utilized for textilebleaching, they preferably comprise, on a weight basis, at least about 36~ of commercial sodium dithionite, at least about ~LZ~7~

3% of sodium hydroxide as the alkali, at least about 0.25% of mixed chelates, and at least about 0.25% of xanthan gum such as Xelzan grade. The viscosity is within the range of 6000-8000 cps~ Quite obviously, lesser amounts of dithionite can be used, e.g., 20% on a pure basis but the above ranges are preferred.
When the slurries are to be utilized for woodpulp bleaching, they additionally comprise at least about 3% by weight of sodium carbonate and approximately 2% by weight of sodium tripolyphosphate; see United States Patent 3,985,674.
For example, the chelate is the tetrasodium salt of ethylenedia-mine tetraacetic acid and is at least about 0.08% by weight of the slurry.
It is precisely when sodium tripolyphosphate is present that difficulties arise for reasons which are not fully understood. As has heretofore been stated, all forms of xanthan gum are operable to produce the tripolyphosphate-free slurries. However, when tripolyphosphate is present, all forms of xanthan are operable at any one data point and two forms of xanthan are operable at all data points. The two forms of xanthan which are broadly applicable are "K9C57 " and "FLOCON
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4800 " previously described. In fact, when these two forms of xanthan gum are used, they can be present in concentrations as low as 0.13 weight percent.

*Trade Mark -1&-~2478~

The one data point whereln all forms of xanthan are operable ls 28% by weight of commercial sodlum dithionite, 0.17% by welght of xanthan gum, 1.92% of Na5P3O10, 3.28 - 3.29% by weight o:E Na~CO3, 0~78% by weight of sodium hyclroxi.de, 0.08% o~ a chel~te, and water.

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EXAMPLES
Experimental work to produce a storable and then pumpable dithionite slurry utilized a formate-derived sodium ¦ dithionite (F/hydrosulfite), having an average content of 88%
to 89% of pure Na2S2O4. In many experiments, the F/hydrosulEite was used as a component of two types of commercially distributed bleaching preparations: (1) a textile bleaching system and l2) a wood pulp bleaching system, both being proprietary blends. These slurries are stable for at least three weeks during storage at 32-40F. The physical properties of the slurries are as follows:
I'extile SlurryWoodpulp Slurry Density 1.4 g/ml 1.4 g/ml pH 13 10-11 ; Appearance White White Viscosity* 6000-8000 cps3000-4000 cps Freezing Point 17-18F 16-17F
*Saybolt Viscosimeter I The lab stirrer used in these experiments was a Model HS of the Jiffy Mixer Company Inc., Irvine, California.

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. Examples l - 13 Attempts to prepare dithionite slurries which would neither settle or gel, thereby creating unpumpable shipments, 1 began by addiny eight materials having either thickeniny or charge repulsion characteristics to the dithionite slurries containing 35% F/hydrosulfite. All of these slurries were determined by observation to be unsatisfactory, as noted in Table I.

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i.l Examples 14 - 23 Experiments were then conducted with guar gum as a suspending agent for two types of E/hydrosulfite slurries that are marketed as bleaching systems. Examples 14 - 19 utilized the textile bleaching type and are given in Table II. Examples 20 - 23 utilized the woodpulp bleaching type and are given in Table III. In both tables, "~ F/hydro" indicates the weight percentage of formate-derived sodium hydrosulfi.te that was used in the slurry. As noted in these two tables, none of the 1.0 guar-thickened slurries was satisfactory, as was readily determinable by observat.ion.

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Examples 24 - 36 A 1% solu-tion of Kelzan xanthan gum in water was first prepared for Examples 24 - 26 and 29 - 35. An appropriate weighed amount o~ this solution was added to predetermined amounts of water, then an alkali and a chelate were added. The alkali was in the form of 50% NaOH. The che:Late was either the sodium salt of ethylenediamine tetraacetic acid (EDTA) or Hampene OH , a liquid mixture of chelating agents sold by the Organic Chemicals Division of W.R. Grace and Company, Nashua, New Hampshire.
The solution was cooled to 32F, and the solid F/hydrosulfite was added at such a rate as to maintain the temperature at 40-45F. The slurry was then returned to the cooling bath and checked in three days for its pourability, etc.
The use of xanthan gum is shown at various levels in a textile bleaching slurry in Table IV. The parts bv weight of the l~ xanthan solution are given as parts by weight of xanthan gum for each example. Although the complete flowing of Example 24 is preferred, the slight settling and flowing of Examples 25 and 26 is suitable for some usages, particularly for shipment situations involving relatively mild vibrations or short times of transit~

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1~ Z N r~ r~l r~ ; r~l r~ r~ rl~ ~Yl rll r~ r~l 7~8 !l Examples 27 and 28 were prepared by dissolving the xanthan gum (Kelzan grade) in 21~ of the total amount of water that was used to form a hydrosol. The NaOH solution and the chelate were then added to the remainder of the water and the admixed solution was chilled to about 35C. The crystalline F/hydrosulfite (88% Na2S2O4) was then added to the costic solution with cooling to maintain a temperature of 45C. To this aqueous slurry, the hydrosol was added to produce a stable j slurry that did not settle within three days. After two days lll of storage at about 35C, the slurry of Example 27 had a i viscosity of 7,120 cps; the slurry of Example 28 had a viscosity o~ 7,9~0 cps.
1~ Examples 29 - 36, taken with Examples 24 - 28, il demonstrate that all forms of xanthan gum are operable.
, Example 29 utilized Kelzan-~i7, Examples 30 - 32 utilized Kelzan-S, Example 35 utilized K9C57 - all products oE Kelco.
, Examples 33 and 35 utilized FLOCON (3.7%) and Examples 34 and l 36 utilized FLOCON (13.84~) - both products of Pfizer.

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Examples 37 - 48 The following examples will illustrate that various concentrations oE commercial sodium hydrosulfite (i.e., 88-89 pure) can be used to form pumpable slurries with xanthan gum 1~ whereas guar is totally inoperable. The results are shown in Table V.

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~30--~2~7~8 Compositions Containiny Tripolyphosphate As indicated earlier, compositions containing polyphosphates show a scatter of results with the exception of FLOCO~ of Pfizer or K9CS7 of Kelco except at one data point where all forms of Kelzan gum are operable. The following Examples will illustrate this.

l - 31 -~ . , ~2~

Examples 49 - 65 Experiments to produce a storable but pumpable slurry, using the woodpulp bleaching system, comE~rised first making a ¦ 1% aqueous solution of Kelzan gum and then adding an appropriate amount of this solution to water for Examples 48 -63. The next additions to the water were solutions of sodium carbonate, sodium tripolyphosphate, chelate, and then 50% NaOH
solution. The entiee solution was then cooled to 32F in an ~l ice bath. The required amount of solid (crystalline ' F/hydrosulfite) was added at such a rate so as to maintain the temperature below 45F. The slurry was then again cooled in the ice bath for three days and observed for separation, settling, and pourability.
¦I Examples 64 and 65 were prepared similarly to Examples 27 and 28 by dissolving the xanthan gum in 12~ of the total water to form a hydrosol and then adding the Na2CO3, the Na5P3O10, the NaOH solution, and the chelate. Then the solution was cllilled to about 35C. The solid, crystalline ~ F/hydrosulfite, was added with cooling to rnaintain a I temperature of 45C. To this aqueous slurry, the xanthan ! hydrosol was added to produce stable slurries which had not settled after three days of storage. The slurry of Example 64 had a density of 1.37 g/ml and a viscosity of 3,900 cps. The slurry of ExaMple 65 had a viscosi:y of 4,040 cps.

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! I ' , The following examples in Table VI illustrate the criticality of the xanthan concentration, all observations being made after three days in a wet ice bath and all I percentages being given on a weight basis, without correcting 'I for the impurity in the F/hydrosulfite. Correcting for the ;; impurity in the F/hydrosulfite by multiplying by 0.885 as an average value, the amount of pure Na2S2O4 in the F/hydrosulfite of the examples is as follows:

Pure F/hydro, % ~a2_2_4 27 23.9 ` 2~3 2~.3 26.6 3.l 27.9 36.4 32.2 As can be seen, only Examples 49, 63 and 64 gave acceptable ~ results.

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xamples 66 - 70 The following two examples in Tables VII illustrate the use of Pfizer's FLOCON Biopolymer 4800 as a 13.8% FLOCON
broth in Example 66 and a 3~7% FLOCON broth in Example 67 at 0.19% Xanthan with a woodpulp slurry, both having been made with F/hydrosulfite.
Experiments with K9C57 xanthan gum are given as Examples 68 - 70 in Table VIII to illustrate the use of this ,I xanthan in woodpulp slurries that would ordinarily set up or be ll in some way unstahle with Kelzan gum. The ratio in the next-to-last column i.s obtained by dividing the total mols of sodium tripolyphosphate (expressed as Na5P3O10) plus mols Na2CO3 into the mols of 100~ Na2S2O4. Ideally, this ratio should be around 4.0 plus/minus 0.2 for the use of Kelzan. Excursions above and below this ratio require the use of K9C57 or the FLOCON Biopolymer.
On a weight basis, lt appears that biopolymer broth is approximately equivalent to the solid xanthan which is sold under the trademark K~C57 in its ability to produce a l hydrosulfite slurry of acceptable storage and pumping , capability.
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--37-- .

Claims (24)

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

1. A storable and pumpable aqueous slurry of sodium dithionate, which comprises solid sodium dithionate crystals, a xanthan gum, a chelate, an alkali and water, wherein the xanthan gum is in such an amount that the solid sodium dithionate crystals are stably suspended in the water and at least 0.13% by weight.

2. The slurry of claim 1, which contains at least about 0.13% by weight of the xanthan gum.

3. The slurry of claim 1, which is free of sodium tripolyphosphate and contains at least about 0.20% by weight of the xanthan gum, at least about 20% by weight of pure sodium dithionate and which has a pH of at least about 10.

4. The slurry of claim 3, wherein the sodium dithionite is commercial grade sodium dithionite.

5. The slurry of claim 1, which contains about 0.13 to about 0.25% of the xanthan gum, about 20 to about 36.5%
by weight of sodium dithionate and which has a pH of at least 10.

6. The slurry of claim 2, 3 or 5, wherein the alkali is sodium hydroxide, potassium hydroxide or soda ash.

7. The slurry of claim 2, 3 or 5, wherein the chelate is the tetrasodium salt of ethylenediamine tetraacetic acid.

8. The slurry of claim 2, which also contains sodium tripolyphosphate, contains carbonate as the alkali.

9. The slurry of claim 1, which comprises at least about 36% by weight of commercial grade sodium dithionite, at least about 3% by weight of sodium hydroxide as the alkali, at least about 0.25% by weight of the chelate and at least about 0.25% by weight of the xanthan gum.

10. The slurry of claim 1, which comprises at least about 20% by weight of sodium dithionite, a chelate, an alkali, and at least about 0.13% by weight of a xanthan gum which is commercial K9C57 (a trademark of Kelco) or FLOCON 4800 (a trademark of Pfizer), the said slurry having a pH of at least 10.

11. The slurry of claim 1, which consists essentially of 28% by weight of commercial grade sodium dithionite, 0.17% by weight of the xanthan gum, 1.92% by weight of sodium tripolyphosphate, 3.28 - 3.29% by weight of sodium carbonate, 0.78% by weight of sodium hydroxide, 0.08% of the chelate and the balance of water.

12. The slurry of claim 1, which comprises commercial grade sodium dithionite, sodium tripolyphosphate, sodium carbonate, the chelate and a xanthan gum formed by bacteria of the species Xanthomonas campestris, wherein the mols of 100% sodium dithionite divided by the sum of mols of sodium tripolyphosphate plus the mols of sodium carbonate is below or above the range 3.8 -4.2.

13. The slurry of claim 12, wherein the said xanthan gum is commercial K9C57 (a trademark of Kelco) or FLOCON 4800 (a trademark of Pfizer).

14. A method for making the storable and pumpable aqueous sodium dithionite slurry defined in claim 1, which comprises:
(A) preparing a dilute hydrosol containing the xanthan gum in an amount sufficient to stably suspend sodium dithionite crystals in the final product;
(B) sequentially adding aqueous solutions of the chelate and the alkali to the dilute hydrosol, while stirring the said dilute hydrosol, to form an alkaline hydrosol;
(C) cooling the said alkaline hydrosol to less than about 7°C in an ice bath to form a cold alkaline hydrosol;
(D) adding anhydrous sodium dithionite to the said cold alkaline hydrosol at such a rate as to maintain its temperature below 7°C and in sufficient amount to provide the said slurry.

15. The method of claim 14 for making a storable and pumpable aqueous tripolyphosphate-free dithionite slurry from crystalline dithionite, which comprises, (A) preparing a dilute hydrosol containing about 0.20 - 0.25% by weight of xanthan gum based on final product;
(B) sequentially adding aqueous solutions of the chelate and sodium hydroxide or potassium hydroxide to the dilute hydrosol, while stirring the said dilute hydrosol to form an alkaline hydrosol;
(C) cooling the said alkaline hydrosol to less than about 7°C in an ice bath to form a cold hydrosol; and (D) adding commercial sodium dithionite to the said cold hydrosol at such a rate as to maintain its temperature below 7°C and in sufficient amount to provide the said dithionite slurry containing at least about 25% of pure sodium dithionite and having a pH of at least 10.

16. The method of claim 15, wherein the said hydrosol of step A is added to a slurry of said commercial sodium dithionite as the last step thereof.

17. The method of claim 15, wherein the said dilute hydrosol is prepared by making a concentrated hydrosol, as a 1% aqueous solution of the xanthan gum, and adding the concentrated hydrosol to water while stirring.

18. The method of claim 15, wherein the resulting slurry is a textile bleaching composition, comprising about 36% of commercial sodium dithionite, at least about 3% of sodium hydroxide or potassium hydroxide, at least about 0.25% of the chelate and at least about 0.25% of the xanthan gum.

19. The method of claim 14 for making a storable and pumpable aqueous dithionite slurry from crystalline dithionite, which comprises:
(A) preparing a dilute hydrosol containing about 0.13 - 0.25% by weight of xanthan gum which is commercial K9C57 (a trademark of Kelco) or FLOCON 4800 (a trademark of Pfizer) based on final product;
(B) sequentially adding aqueous solutions of a chelate and sodium hydroxide or potassium hydroxide to the dilute hydrosol, while stirring the said dilute hydrosol to form an alkaline hydrosol;

(C) cooling the alkaline hydrosol to 0°C in an ice bath to form a cold hydrosol; and (D) adding commercial sodium dithionite to the cold hydrosol at such a rate as to maintain its temperature below 7°C and in sufficient amount to provide the said dithionite slurry containing at least about 25% of pure sodium dithionite and having a pH of at least 10.

20. The method of claim 19, wherein the commercial sodium dithionite is anhydrous and is added in such an amount that the resulting slurry contains from 28 to 36% by weight of the commercial sodium dithionite, provided that the slurry contains at least about 25% of pure sodium dithionite.

21. The method of claim 19, wherein the ingredients are employed in such amounts that the resulting slurry contains on a weight basis:
at least about 36% of commercial grade sodium dithionite;
at least about 3% of sodium or potassium hydroxide;
at least about 0.25% of the chelate; and at least about 0.25% of the xanthan gum.

22. The method of claim 19, wherein the said hydrosol of step A is added to a slurry of the said commercial sodium dithionite as the last step thereof.

23. The method of claim 22, wherein the said dilute hydrosol is prepared by making a concentrated hydrosol, as a 1% aqueous solution of said xanthan gum, and adding the said concetrated hydrosol to water while stirring.

24. The method of claim 23, wherein the said slurry is a woodpulp bleaching composition and wherein step B add-itionally comprises sequentially adding solutions of sodium carbonate and sodium tripolyphosphate to the said dilute hydro-sol while stirring.