CA1117731A

CA1117731A - Washing procedure in chlorine dioxide production

Info

Publication number: CA1117731A
Application number: CA000317998A
Authority: CA
Inventors: Peter D. Dick; Gerald Cowley
Original assignee: Erco Industries Ltd
Current assignee: Erco Industries Ltd
Priority date: 1978-01-03
Filing date: 1978-12-14
Publication date: 1982-02-09
Also published as: CA1118184A; PT68985A; FI69822C; CS10279A2; JPS54128998A; NO784406L; OA06144A; FR2413320B1; ZA786778B; CU35046A; DE2856504A1; ES476508A1; PL120161B1; FR2413320A1; FI790007A; FI69822B; PL212594A1; AU4303979A; AR220364A1; BR7808556A

Abstract

IMPROVED WASHING PROCEDURE IN CHLORINE
DIOXIDE PRODUCTION

ABSTRACT OF THE DISCLOSURE
Washing of by-product salts from chlorine dioxide generation processes is effected using a multi-stage decantation-washing operation. Metathesis pro-cessing of certain by-products also is disclosed.

Description

~i~7731 The present invention relates to the production of chlorine dioxide.
It is known to produce chlorine dioxide by reduc-tion of chlorate ions with chloride ions in the aqueous phase in the presence of free hydrogen ions, in accordance with the equation:
C103 + Cl + 2H --~ C102 + 1/2C12 + H20 This process may be effected in a number of ways, broadly falling into two groups. In the.first group, the chloride ions are added to the reaction medium, such as, in the form of a chloride salt, usually an alkali metal chloride, preferably sodium chloride, or as hydrochloric acid, while, in the second group, the chloride ions are formed in situ by reduction of the chlorine, usually using sulphur lS dioxide or methanol as the reducing agent.

111~73~

A variety of strong acids may be used, alone or in admixture, to provide the free hydrogen ions required in the chlorine dioxide-producing reaction, such as, sul-phuric acid, hydrochloric acid and phosphoric acid. Whére hydrochloric acid alone is used as the source of the free hydrogen ions, it also acts as the source of chloride ion reducing agent. Where hydrochloric acid is used in ad-mixture with other acids, it may also provide part or all of the chloride ion reducing agent, depending on the molar lQ quantity used.
The chlorate ions usually are introduced to the reaction medium in the form of an alkali metal salt, preferably sodium chlorate. The cation of the chlorate, along with any other cations introduced to the reaction medium, combines with the anion of the acid to form a by-product salt. The following equations illustrate the formation of these by-products:

NaCl03 + NaCl + H2S04 2 / 2 2 2 4 NaCl03 + 2HCl ~ 2 / 2 2 2NaC103 + S2 + H2 4 ~ 2Cl02 + 2NaHS04 The by-product is removed from the reaction medium, on a continuous or intermittent basis, by crys-tallization as a solid phase. Such crystallization may be effected inside or outside the reaction vessel. Removal of the crystalline material from the mother liquor usually results in entrainment of some mother liquor.
Conventional separation techniques, such as, fil-tration and centrifugal separation, often in combination with wash water, have been used to separate the mother liquor and purify the by-product crystals, the separated mother liquor and spent wash water usually being returned to the generator containing the reaction medium to avoid loss of the chemical values thereof.
The present invention is directed to an improved washing pro-cedure and apparatus therefor, for use in conjunction with the prior art chlorine dioxide processes which results in a more efficient washing of the crystals free from entrained reac-tion medium. In the present invention, the by-product crystals are subjected to a multistage decantation-washing operation and purified crystals are removed from the decantation-washing.
The multistage decantation-washing is effected in a plurality of superimposed liquid- and solid-filled cylindrical compartments to which thickened crystal slurry from the next higher compartment is fed concurrently with wash water forwarded from the liquid overflow from the next lower compartment. In each compartment, the crystals fall to the bottom and are thickened prior to transfer to the next lower compartment while the aqueous phase is decanted from adjacent the top of the compartment for pass-age to the next-higher compartment.
Accordingly, in one aspect, the present invention is directed to an improvement in a process for the production of chlorine dioxide by reducing chlorate ions in an aqueous acid reaction medium containing chloride ions and recovering a by-product salt from the reaction medium as a slurry with entrained reaction medium. In this one aspect, the improve-3ament comprises feeding the slur~y to the upper-most compartment of a plurality of superimposed compartments;
feeding thickened slurry consecutively downwardly through the plurality of superimposed compartments f~om the top-most compartment to a lower-most compartment; contacting the slurry entering the lower-most compartment with wash water to dilute and wash the same and thereafter permitting crystals in the thus-contacted slurry to settle to the bottom of the lower-most compartment and thicken; overflowing spent wash water from the lower-most compartment adjacent an upper region thereof and feeding the overflowed spent wash water to the next-higher compartment to contact the slurry entering the next-higher compartment, ~y using a wash water transfer means extending between the upper region of the lower com-partment to the slurry transfer means of the next higher compartment, to dilute and wash the same andthereafter permitting crystals in the thus-contacted slurry to settle to the bottom the next-higher compartment and thicken for passage to the next-lower compartment; repeating the steps of spent wash water overflow, feed thereof to the next-higher compartment, slurry contact for dilution and washing, and crystal settling and thickening for each successively higher compartment in the plurality of compart-ments; removing spent wash water from the upper-most 25 compartment cont~ining reaction medium separated from the slurry for forwarding to the chlorine dioxide production;
and removing a slurry having a decreased entrained reaction medium content from the lower-most compartment; whereby the slurry is subjected to a multistage decantation-washing ill7731 3a(i) operation in a plurality of superimposed liquid-and solid-filled compartments.
The invention also includes apparatus for effecting the process and accordingly, in another aspec~ of the 3b present invention, there is provided an improvement in a chlor-ine dioxide generating apparatus including at least one chlor-ine dioxide generating vessel for the production of chlorine dioxide and a by-product salt by reducing chlorate ions in an aqueous acid reaction medium containing chloride ions and located in the at least one vessel and at least one washing means for washing a slurry of solid by-product salt recovered from the reaction mRdium to decrease the concentra-tion of entrained reaction medium therein.
lQ In this aspect, the improvement is that wherein the washing means is constituted by a decantation-washing column comprising a plurality of superimposed decantation-washing compartments; upper inlet means for feeding slurry to the upper-most compartment, lower outlet means for removal of washed slurry from the column, lower inlet means for feeding wash water to the column and upper outlet means for removal of used wash water from the column; slurry transfer means extending from one compartment to the next-lower compartment for each of the compartments from the upper-most compartment to the lower-most compartment; and wash water transfer means extending between an upper region of one compartment to the slurry transfer means of the next-higher compartment for each of the compartments from the lower-most compartment to the upper-most compartment.
A particularly useful decantation washing apparatus utilizable in the second aspect of the invention consists essentially of an elongate generally cylindrical upright tower; a plurality of superimposed compartments within the tower extending from an upper-most compartment to a lower-most compartment, each separated from the next-lower 3c compartment by a downwardly-sloping frusto-conical bottom wall; each bottom wall having a central opening and being joined to a short axially downwardly-directed unobstructed pipe having an inlet coinciding with the central opening in the bottom wall, the short pipe establishing a fluid relationship between adjacent superimposed compartments;
slurry inlet means communicating with an upper region of the upper-most of the superimposed compartments for feeding thereto a slurry of solid materials to be treated in the apparatus; slurry outlet means communicating with a lower region of the lower-most compartment for removing therefrom a slurry of solid materials treated in the apparatus; treat-ment liquid inlet means communicating directly with the short pipe extending between the lower-most compartmen~ and lS the next-higher compartment for feeding treatment liquid to the apparatus; spent treatment liquid outlet means communicating directly with an upper region of the upper-most compartment for removing spent treatment liquid from the apparatus; first liquid transfer means extendingdirectly between an upper region of each compartment in the column, with the exception of the two upper-most compartments, and the short pipe entering the next-higher compartment for transfer o~ liquid therethrough from one compartment to the next-higher compartment; and second liquid trans f er means extending directly between an upper region of the next-lower than the upper-most compartment and an upper region of the upper-most compartment for trans4er~ of liquid therethrough to the upper-most compartment.
While the present invention has wide applica-bility to any chlorine dioxide genexating process that produces a crystalline by-product, the invention is 1~7731 particularly useful in a number of specific chlorine dioxide-generating systems which are now described.
In U.S. Patent No. 3,864,456, there is described the production of chlorine dioxide in admixture with chlorine and water vapor in a continuous process wherein sodium chlorate is reduced with chloride ions in an acid a~ueous reaction medium containing sulphuric acid and having an acid normality of about 2 to about 4.8 normal.
The reaction medium is maintained at its boiling point under a subatmospheric pressure applied to the reaction zone.
The chloride ions may be provided by sodium chloride, hydrogen chloride or mixtures thereof. Where hydrogen chloride is used to provide at least part of the chloride ion, this chemical also is used to provide part of the acid requirement, so that the acid is provided by a mixture of sulphuric and hydrochloric acids.
In this process, the by-product sodium sulphate deposits in crystalline neutral anhydrous form from the reaction medium once the reaction medium has become satur-ated therewith after start up.
In U.S. Patent No. 3,563,702, it is indicated that the efficiency of chlorine dioxide production accor-ding to the procedure of U.S. Patent No. 3,864,456, i.e.
the extent to which chlorate in the reaction medium reacts to form chlorine dioxide, can be increased by the use of small amounts of catalyst~ such as, vanadium pentoxide, silver ions, manganese ions, dichromate ions and arsenic ons .
The latter processes are effected above about 30C and below the temperature above which substantial decomposition of chlorine dioxide occurs, preferably in the range of about 50 to about 85C, and particularly at about 65 to about 75C. The vacuum applied to the reac-tion zone to provide the subatmospheric pressure therein is that necessary to achieve boiling at the reaction tem-perature and generally ranges from about 100 to about 400 mm Hg absolute.
At acid normalities above about 5 normal and up to about 12 normal in the reaction mediur.t, chlorine dioxide may be produced at high efficiency in the absence of added catalyst using a procedure equivalent to that described in lS U.S. Patent No. 3,864,456. At these higher acid normal-ities, however, the sodium sulphate crystallizes from the reaction medium in the form of acid sulphate, such as, sodium sesquisulphate or sodium bisulphate, depending on the acid normality of the reaction medium.
The sodium sulphate which is deposited from the reaction medium in the chlorine dioxide generator in these processes usually is withdrawn from the generator, on a continuous or intermittent basis, in the form of a slurry with reaction medium.
In Canadian Patents Nos. 913,328 and 956,784, there is described the production of chlorine dioxide by reaction of sodium chlorate with hydrochloric acid at a total acid normality below about 1 normal, preferably in a single vessel generator-evaporator-crystallizer, in similar manner to that described in the aforementioned U.S.
No. 3,864,456 with respect to sulphuric acid. The crystalline by-product in this case is sodium chloride.
Another process to which the present invention may be particularly applied is that described in U.S.
Patent No. 4,081,520 wherein methanol, sodium chlorate and sulphuric acid are reacted at high acid normality in a single vessel generator-evaporator-crystallizer to crystallize sodium bisulphate.
In U.S. Patent No. 3,975,505 there is described a crystal washing procedure for neutral anhydrous sodium sulphate, such as is obtained by the process of U.S. Patent No. 3,864,456, while in U.S. Patent No. 4,045,542 there is described a crystal washing procedure for sodium chloride, such as is obtained by the process of Canadian Patent Nos.
913,328 and 956,784. In both processes, the slurry is in-troduced to the top of a separator column and warm water, of temperature from about 30 to about 70C, is introduced to the bottom of the column in countercurrent flow to the downward flow of slurry. The crystals contained in the downward flowing slurry are continuously washed by the water, the washed aqueous slurry is removed from the bottom of the column and the wash water containing chemicals wash-ed from the slurry is passed to the generator.
The washing efficiency of the decantation washingprocedure of this invention is considerably higher than that of the latter countercurrent washing operation. Thus, each 11~7731 stage of the decantation washing is at least 50~ efficient while each stage in the prior patent procedure is about 20% efficient. Thus, to achieve at least the same overall washing efficiency as is achieved in 10 stages in the prior patent washer requires no more than four stages in the pro-cedure of the invention.
The crystal slurry which is washed following the procedure of this invention may have a concentration vary-ing widely but generally within the range of about 1 to about 30~ w/w.
Prior to subjecting the slurry to the decantation washing, it first is thickened to remove some of the asso-ciated aqueous phase, such as, by use of a cyclone separa-tor, or by suitable design of the top-most compartment of the decantation washing apparatus. This initial thicken-ing generally results in a slurry of thickness of about 50 to about 80% w/w and this slurry concentration is that of the product from the washing operation.
The use of a cyclonic thickening operation is preferred in that the thickening operation is a rapid one and the aqueous phase of the thickened slurry is diluted by wash water in the first washing stage, which will de-crease the concentration of reactants in the aqueous phase to a level insufficient to sustain any residual chlorine dioxide generation. Such residual chlorine di-oxide generation otherwise may disturb proper settling of the crystals i~ the deca~tation-washing stages.

The thic~ened slurry is passea through the wash-ing compartments of the multistage decantation washer while wash water which has passed through the plurality of compartments is removed usually for recycle to the chlorine dioxide generation system. Into the lowest compartment is fed fresh water while washed product is removed from that compartment.
The temperature of the wash water used in the washing operation may vary widely, generally from ambient temperature (about 20C) to about 80C, depending upon the solid phase contained in the slurry being washed. For example, in the washing of sodium chloride crystals, the range of wash water temperatures may be from ambient tem-perature to about 80C. In the case where anhydrous neutral sodium sulphate is washed, the temperature of the wash water may vary from about 30C to about 80C, or pre-ferably from about 38C to about 80C. In the case where hydrated neutral sodium sulphate is washed, the temperature of the wash water may vary from ambient temperature to about 30C or preferably from about 20C to about 30C.
In addition to being washed free of entrained contaminants, various sodium sulphates may be converted to other products during the washing operation. For example, when the solid product of the generator is a sodium acid sulphate, such as, sodium sesquisulphate or sodium bi-sulphate, which is the case when acid normalities above about 5 normal are employed in the reaction medium, the washing of the crysta7s with water is accompanied by con-version of the acid sulphate to neutral sodium sulphate asfollows:

2NaHSO4 ~ 2 4 H2SO4 Thus, no acid is lost with the acid sulphate but rather is returned to the generator, upon return of the used wash water to the generator.
The latter procedure is particularly beneficial since high acid normality chlorine dioxide generating pro-cesses are inherently highly efficient and catalysts are not required~ Since the solid by-product of the process can be washed and at the same time converted to the neutral form with excess acid being returned to the generator, there is no economic penalty in lost acid resulting from the high acidity system.
- The temperature of the wash water used in the latter process varies depending on whether the hydrated or anhydrous form of the neutral sodium sulphate is desired.
If the hydrated form is desired, the wash water temperature is about 20 to about 30C, while if the anhydrous form is desired, the wash water temperature is about 30 to about 80C, preferably about 38 to about 80C.
The washing process of the invention also can be used to result in hydrated neutral sodium sulphate from a slurry containing anhydrous neutral sodium sulphate by using wash water having a temperature of about 20 to about 30C.

lQ/11/12 The size of the decantation washer varies widely, depending on the size of the chlorine dioxide generator with which it is associated and the number of washing and/or reaction stages. Typical dimensions are a diameter of about 1 to about 3 feet and a height of about 12 to 16 feet.
The invention is described further, by way of illus-tration, with reference to the accompanying drawings, wherein:
Figure 1 is a schematic flow sheet of an apparatus for effecting the washing process of the invention in conjection with a specific chlorine dioxide generating system.
Referring to Figure 1 of the drawings, a chlorine dioxide generator 10 contains an acid reaction medium of total acid normality of about 2 to about 4.8 normal to which sodium chlorate and sodium chloride reac-tants are fed by line 12 and sulphuric acid by line 14.
Separate feeds of sodium chlorate and sodium chloride may be utilized, if desired. A catalyst may also be fed to the generator 10.
The reaction medium is maintained at a boiling temperature above about 30C to below the temperature above which substantial decomposition of chlorine dioxide occurs, while a subatmospheric pressure is applied to the generator.
A gaseous mixture of chlorine dioxide, chlorine and steam evaporated from the reaction medium is removed from the generator 10 by line 16 for processing in known manner to provide an aqueous solution of chlorine dioxide.

., .
~ ,, 111~731 After start up, the reaction medium achieves saturation with sodium sulphate which crystallizes from the reaction medium. Under the reaction conditions speci-fied, the sodium s~lphate crystallizes as anhydrous neutral sodium sulphate. The sodium sulphate is removed from the generator 10 by line 18 as a slurry wi$h the reaction medium.
The slurry is forwarded to a cyclone separator 20 for thickening, such as from a slurry concentration of about 1 to about 30 wt.% to about 50 to about 80 wt.%.
The cyclone underflow consisting of thickened slurry is forwarded by line 22 to a decantation-washing apparatus 24. In some instances, an excess of sodium sulphate crystals to that to be processed by the washing apparatus 24 may be forwarded from the cyclone separator 20 by line 22 to ensure that the washing apparatus 24 always remains full of crystals. The excess crystals recycl~ to the generator 10 with overflow wash water from the washing apparatus 24.
The washing apparatus 24 may be located in any desired relationship with respect to the generator 10. For example, the washing apparatus 24 may be located directly below the generator 10, or alternatively, the washing apparatus 24 may be located alongside the generator 10.
The washing apparatus 24 comprises a plurality of superimposed decantation washing and thickening compart-ments 26 which are ~luidly connected to each other, as described below.

The thickened slurry in line 22 enters the top-most compartment 26 while wash water forwarded from the next-lower compartment 26 by line 28 is also fed into the top-most compartment 26. The solid phase settlesonto a conical surface 30 which separates the top-most compart-ment 26 from the next-lower compartment 26 while the liquor phase is removed from the top-most compartment 26 by line 32 for recycle to the generator 10 by line 34 along with the overflow liquid phase from the cyclone separator 20 in line 36. The solids in slurry form are permitted to flow under the influence of gravity to the next-lower compartment 26 through an unobstructed transfer pipe 38.
Slurry transfer under the influence of gravity in this way has been found to be successful and is pre-ferred. Suitable settled crystal disturbiny means, such as,vibrators and rakes, however, may be used to assist in the transfer operation.
Wash water enters the transfer pipe 38 by line 40 communicating therewith and mixes with the transferred slurry. The wash water fed in this way is decanted from an upper region of the next-lower compartment 26, by line 40. The process of m~ing, decantation ~nd thickening ~n is repeated in this next-lower compartment 26.
The direct communication between line 40 and the transfer pipe 38 for feeding wash water to the respective compartment 26 has been found to operate satisfactorily and is preferred. Other wash water feeding operations may be used, however, such as, the use of a ring manifold 111773~

located below the transfer pipe 38, or multiple wash water ~ inlets to the transfer pipe ~.
The operations of transfer, dilution, decanta-tion and thickening are repeated in each successively lower compartment 26 until the finally washed and thick-ened sodium sulphate product results and is removed from the lower-most compartment 26 by line 42 communicating therewith through a suitable extractor. device 44. The ex-tractor device 44 preferably is a variable capacity ex-tracting device, such as, a rotary valve, a pinch valveor a Moyno pump. The product slurry in line 42 typically comprises 50 to 80% by weight solid neutral anhydrous sodium sulphate, 40 to 16~ by weight water and 4 to 10~ by weight dissolved sodium sulphate.
The rate of removal of product slurry by line 42 is substantially the same as the rate of flow of slurry through the column 24, so that the column 24 remains full of slurry at all times. As already mentioned, inter-mittent variations in flow rate which may tend to deplete the column may be smoothed out by overfeeding slurry to the top-most compartment 26, with the excess crystals recycling by lines 32 and 34.
Fresh warm wash water is fed by line 46 to the transfer pipe 38 connecting the lower-most compartment 26 with the next-higher compartment 26. The wash water used in each successively-higher compartment 26 is the increasingly contaminated ~iquid overflow from the next-lower compartment 26.

~117731 The liquor removed from the top-most compartment 26 by line 32 is equivalent in volume to that introduced to the apparatus 24 by line 46 and by line 22, less liquor exiting with the product through exit pipe 42.
In each compartment 26 of the decantation-washing apparatus 24, therefore, the thickened slurry entering by line 22 is mixed and diluted with a weak wash solution overflowing or decanting from a next later stage of decan-tation-washing. In each case, after thickening, the crystals flow through the transfer pipe 38 to the next stage of washing and the overflow is collected and utilized for washing purposes in an earlier stage of decantation-washing.
Small diameter crystals not settled in compart-ment 26 decant with the wash water and are transferred by pipe 40 to the next higher compartment. The flow velocity through pipe 40 should be such as to minimize plugging by the crystals and generally a flow velocity in excess of about 3 ft/sec is used.
The flow of crystals from one compartment to the next through the transfer pipe 38 is effected by gravitation-al forces. This transfer mechanism, which is preferred, has been found to be effective in ensuring that liquor from the next-lower compartment 26 does not pass to the next-higher compartment 26 through the transfer pipe 38.
It is preferred to design the individual compartments 26 to act as mass flow cones to permit very ready complete gravitational transfer of the solids settling on the particular conical surface 30 into the next-lower compartment 26 through the respective transfer pipe 38.

111'~731 The conical surface 30 and the transfer pipe 38 may be suitably shaped and dimensioned to achieve this result, with the transfer pipe 38 having any desired cross-sectional shape, such as, round or oval.
However, a positive flow control device may be used, if desired, to achieve this purpose and suitable flow control devices include a weir overflow device, a liquid seal which alternatively communicates with the upper and lower compartments but at no time with both, or an auger or similar helical screw device.
The decantation-washing apparatus 24 in the illustrated preferred embodiment effects its operations without the necessity for any moving parts and hence is simple in construction and virtually maintenance free.
The use of a cyclone separator 20 to effect the initial thickening of the slurry in line 18 is preferred since the thickening operation is 2 rapid one and the aqueous phase of the thickened slurry is diluted by the wash water fed by line 28 in the top-most compartment 26, so that residual chlorine dioxide generation is inhibited.
However, thickening may be effected in the top-most compartment 26 of the decantation-washing column 24, if desired, with the slurry in line 18 being fed directly thereto.
The decantation-washing procedure in apparatus 24 has been described with reference to one particular chlorine dioxide generating system effected in generator 10. As already mentioned, the washing procedure may be effected on any solid by-product of a chlorine dioxide ~1~7731 generating process effected in either a single vessel generator-evaporator-crystallizer or where the solid product is precipitated in a separate crystallizing zone.
There are several parameters which are important to successful decantation-washing in the column 24. In each compartment 26, two separate operations are required to obtain efficient washing, namely, effective mixing of the transferred slurry and decanted wash water and effective settling and thickening of the crystals on the surface 30.
The mixing of the transferred slurry and decanted wash water is dependent on several variables, including the dimensions of the transfer pipe 38, the throughput of solids and wash water in the column 24, the wash water velocity engaging the transferred slurry and the number of wash water inlets.
One of the important features of the invention is prevention of wash water entering a given compartment from flowing upwardly through the slurry transfer pipe to the next-higher compartment, otherwise effective decantation-washing is not possible. In order to prevent such upwardwash water flow in the illustrated embodiment, the slurry flow downwardly through the transfer pipe 38 is maintained considerably in excess of the upward flow rate of wash water which would occur through a static bed of crystals, as determined from permeability tests. A factor of at 'east 1000 times the permeability rate often is used, although any excess theoretically may be used to ensure that no upward wash water flow occurs. To achieve this condition, flow rates through the transfer pipe 38 in the range of about 2 to about 6 ft/min/sq.ft. of pipe cross section preferably are employed, suitably about 4.0 ft/min/sq.ft. of pipe cross section. These values correspond to a cross-sectional area 5 of transfer pipe 38 of about 0.005 to about 0.015 ft.2/ton solids/day, suitably about 0.01 ft.2/ton solids/day.
The dimensions of the transfer pipe 38 are also designed to maintain the slurry velocity in the pipe below a value above which fluidization occurs and, in this regard, 10 the transfer pipe 38 is generally dimensioned to provide a Froude Number value in the range of about 0.05 to about 0.15.
The Froude Number is determined by the equation:

N = Vc2 gd wherein Vc is the flow velocity, d is the diameter of the smallest particle to be retained, and g is acceleration due to gravity.
The unit æea of the oolumn required for any given material is determined from batch settling test experiments carried 20 out at the desired slurry concentration. This method is well known and was first outlined by Coe and Clevenger in Trans.
Am. Inst. Mining Engineers, Vol 55, p. 356 (1916) and a mathematical model and analysis for determination of thickener unit areas has ~een presented by Talmage and 25 ~itch, Industrial and Engineering Chemistry, vol 47 (1), p,38 (19553.

The latter model provides the following equation:

Unit area = tu sq.ft/lb solids/sec.
Co Ho wherein tu is the time at the intersection of the tangent to the settling curve for the slurry taken at an arbitrary-settling time and the minimum slurry height at maximum concentratiGn Co is the initial concentration of slurry (lb/cu.ft) Ho is the initial slurry height for settling.
The unit area may vary widely depending on the tu value chosen and conveniently is in the range of about 0.07 to about 0.11 ft2/ton solids/day, for example, about 10 0.092 ft2/ton solids/day.
The overall height of the column 24 depends on the number of compartments 26 present in the column and the individual height of each compartment. The number of compartments 26 is dependent on the washing efficiency 15 of each individual stage. The efficiency of any individual stage can be calculated from the equation:

E = Solute in - Solute out x 1.25 Solute in where Solute in and Solute out are the solute flows 20 associated with the solids flows into and out of the compartment. The factor 1.25 is used in the foregoing equation because, for a fully settled bed of solids in a saturated solution of the salt, the composition of the mix-ture is about 80% salt (both solid and dissolved) and 20%
25 water.
From these values, given a tray efficiency of about 60~, a unit area rate of 0.092 ft/ton/day and a designed overall efficiency of greater than 96~, a column having 5 compartments 26 would be required.

The invention is illustrated by the following Example:
Example A thickener-washing column of the type illustra-ted in Figure 1 was set up with five plates 30, a diameter of 10 inches and a length of 10 feet. The column was operated at an ambient temperature of about 20 to 25C
with a slurry o~ sodium chloride of concentration 17% by weight being fed to the top of the column, thickening of the slurry being effected in the top-most compartment and a slurry of about 65% by weight solids being removed from the lower end at an overall extraction efficiency of about 98%.
The column was operated on a continuous basis at a solids rate equivalent to about 4 tons per day of chlorine dioxide production and a ratio of wash water to salt of about 1. The average washing efficiency for each stage of the column was determined to be about 67%.
In summary of this disclosure, the present invention provides an efficient washing operation for crystalline by-products of chlorine dioxide processes. Modifications are possible within the scope of the invention.

.~

Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. In a process for the production of chlorine dioxide by reducing chlorate ions in an aqueous acid reaction medium containing chloride ions and recovering a by-product salt from the reaction medium as a slurry with entrained reaction medium, the improvement which comprises feeding said slurry to the upper-most compartment of a plurality of superimposed compartments, feeding thickened slurry consecutively downwardly through said plurality of superimposed compartments from said top-most compartment to a lower-most compartment, contacting said slurry entering said lower-most compartment with wash water to dilute and wash the same and thereafter permitting crystals in the thus-contacted slurry to settle to the bottom of said lower-most compartment and thicken, overflowing spent wash water from said lower-most compartment adjacent an upper region thereof and feeding said overflowed spent wash water to the next-higher compart-ment to contact said slurry entering said next-higher compartment, by using a wash water transfer means extending between said upper region of the lower compartment to the slurry transfer means of the next-higher compartment, to dilute and wash the same and thereafter permitting crystals in the thus-contacted slurry to settle to the bottom of said next-higher compartment and thicken for passage to the next-lower compartment, 22a repeating said steps of spent wash water over-flow, feed thereof to the next-higher compartment, slurry contact for dilution and washing, and crystal settling and thickening for each successively higher compartment in said plurality of compartments, removing spent wash water from said upper-most compartment containing reaction medium separated from said slurry for forwarding to said chlorine dioxide production, and removing a slurry having a decreased entrained reaction medium content from said lower-most compartment, whereby said slurry is subjected to a multi-stage decantation-washing operation in a plurality of superimposed liquid-and solid-filled compartments.

2. The process of claim 1, wherein said by-product salt slurry has a consistency of about 1 to about 30% w/w and the slurry is thickened to a consistency of about 50 to about 80% w/w prior to said feed to said upper-most compartment.

3. The process of claim 2, wherein said initial thickening operation is effected in a cyclone separator and the aqueous phase separated from the slurry in this way is returned to the generator with said spent wash water.

4. The process of claim 1, 2 or 3, wherein the feed of thickened slurry from one compartment to the next-lower compartment of said plurality of compartments is effected in non-fluidized manner.

5. The process of claim 1, 2 or 3, wherein the feed of thickened slurry from one compartment to the next-lower compartment of said plurality of compartments is effected in non-fluidized manner and at a Froude Number Value of about 0.05 to about 0.15.

6. The process of claim 1, wherein said wash water has a temperature of about 20° to about 80°C.

7. The process of claim 1, wherein said wash water has a temperature of about 30° to about 80°C.

8. The process of claim 1, wherein said chlorine dioxide is produced by reducing sodium chlorate in an aqueous acid reaction medium containing chloride ions in a reaction zone at the boiling point of the reaction medium under a subatmospheric pressure.

9. The process of claim 8, wherein said chlorine dioxide production is effected using sodium chloride and/or hydrogen chloride as the source of said chloride ions, said reaction medium contains sulphuric acid and said by-product salt is sodium sulphate.

10. The process of claim 9, wherein said reaction medium has a total acid normality of about 2 to about 4.8 normal and said sodium sulphate is anhydrous neutral sodium sulphate.

11. The process of claim 9, wherein said reaction medium has a total acid normality of greater than about 5, said sodium sulphate is a sodium acid sulphate and the sodium sulphate contained in the slurry removed from said multistage operation is anhydrous neutral sodium sulphate.

12. The process of claim 8, wherein hydrochloric acid is used as the source of all the acidity of the reaction medium, and said by-product salt is sodium chloride.

13. The process of claim 1 or 11, wherein said chlorine dioxide production is effected in the presence of methanol to form said chloride ions in situ by reduction of generated chlorine, the acidity of said reaction medium is supplied exclusively by sulphuric acid at high total acid normality and said by-product sodium salt is a sodium acid sulphate.

14. The process of claim 1, wherein a catalyst is used to increase the efficiency of chlorine dioxide generation.

15. The process of claim 1, 2 or 3, wherein said super-imposed compartments are of cross-sectional dimension corresponding to a unit area value of about 0.07 to about 0.11 ft2/ton of solids/day and the thickened slurry is fed from one compartment to the next-lower compartment through a transfer pipe in non-fluidized manner at a Froude Number Value of about 0.05 to about 0.15 and at a flow rate of about 2 to about 6 ft/min/ft2 of transfer pipe.

16. In a chlorine dioxide generating apparatus including at least one chlorine dioxide generating vessel for the production of chlorine dioxide and a by-product salt by reducing chlorate ions in an aqueous acid reaction medium containing chloride ions and located in the at least one vessel and at least one washing means for washing a slurry of solid by-product salt recovered from the reaction medium to decrease the concentration of entrained reaction medium therein, the improvement wherein:
said washing means is constituted by a decantation-washing column comprising a plurality of superimposed decantation-washing compartments, upper inlet means for feeding slurry to the upper-most compartment, lower outlet means for removal of washed slurry from said column, lower inlet means for feeding wash water to said column and upper outlet means for removal of used wash water from said column, slurry transfer means extending from one compartment to the next-lower compartment for each of said compartments from said uppermost compartment to said lower-most compartment, and wash water transfer means extending between an upper region of one compartment to said slurry transfer means of the next-higher compartment for each of said compartments from the lower-most compartment to the upper-most compartment.

17. The apparatus of claim 16 wherein said decantation-washing column is generally cylindrical, each of said decantation-washing compartments is separated from the next-lower compartment by downwardly-sloping frusto-conical baffle means, and said slurry transfer means is constituted by a slurry transfer pipe communicating with a central opening in said baffle means.

18. The apparatus of claim 17 wherein said slurry transfer pipe is unobstructed and flow control means between successive compartments are absent.

19. The apparatus of claim 17 including flow control means associated with said slurry transfer pipe for controlling the flow of slurry from one compartment to the next.

20. The apparatus of claim 18, wherein said slurry transfer pipe has a cross-sectional dimension corresponding to a unit area value of about 0.005 to about 0.015 ft2/ton solids/day and said column has a cross-sectional dimension corresponding to a unit area value of about 0.07 to about 0.11 ft2/ton solids/day.

21. The apparatus of claim 16, 17 or 18, including cyclone separator means communicating with said column for thickening said by-product slurry prior to feed thereof to said column.

22. The apparatus of claim 20 including cyclone separator means communicating with said column for thickening said by-product slurry prior to feed thereof to said column.

23. The apparatus of claim 16, 17 or 18 including a thickening compartment superimposed on and communicating with the upper-most decantation-washing compartment of said column.

24. An apparatus, consisting essentially of:
an elongate generally cylindrical upright tower, a plurality of superimposed compartments within said tower extending from an upper-most compartment to a lower-most compartment, each separated from the next-lower compartment by a downwardly-sloping frusto-conical bottom wall, each said bottom wall having a central opening and being joined to a short axially downwardly-directed unobstructed pipe having an inlet coinciding with the central opening in said bottom wall, said short pipe establishing a fluid relationship between adjacent super-imposed compartments, slurry inlet means communicating with an upper region of the upper-most of said superimposed compartments for feeding thereto a slurry of solid materials to be treated in the apparatus, slurry outlet means communicating with a lower region of the lower-most compartment for removing there-from a slurry of solid materials treated in the apparatus, treatment liquid inlet means communicating directly with the short pipe extending between the lower-most compartment and the next-higher compartment for feeding treatment liquid to said apparatus, spent treatment liquid outlet means communicating directly with an upper region of the upper-most compartment for removing spent treatment liquid from said apparatus, first liquid transfer means extending directly between an upper region of each said compartment in said column,with the exception of the two upper-most compartments, and said short pipe entering the next-higher compartment for transfer of liquid therethrough from one compartment to the next-higher compartment, and second liquid transfer means extending directly between an upper region of the next-lower than said upper-most compartment and an upper region of said upper-most compartment for transfer of liquid therethrough to said upper-most compartment.

25. The apparatus of claim 24 wherein said treatment liquid inlet means comprises a pipe extending radially through the side wall of the tower and terminating at said short pipe joining said lower-most compartment and the next-higher compartment in fluid flow communication therewith.

26. The apparatus of claim 24 wherein each said first liquid transfer means comprises a pipe having a first portion extending from said upper region of the respective compartment externally of said tower and a second portion extending radially through the side wall of the tower and terminating at the short pipe entering the next-higher compartment in fluid flow communication therewith.

27. The apparatus of claim 24 wherein said treatment liquid inlet means comprises a pipe extending radially through the side wall of the tower and terminating at said short pipe joining said lower-most compartment and the next-higher compartment in fluid flow communication therewith and wherein each said first liquid transfer means comprises a pipe having a first portion extending from said upper region of the respective compartment externally of said tower and a second portion extending radially through the side wall of the tower and terminating at the short pipe entering the next-higher compartment in fluid flow communication therewith.

28. The apparatus of claim 25, 26 or 27, wherein said short pipe has a cross-sectional dimension corresponding to a unit area value of about 0.005 to about 0.15 ft2/ton solids/day and the tower has a cross-sectional dimension corresponding to a unit area value of about 0.07 to about 0.11 ft2/ton solids/day.