CA2050838A1

CA2050838A1 - Process for making improved sodium dithionite suspensions

Info

Publication number: CA2050838A1
Application number: CA 2050838
Authority: CA
Inventors: Leonard Diaddario Jr.
Original assignee: Individual
Current assignee: CNA Holdings LLC
Priority date: 1990-09-07
Filing date: 1991-09-06
Publication date: 1992-03-08
Also published as: GB9119024D0; GB2247674A; DE4129738A1

Abstract

ABSTRACT

A process for producing stabilized suspensions or slurries of sodium dithionite is disclosed wherein the process comprises mixing water, sodium hydroxide, chelate, a modified xanthan gum and sodium dithionite in any order, provided that the temperature of the mixture is kept at or below 45 degrees F
during and after the addition of sodium dithionite.

Description

20~0~38 PROC15~8 FOR MA~ING I~IPROVE:D BODIIJ~I DIq!~IONIq~E: BlJ8PENBIo21~;

Backqround of the Invention This invention relates to aqueous 61urries and particularly to non-settling and flowable aqueous 6uspensions or slurries of sodium dithionite that remain in pumpable form without significant expansion, settling or gelation.
- Sodium dithionite, also called sodium hydrosulfite or Na2S204, is a powerful reducing agent that has historically been used for bleaching, particularly in the bleaching of textiles and wood pulps, such as ground wood and semi-chemical pulps.
Various problems have been recited for the preparation and handllng of sodium di~hionite or compositions, slurries, ~uspensions, solutions, etc., containing sodium dithionite.
These include the problems of decomposition, which require the sodium dithionite to be stored and shipped as a suspension, ~olution or slurry, the expense o preparing such suspensions, solutions or slurries, the expense of shipping such suspensions, solutions or slurrie6, and the difficulties encountered in maintaining adequate suspension so that pumping can be done to empty the transport truck.
A variety of problems have been discussed in the patent literature and attempts made to create improved 61urries and disper6ions or improve the stability of sodium dithionite itself. These patents include U.S. Patent No~. 3,536,445;
3,804,944; 3,839,217; 3,839,218; and 4,283,303.

20~08~8 u.s. Patent No. 4,729,~86 to Little, et al, describes ~odium dithionite 61urries or 6uspen~ions which are non-settling during shipment and pumpable after storage at 32-40 degrees F. These slurries or suspensions comprise, on a weight basis, 34% ~odium dithionite, 0.17% of a xanthan g~m, 5.5-6.5% of 50~ sodium hydroxide, ~.26% of a chelate, and 1.96% of sodium tripolyphosphate.
U.S. Patent No. 4,534,g54 to Little, et al describes agueous ~odium dithionite slurries which are non-settling and pumpable, and which contain at least 25% by weight of crystalline pure sodium dithionite and at least about 0.13% by weight of a xanthan gum. Textile bleaching compositions and wood pulp bleaching compositions are also disclosed.
In many of the above-described formulations, viscosity modif~ers are used to prevent the dispersed solids from ~ettling. Numerous natural and synthetic gums are used for this purpose, including xanthan gums.
Xanthan gums are excellent and widely used suspending and viscosity building agents. Xanthan gums are polysaccharides having a repeating unit containing five sugar residues: two glucose, two mannose and one glucuronic acid (See "Xanthan Gum-Natural Biogum for Scientific Water Control" Third Edition, published by Kelco Division, ~erck & Co., Inc, Rahway, New Jersey, 1988 for further information on xanthan gums). Various proprietary xanthan gums having ~lightlY
different molecular structures and rheological properties are available from several manufacturers. Xanthan gums in general, however, have a few disadvantages. It is very difficult to disperse and wet them in water or brine so that hydration can take place. A high degree of 6hear is usually necessary in order to wet each gum particle. Once dispersed ~nd wetted, the hydration of the gum is manifested by a rapid increase in the viscosity. Additionally, a typical xanthan gum ~uch as Æ LZAN4 (Kelco Division of Merck & Co.) does not readily hydrolyze under alkaline pH conditlons. This dictates the process 6equence which must be followed in order to produce an aqueous sodium dithionite suspension. The use of a xanthan gum such as KELZAN~ requires that the xanthan gum be dissolved and hydrolyzed to produce a hydrosol prior to mixing with alkali.
Several modifled versions of xanthan gum are available which are characterized by ease of dispersion under low shear mixing. Some modified xanthan gums do not rapidly or readily hydrolyze unless triggered by the addition of alkali. A
commercially available example of a xanthan gum with these properties (also called "modified x~nthan gum" herein) is KELZAN S (Kelco Division of Merck ~ Co.). (KELZAN~ S is listed as having a Chemical Abstracts' Number of 86924-54-1 and descrlbed as xanthan gum acetate, however, some references have described it as being glyoxal-coated.) There has still been difficulty, however, in obtaining ~tabilized aqueous slurries of 60dium dithionite for textile bleaching and for wood pulp bleaching which do not experience gelling problems and which do not result in batch variation, 20~08~

especially with regard to the property known as settling.
Add$tionally, it would be advantageous to h~ve a process for forming ~table, aqueous slurries of sodium dithionite for textile or wood pulp bleaching which have greater flexibility in the order of the process steps and which provides a product with reduced variation of properties from batch to batch.

Obiects and Summary of the Invention The object of this invention is to provide a process for producing stabilized aqueous 6uspensions or slurries of sodium dithionite which are non-settling and flowable, which remain in pumpable form without significant expansion, settling or gelation. It is a further object of this invention to provide à process which i6 flexible in the order in which the processing steps are performed and which yields a consistent product under 6uch flexible processing conditions.
It has been discovered that by utilizing a modified xanthan gum known as KELZAN~ S, a new process for the preparation of an aqueous sodium dithionite ~lurry can be created which overcomes the processing difficulties and product variability produced by procedures disclosed in prior art processes, such as those disclosed in U.S. Patent Nos.
4,534,954 and 4,729,886.
The process comprises forming a stabilized 61urry or suspension of sodium dithionite by mixing water, ~odium hydroxide, chelate, a modified xanthan gum and sodium dithionite with cooling. These materials may be combined in 2Q~0~8 any order to form the slurry or suspension as long as the temperature of t~e mixture is kept at or below 45 degrees F
during and after the addition of sodium dithionite.
Optionally, if the material i~ to be u~ed for bleaching wood pulp, sodium tripolyphosphate also called Na5P3010 or, STPP, may be added at any point in the method. Unlike U.S.
Patent No. 4,729,886, the point in the process at which sodium tripolyphosphate is added is not critical. Also, other additives may b~ added such as biocides, preservatives, etc., as may be known in the art.

De~ailç~ scription of the I~vention ~ he method of this invention comprises forming a stabilized suspension or slurry of eodium dithionite by mixing water, sodium hydroxide, chelate, a modified xanthan gum and sodium dithionite with cooling, and the ability to combine the ingredients in no particular order to form the slurry as long as the temperature of the mixture is kept at or below 45 degrees F, and preferably below 45 degrees F, during and after the addition of sodium dithionite.
This method may be practiced in a variety of ways such as the following:
~ çtho~ ~
(1) mixing appropriate amounts of water, sodium hydroxide and chelate and cooling the mixture to below 45 degrees F; (2) adding solid sodium dithionite (for example, crystalline sodium dithionite) to form a suspension at a rate to maintain 20~08~g the temperature below 45 degree F.; (3) adding to the 6uspension a modified xanthan gum which can hydrolyze readily in alkali; and (4) cooling the suspension and sto~ing it a~ a temperature below 40 degrees F.
Method B
(1) adding sodium hydroxide and chelate to a source of liquid sodium dithionite; (2) adding solid 60dium dithionite (such as crystalline sodium dithionite) to form a suspension;
and (3) stabilizing the suspension with a dispersible xanthan gum which can hydrolyze readily in alkali.
Method C
(1) adding sodium hydroxide and chelate to liquid sodium dithionite and forming a suspension by flash evaporation or vacuum evaporation; and (2) 6tabilizing ths suspension with a dispersible xanthan gum which can hydrolyze readily in alkali.
The slurry can be prepared from sodium dithionite manufactured by any known process, ~uch as zinc-derived, ~ormate-derived, borohydride-derived and electrochemically-derived sodium dithionite. While Na2S204 of varying purity may be used, it is preferred that the sodium dithionite be at least about 85-88% pure, especially at least 88% pure. One ~uch example of formate-derived sodium dithionite is VIRTEX~D
(~oechst Celanese, Specialty Chemicals Group). The rate of addition of sodium dithionite is dependent upon the amount of cooling capacity of the process eguipment, guch as the capacity of a heat exchanger used.

20~08~8 The sodium dithionite may be added in various ways such as in ~olution (also called liquid sodium dithionite herein) or in colid form, for example, crystalline form. Solutions may be made in varying concentrations, ~uch as up to 24%

Na2S24 -Under optimal conditions, processing times as brief as five hours per 40,000 pound batch have been achieved for the process of this invention. This compares to eight to twelve hour process times when the prior art process described in Example 7 of U.S. ~atent No. 4,729,886 was scaled up to a 40,000 pound batch size.
The chelating agents used are not critical; typical chelating agents include nitriletriacetic acid (NTA), ethylenediaminetetraacetic acid tEDTA), hydroxyethylene-diaminetriacetic acid, diethylenetriaminepentaacetic acid, and the sodium and potassium 6alts of these agents. one example of a particular chelating agent is Worthene~ 120, a 40~
solution of tetrasodium ethylenediaminetetraacetic acid trisodium salt (Na3HEDTA) (available from Ciba Geigy). While many of the examples herein use a 40~ ~olution of chelate, it i~ within the scope o~ this invention, to use greater or lesser concentrations of chelating agents and vary the amount of water used to add an equivalent amount of chelate.
While either sodium hydroxide or potassium hydroxide may be used, sodium hydroxide is preferred. A convenient concentration of NaOH io a 50% solution, ~ut it is within the scope of this invention to use greater or lesser 20~8~8 concentrations and vary the amount of water used to add an equivalent amount of NaOH.
In practicing the method of this invention, suspensions may be made using from about 5% to about 9% of a 50% solution of NaOH, from about 0.2% to about 0.8% of a 40% solution of a chelate such as Na3HEDTA, from about 30% to about 50% of Na2S204 ~such as a commercial grade having at least 88%
purity) and from about 0.1% to about 0.4% of a modified xanthan gum (that is, a dispersible xanthan gum which can hydrolyze readily in alkali such as KELZAN~ S~, with the remainder of the composition being composed of water up to a 100% weight basis. All percents are by weight percent. More particular ranges and preferred ranges are given in Table A.

TA~E A
More ~Q~
Componen~,P~e,ferred Ranae ~ er~e~_RAnqe NaOH (50% solution)6-8% 6.5-7.5%
Chelate (40% solution) 0.3-0.6% 0.45-0.55%

2S24 35-45% 38-42%
Modified Xanthan Gum 0.15-0.35% 0.2-0.3%

Water ' Remainder Remainder up to 100% up to 100%

These suspensions are suitable for textile bleaching ~pplications.
In practicing the method of this invention, guspensions suitable for use in wood pulp bleaching applications may be made using from about 5% to about 8~ of a 50% solution of 2Q~0838 NaOH, from about 0.2% to about 0.8S of a 40~ solution of a chealate such as Na3HEDTA, from about 30% to ~bout 4s% of Na2S204 (such as a commercial grade having at least 88%
purity), from about 4% to about 6~ of sodium tripolyphosphate, ~rom about 0.1% to about 0.4% of a modified xanthan gum (that is, a dispersible xanthan gum which can hydrolyze readily in alkali such as Æ LZAN~ S), with the remainder of the composition being composed of water up to a 100% weight basis.
All percents are by weight percent. More particular ranges and preferred ranges are given in Table B.

~A~L~ ~
~Q~ Most Com~o~Prefer~ed Ranae ,Preferred Range NaOH (50% solution)6.0-7.5% 6.5-7.0%
Chelate (40% solut~on) 0.3-0.6% 0.45-0.55%
Na2S204 35-42% 38-40%
S~PP 4.5-s.5% 4.8-5.2%
Modified Xanthan Gum0.15-0.35% 0.2-0.3%

Water Remainder Remainder up to 100% up to 100%

In using the method of this invention, ,examples of formulations of suspensions which can be made and which are useful in the bleaching of wood pulp are as follow~:

2~08'~8 compon~ent Partic~lar Ranae More Particular R~nge H20 45.77-57.93~ 47.30-51.37%
NaOH (50% solution)5.7-7.5% 6.5-7.2%
Na3HEDTA (40~ solution) 0.25-0.55% 0.48-0.52%
Na2S204 (purity greater 34.0-42.5~ 38.5-41.5%
than or equal to 88%) Na5P3010 2.0-3.3% 2.9-3.1%

Modified Xanthan Gum0.12-0.38% 0.25-0.38%
(KELZAN~ S) (All percents are by weight) An essential property of the 61urries made by the method of this invention are their settling characteristics, i.e., minimizing the extent to which the dispersed solids settle out under gravity. The specific product application defines the physical stability ~equirements of the ~uspension. In the development and formulation of these products, correct rheological properties must be imparted to ensure that the solid phase remains suspended for the desired shelf-life of the product. An acceptable shelf-life for such slurries is commonly regarded as being at least about two weeks.
The physical stability of the agueous Bodium dithionite 6uspensions were evaluated on the basis of the percent settling after 15 and 30-day storage periods and ~he rate of settling over the 30-day storage period. Three to six 16 ounce samples were retained from manufacturing trials and were held at 40 degrees ~ for 30 days. At five-day intervals the 20~08~8 total height of the sample and the height of the clear 6upernatant layer were measured. The percent 6ettling is equal to the height of the clear layer divided by the total height of the sample times 100%. From a plot of the average percent settling versus the age of the 6ample, the settling rate is calculated by the least squares evaluation of the following equation:

% Settling = (Settling Rate) (Sample Age in Days) Acceptable settling rates are less than or equal to 0.7% per day, and good settling rates are less than or equal to 0.25%
per day.
Suspensions which were found to be acceptable after a 30-day storage period contained, on a weight basis, 0.1-.4%
modified xanthan gum and 38-48% solids with the remainder of the formulation being water. The solids are composed of sodium dithionite, sodium hydroxide, chelate and any additional components necessary for the specific end use application, such as sodium tripolyphosphate (STPP).
A typical suspension formulation useful for wood pulp bleaching consists of 0.25% modified xanthan gum (Æ LZAN~ S), 40.0% commercial grade formate-derived ~odium dithionite, 0.50% chelate (40% Na3HED~A solution), 7.00~ 60dium hydroxide (50% solution), 3.00% sodium tripolyphosphate, ~nd 49.25%
water.

~Q~O~

A typical ~uspension formulation for textile bleaching applications consists of 0.25% modified xanthan gum (XELZAN~
S), 40.00% commercial grade formate-derived 60dium dithionite, 0.50% chelate (40~ Na3HEDTA solution), 6.50S 60dium hydroxide (50S ~olution), and 52.75% water.
Experimental studies were conducted to define the optimum ranges for the components of the agueous sodium dithionite suspension. The object of these studies was to obtain a formulation which produced a suspension which was physically ~table for 30 days when stored at 40 degrees F and which still remained flowable and pumpable. Approximately 40,000 pound test batches o~ the suspensions were prepared as described in the following Examples. For all the Examples described herein the equlpment used was a ~acketed, 3500 g~llon stainless steel tank (also called a reactor~ agitated with: a Lightnin Series 70 mixer Model 75S25 with a right-angle double reduction gear r~ducer and rigid/rigid coupling to the lower shaft; a 3.5 inch diameter by 144 inch shaft; a 30 inch diameter, 4-blade, upperl axial impeller Model A200; and a lower 26 inch diameterj 8-blade, radial-~low impeller with stabilizer Model R100 rotating at 100 RPM. Additional cooling capacity was provided by an American Vicarb Corporation plate heat exchanger Model V55 located in a recycle loop. ~he reaction mixture was circulated from the make tank, through the plate heat exchanger, and back to the maks tank with a Waukesha pump. The xanthan gum was dispersed using a Schutte and Koerting Type 267 eductor.

20~0~38 In general, Examples 1-19 were made to optimize the composition of the ~odium dithionite slurries useful for wood pulp bleaching applications and Examples 20-28 were made to optimize the compositions of such slurries for textile bleaching applications. Examples 29-34 were made as refined optimized slurries for the bleaching of wood pulp. Examples 35-41 are comparative examples made by the method of Example 7 of U.S. Patent No. 4,729,886.

~YAMPLE 1 An approximately 40,000 pound batch of a suspension of sodium dithionite was prepared by adding 11 gallons of Na3HEDTA, 40% solution (Worthene~ 120 from Ciba Geigy) and 187 gallons of a 50% eodium hydroxide 601ution to 2250 gallons of water in a ~acketed, 3500 gallon, stainless steel reactor as described above with agitation. All materials were at normal storage temperatures (about ambient) when they were added.
The mixture was cooled to 30-35 degrees F while being agitated. While maintaining the temperature of the slurry at or below 45 degrees F, 14,180 pounds of sodium dithionite were added slowly with good agitation to insure adequate wetting.
After all the sodium dithionite wzs added, 800 pounds of sodium tripolyphosphate was added while ctirring was maintained. Next, 100 pounds of a modified xanthan gum (KELZAN~ S) was added to the reaction vessel through an eductor funnel with 262 gallons of water with stirring. This 20~08~8 addition of modified xanthan gum was followed by stirring the mixture for at least one hour at maximum agitation to allow the modified xanthan gum to hydrolyze. Samples were stored in a low temperature incubator at 40 degrees F to test for physical and chemical stability.

Examples 2-19 were prepared using the method described for Example 1, but substituting the amounts of material shown in Table I. The settling data is also listed in Table I.

.`8 ,.
U~ O O ~D ~r ~ t~ o r~ ~ o u~ O ~ O
,a ~ ~ u~ In o o~ ~1 ~ O ~ Ul ~ a~ D ~ O O
~ 5 .~ ~ ~1 ~1 ~I N N .-1 0 ~1 U~ O ~ ~ U~ _~ ~ O
J~ ~ ..................
U~Y~ OOOOOOOOOOC~OOOOOOO
V V ~ V
O ~q tJ~ ~ al ,1: .C r .C
a ., ~
~U _ 1~ ~ ~ O O t` tO O Q N ~ O u7 v~
U - -- ~ O ~ CD It~ N 1~ r~ N ~ O Q O 61 :~: X ~ `~
O U~ V V V V
'a :- 3 3 3 a _ 0 ~ O O O O ~r O t~ o IJ ~ D. ~ -- O tO ~1 ~q i N 1 0 0 ~ ~O O O~ U~ f~ O ~ ~ ' c ~ In :~ ~ 3 U~
c c 6~ O O O ~ O O O O O O O O O O O O O 0 3 4-~ ,q t~ .4 111 0 1~ 0 1~ 0 It O U~ o U~ O ~ O U~ E! E E e ~I x x ~ ~ ~ .. v ~v ~ ~ In o o o o o o o o o o o o o o o o o o '~ '~ '~ '~
i~ E~ ~ o~ c c ~ 3 c c c o o ~ u~ o o o o o o u~ o o u~ u~ o o ~ r b~
~ u) co u~ u~ o o ~ ~ o o ~ ~ ~ r r 1 r o~ cn o o o o u~ ~,~ r 0 o~ o o o o o r~ O O

30 :~ 3 3 _~ -- V ~J V V
Il ~ 1 r1 0 0 0 0 3 3 3 3 _ O O O O
Z
0 ~ 0 ~ C C
~ _ ~
:~ O _1 0 ~ 0 0 o u~ ra r r r ~ ~ o o o o ~D o o o o ~ ~ ~ ~ ~v v v v c~ co N 1~ ~ N O O O O ~ ~
O ~1 ~ ~1 ~1 ~1 ~I N N N N N N N N N ~ N C C C C
U~
.. .. .. ..
I 1~ o o o o ~ ~ o f~ o D O _~ 0 In O ~r O O 11'7 U~ N O~ O O~ O 0' 0 ~ O ~ ~
v ,a r- ul r u~ 1~ N ~r ~ N ~1 N r~ N U~ N 10 3 ~ N N N N N N N ~ N N Ol N N N N N N ~1 fll f C~. ~ ~2.
¦~i N ~ ~ u~ O ~ 8 g ~
X ~ ~ ~ ~ ~ ~ ~ ~ ~ ~

2 ~

~X~MPLE8 20 - 28 Examples 20-28 were made following the method of Example 1 using the amounts of materials listed in Table II, but without the addition of sodium tripolyphosphate. The settling data i5 also listed in Table II.

o ~ o o _~ ~ ~ ~ O
o ~
........
~ ~ ~ o o ~ o o o o o o v v o ~ u~ v~

.~) ~ c ~ -- ~ ~i ~ y y ~ .e ~
~o 3 ~
_ U~ D ~ ~ CO N ~1 0 0 ~J Q, Cl _ N ~ O 0~ O ~ r ~

~\ O
.~ ~ ~ 0 O O O O O O O O O
-1 ~ ,C N ,.q O O O O O O O O O V V
~ ~E ~ --~ C 1: 1 O _ ~ 0 N ~ 0 0 ~"~ O ~ OD
U~ .4 11~ O 1` C~ 1` t` N ~ C\ O
~ _~ ~r U') ~ d ~ ~ ~

C: ~ ~
~ r~ V V
~ 4~ ~i . Z O O O
C C
OU~ ~OOOOOOOO 0~
Z O~> _ N N N N N N ~ N N V V
O t:

C~ _1 U~ N N ' ' O ~ ~D
N N N N N N ~ N

~ I e e N N N N N N N N t'~l ~

8~

~A~PLE 29 An approximately 40,000 pound batch of a suspension of sodium dithionite was prepared by adding 18 gallons of Na3HEDTA (40% solution) (Worthene~ 120), and 220 gallons of a 50% sodium hydroxide solution to 1950 gallons of water in a ~acketed, 350~ gallon, 6tainless ~teel reactor with agitation as described above. The initial temperatures of all ingredients were normal storage temperatures. The mixture was cooled to 30-35 degrees F while being agitated. While maintaining the temperature of the slurry at or below 45 degrees F, 17,160 pounds of ~odium dithionite were added slowly with good agitation to ensure adequate wetting. After all the sodium dithionite was added, 1200 pounds of sodium tripolyphosphate were added while stirring was maintained.
Next, 100 pounds of a modified xanthan gum ~KELZAN~ S) were added to the reaction vessel through an Qductor funnel with 411 gallons of water while stirring was continued. ~his addition of modified xanthan gum was followed by stirring the mixture for at least one hour at maximum agitation to allow the modified xanthan gum to hydrolyze. Samples were stored in a low temperature incubator at 40 degrees F to test for physical and chemical stability.

~2AMPL~B 30 - 34 Examples 30-34 were prepared using the Dethod described for Example 29, but substitu~ing the amounts of material ~hown in Table III. The settling data is ~lso given in Table III.

8~

_ o~ ~ o o~ o ~
, ~ ~ ~ o ~ o U-, o o o o o o o o o o o o U~ X
~a 0 o a n ~ ~ ~ o ~ o o~
I ~ ~ ~ O ~1 0 0 la~o o a oooooo 0~I ~ r~ooooo o.

V~ ~ _ ~1 ~1 ~ H ~1 ~1 H U~ ~ X ~ --3 U~ _ O O O o o o O

O _ o o In o o o 01 ~ t` ~ 0 0 0 ~n ,4 _I G H ~ ~ O
Z; ~ ~ ~ ~

a o _l a:~ 0 ~
o Z ~r :~O ~ oooooo o u~ ~a ~ 1 N
~ ~ ~ t~ ~ ~ ~ ~ ~
o U~
_~ ~ ~
~D ~ ~ ~D ~ ~D
~ ~ ~ ~ ~ r~
3 _ ~ t'`l N t~

O~ O ~1 r~

2~

~A~PL~8 35 - ~1 For comparision, batches of aqueous ~odium dithionite cuspensions useful for wood pulp bleaching applications were prepared by the method described in Example 7 of U.S. Patent No. 4,729,886.

~XAMPLE 35 Using the same equipment described in the previous Examples, 100 pounds of xanthan gum (KELZAN~ was dispersed in 1512 gallons of water followed by at least one hour of mixing and circulating to allow the xanthan gum to hydrolyze. Once all the water was added to the slurry make tank and the xanthan gum was hydrolyzed, 6 gallons of Na3HEDTA (40%
solution) and 103 gallons of NaOH (50% ~olution) were added to the slurry make tank with stirring. The mixture was cooled to 30-35 degrees F while being agitated and circulated. While maintaining the temperature oS the slurry at or below 45 degrees F, 7570 pounds of a formate-derived Na2S204 was added slowly with agitation and circulation to insure adequate wetting and temperature control.
After all of the solids were added the suspension was allowed to mix and circulate for at lea~t one hour be~ore ~amples were obtained and stored in a low temperature 8~

incubator at 40 degrees F to track the physical and chemical ~tability of the suspension.

E~AMPLE8 36 - 41 Examples 36-41 were prepared by the method of Example 35 using the amounts of material given in Table IV.
~ he 14-day settling data for the Examples 36-41 given in Table IV showed a large amount of variability. By comparison, Examples 29-34 prepared by the process of this invention had much les6 variability in 6ettling properties and ~howed very little or no separation after 15 and 30-day storage periods.

2~08~
o o~ r t~ ~ o ~ o~
~ 1; C ~ dl . . . . . .
~ la c~ a) a ~I N r1 C C
-U~

.C N .. 4 If) If~ 0 In O
_I ~

1:4 Ul I O O O O ~ O
P~ R ~ r~ ~ ~ o o o U~ _ .

'~ o o o ~ o o o ul I~ r 1~ ~ o r~
u~ ~ ~ o _I I~
i~3 Z ~

E~ ,1 _ a o _~ ~
o Z ~r c _I
O _ r~ ~ ~ ~ r r 1~
o o o o a~ co O ~ D`
~o o Z ~
Ll ~ ~ N ~ ~ O ~D O
J~ ~a u~ r ~I
In ~ eo ~ O
X ~ ~ ~ ~ ~ ~r

Claims

1. A method of making a stabilized suspension of sodium dithionite, wherein the method comprises mixing, in any order, water, sodium hydroxide, chelate, sodium dithionite, and modified xanthan gum, wherein said gum can hydrolyze readily when exposed to alkaline conditions, and provided said mixture is maintained at a temperature less than or equal to 45 degrees F during and after the addition of sodium dithionite.

2. The method as claimed in Claim 1 wherein the amounts of materials mixed to form said suspension comprise NaOH in an amount equivalent to between about 5% to about 9% of a 50% solution of NaOH, chelate in an amount equivalent to between about 0.2% to about 0.8% of a 40% solution of chelate, from about 30% to about 50% of Na2S2O4, from about 0.1% to about 0.4% of said modified xanthan gum, and water, wherein all percents are weight percents and the total is 100%.

3. The method as claimed in Claim 1 wherein the amounts of materials mixed to form said suspension comprise NaOH in an amount equivalent to between about 5% to about 8% of a 50% solution of NaOH, chelate in an amount equivalent to between about 0.2% to about 0.8% of a 40% solution of chelate, from about 30% to about 45% of Na2S2O4, from about 4% to about 6% of sodium tripolyphosphate, from about 0.1% to about 0.4% of said modified xanthan gum, and water, wherein all percents are weight percents and the total is 100%.

4. The method as claimed in Claim 2 wherein the amounts of material comprise from about 6% to about 8% of said NaOH, from about 0.3% to about 0.6% of said chelate, from about 35% to about 45% of Na2S2O4, from about 0.15% to about 0.35% of said modified xanthan gum, and water.

5. The method as claimed in Claim 3 wherein the amounts of material comprise from about 6.0% to about 7.5% of said NaOH, from about 0.3% to about 0.6% of said chelate, from about 35% to about 42% of Na2S2O4, from about 0.15% to about 0.35% of said modified xanthan gum, from about 4.5%
to about 5.5% sodium tripolyphosphate, and water.

6. The method as claimed in Claim 4 wherein the amounts of material comprise from about 6.5% to about 7.5% of said NaOH, from about 0.45% to about 0.55% of said chelate, from about 38% to about 42% of Na2S2O4, from about 0.2%
to about 0.3% of said modified xanthan gum, and water.

7. The method as claimed in Claim 5 wherein the amounts of material comprise from about 6.5% to about 7.5% of said NaOH, from about 0.45% to about 0.55% of said chelate, from about 38% to about 42% of Na2S2O4, from about 0.2%
to about 0.3% of said modified xanthan gum, from about 4.8% to about 5.2% sodium tripolyphosphate, and water.

8. The method as claimed in any of Claims 1, 2, 3, 4, 5, 6 or 7 wherein said chelate is selected from the group consisting of nitriletriacetic acid, ethylenediamine-tetraacetic acid, hydroxyethylenediaminetriacetic acid, diethylenetriaminepentaacetic acid, and the sodium and potassium salts of said chelates.

9. The method as claimed in any one of Claims 1, 2, 3, 4, 5, 6 or 7 wherein said chelate is tetrasodium ethylene-diaminetetracetic acid trisodium salt.

10. The method as claimed in any of Claims 1, 2 or 3 wherein said method comprises: (a) mixing said water, said sodium hydroxide and said chelate and cooling the mixture to below 45 degrees F; (b) adding solid sodium dithionite to form a suspension at a rate to maintain the temperature below 45 degrees F; (c) adding to said suspension said modified xanthan gum; and (d) cooling said suspension to a temperature below 40 degrees F.

11. The method as claimed in any of Claims 1, 2 or 3 wherein said method comprises: (a) adding said sodium hydroxide and said chelate to a source of liquid sodium dithionite;
(b) adding solid sodium dithionite to form a suspension;
and (c) stabilizing said suspension with said modified xanthan gum.

12. The method as claimed in any of Claims 1, 2 or 3 wherein said method comprises: (a) adding sodium hydroxide and said chelate to liquid sodium dithionite and forming a suspension by flash evaporation or vacuum evaporation;
and (b) stabilizing said suspension with said modified xanthan gum.

13. A stabilized suspension of sodium dithionite made by a method comprising mixing, in any order, water, sodium hydroxide, chelate, sodium dithionite, and modified xanthan gum, wherein said gum can readily hydrolyze when exposed to alkaline conditions, provided the temperature of said mixture is kept less than or equal to about 45 degrees F during and after the addition of sodium dithionite.

14. A stabilized suspension of sodium dithionite as claimed in Claim 13 made by a method comprising mixing NaOH
in an amount equivalent to between about 5% to about 9%
of a 50% solution of NaOH, chelate in an amount equivalent to between about 0.2% to about 0.8% of a 40%
solution of chelate, from about 30% to about 50% of Na2S2O4, from about 0.1% to about 0.4% of said modified xanthan gum, and water, wherein all percents are weight percents and the total is 100%.

15. A stabilized suspension of sodium dithionite as claimed in Claim 13 made by a method comprising mixing NaOH in an amount equivalent to between about 5% to about 8% of a 50% solution of NaOH, chelate in an amount equivalent to between about 0.2% to about 0.8% of a 40% solution of chelate, from about 30% to about 45% of Na2S2O4, from about 4% to about 6% of sodium tripolyphosphate, from about 0.1% to about 0.4% of said modified xanthan gum, and water, wherein all percents are weight percents and the total is 100%.

16. A stabilized suspension of sodium dithionite as claimed in Claim 13, 14 or 15 made by a method comprising: (a) mixing said water, said sodium hydroxide and said chelate and cooling the mixture to below 45 degrees F; (b) adding solid sodium dithionite to form a suspension at a rate to maintain the temperature below 45 degrees F; (c) adding to said suspension said modified xanthan gum; and (d) cooling said suspension to a temperature below 40 degrees F.

17. A stabilized suspension of sodium dithionite as claimed in Claim 13, 14 or 15 made by a method comprising: (a) adding said sodium hydroxide and said chelate to a source of liquid sodium dithionite; (b) adding solid sodium dithionite to form a suspension; and (c) stabilizing said suspension with said modified xanthan gum.

18. A stabilized suspension of sodium dithionite as claimed in Claim 13, 14 or 15 made by a method comprising: (a) adding said sodium hydroxide and said chelate to liquid sodium dithionite and forming a suspension by flash evaporation or vacuum evaporation; and (b) stabilizing said suspension with said modified xanthan gum.

19. A method of bleaching wood pulp comprising exposing a quantity of wood pulp to the suspension as claimed in Claim 15.