FI69822B - FOERFARANDE FOER UTVINNING AV ETT BIPRODUKTSALT FRAON SUSPENSIONEN DAERAV I ETT VATTENHALTIGT SURT REAKTIONSMEDIUM SOM ANVAENDS FOER FRAMSTAELLNING AV KLORDIOXID - Google Patents

Description

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(51) Kv.ik. * / Int.ci. * C 01 B 11/02 FINLAND —FINLAND (21) Patent application - Patentansökning 790007 (22) Application date - Ansökningsdag 02.01 .79 (* = «) (23) Starting date - Giltighetsdag 02.01 .79 (41) Has become public - Blivit offentlig 0 * 4.07.79

National Board of Patents and Registration Date of publication and publication. - 31.12.85

Patent- och registerstyrelsen '' Ansökan utlagd och utl.skriften publicerad (32) (33) (31) Privilege requested - Begärd priority 03.01.78 O7.O3.78, i2.O5.78 England-England (GB) 00029/78 , Ο9Ο5Ο / 78, 19235/78 (71) Erco Industries Ltd, 2 Gibbs Road, Islington, Ontario, Canada (CA) (72) Gerald Cowley, Mississauga, Ontario,

Peter David Dick, Brompton, Ontario, Canada (CA) (7 * 0 Oy Kolster Ab (5 * 0 Method for obtaining a by-product salt from its suspension in an acidic aqueous reaction medium used for the preparation of chlorine dioxide - Preparation of the utilization of a biproduct suspension) i ett vattenha1tigt, surt reaktionsmedium som används för framstä11 Ning av klordioxid

It is known to prepare chlorine dioxide by reducing chlorate ions with chloride ions in the aqueous phase in the presence of free hydrogen ions according to the following equation: C103 “+ Cl“ + 2H + -> C102 + 1/2 Cl2 + H 2 O This method can be carried out in many ways and broadly separates the two groups. In the first group, chloride ions are added to the reaction medium, e.g. in the form of a chloride salt, usually an alkali metal chloride, preferably sodium chloride or hydrochloric acid, and in the second group, chloride ions are generated during the reaction by reducing chlorine, usually with sulfur dioxide or methanol.

2 69822

Various strong acids can be used alone or in admixture to form the free hydrogen ions required in the chlorine dioxide production reaction, such as sulfuric acid, hydrochloric acid and phosphoric acid. When hydrochloric acid alone is used as the source of hydrogen ions, it also acts as a chloride ion reducing agent. When mixtures of hydrochloric acid and other acids are used, hydrochloric acid may also act in part or in whole as a chloride ion reducing agent, depending on the molar amount used.

Usually chlorate ions are added to the reaction medium in the form of an alkali metal salt, preferably sodium chlorate. The chlorate cation and other cations added to the reaction medium combine with the acid anion to form a by-product salt. The following equations describe the formation of these by-products:

NaC 10 ^ + NaCl + HSO 2. -> C100 + h Cl0 + Ho0 + Nä S0 „o 24 2 2224

NaClO3 + 2 HCl -> C102 +% Cl2 + H2O + NaCl2 NaC102 + SO2 + H2SO4 -> 2 C1C> 2 + NaHSO4

The by-product is removed from the reaction medium continuously or in batches by crystallization as a solid phase. Such crystallization can be performed inside or outside the reaction vessel. When the crystalline material is removed from the mother liquor, there is usually some mother liquor left in it.

Conventional separation methods, often combined with water-washing, have been used to separate the mother liquor and purify the by-product crystals. Usually, the separated mother liquor and spent wash water are returned to the generator containing the reaction medium to avoid the loss of the chemicals they contain.

The invention relates to a process for recovering a by-product salt from its suspension in an aqueous acidic reaction medium used for the production of chlorine dioxide containing chlorate and chloride ions, in which part of the suspension is allowed to flow downwards by gravity over several overlapping liquids and solids.

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The process according to the invention achieves a more efficient washing of the included reaction medium from crystals. In the present invention, the by-product crystals are subjected to a multi-stage decantation-washing treatment and the purified crystals are removed from this treatment.

The method according to the invention is characterized in that the suspension is passed downstream of the washing water from one chamber to the next lower chamber, whereby the crystals settle to the bottom of the next lower chamber and thicken before being transferred to the next lower chamber. located near the upper end of the next upper chamber, for use therein as a downstream wash water.

Although the present invention is broadly applicable to any process for generating chlorine dioxide and forming a crystalline by-product, the invention is particularly useful in the following specific systems for generating chlorine dioxide, described below.

U.S. Patent No. 3,864,456 describes the preparation of a mixture of chlorine dioxide, chlorine and water vapor in a continuous process in which sodium chlorate is reduced with chloride ions in an acidic aqueous reaction medium containing sulfuric acid with an acid normality of about 2 to about 4.8. The reaction medium is kept at its boiling point while the reaction zone is under reduced pressure.

The chloride ion source may be sodium chloride, hydrogen chloride or mixtures thereof. When hydrogen chloride is used as at least a partial source of chloride ion, it can also fill part of the acid requirement, whereby the acid consists of a mixture of sulfuric acid and chloric acid.

In this process, the by-product, i.e. sodium sulfate, precipitates from the reaction medium in crystalline, neutral and anhydrous form after saturation of the sodium sulfate with the reaction medium after the start of the reaction.

U.S. Patent No. 3,563,702 demonstrates that the production efficiency of the chlorine dioxide production process of U.S. Patent No. 3,864,456, i. how far the chlorate in the reaction medium reacts to chlorine dioxide can be increased by using small amounts of catalyst 4 69822 such as vanadium pentoxide, silver ions, manganese ions, dichromate ions and arsenic ions.

The latter methods are carried out at a temperature above about 30 ° C and below a temperature at which the chlorine dioxide begins to decompose considerably, preferably in the range from about 50 ° to about 85 ° C and especially at about

65 ° to about 75 ° C. The reaction zone uses the vacuum required to reach the boiling point at the reaction temperature and is generally about 100 to about 400 mm Hg in absolute pressure.

When the acidity of the reaction medium is above about 5 and up to about twelve, chlorine dioxide can be produced from high power without the addition of a catalyst using a process similar to that described in U.S. Patent No. 3,864,456. But at these higher acid normalities, sodium sulfate crystallizes from the reaction medium in the form of an acid sulfate such as sodium sesquisulfate or sodium sulfate depending on the acid normality of the reaction medium.

Sodium sulfate precipitated from the reaction medium of the chlorine dioxide reactor of these methods is usually removed from the generator continuously or in batches in the form of a slurry with the reaction medium.

Canadian Patent Nos. 913,328 and 956,784 describe the preparation of chlorine dioxide by reacting sodium chlorate with hydrochloric acid at a total acidity of less than about 1, preferably in a single generator-evaporator-crystalline vessel in the same manner as described in the aforementioned U.S. Patent No. 3,864,456. In this case, the crystalline by-product is sodium chloride.

Another method to which the present invention may be particularly applicable is described in U.S. Patent No. 4,081,520, which allows methanol, sodium chlorate and sulfuric acid to react with high acid normality in a single generator-evaporator-crystalline vessel to crystalline sodium bisulfate.

U.S. Patent 3,975,505 describes a crystal washing process for neutral, anhydrous sodium sulfate obtained, e.g., by the process of U.S. Patent No. 3,864,456. U.S. Patent No. 4,045,542 describes a crystal washing process for sodium chloride obtained, e.g., by the process of Canadian Patent Nos. 913,328 and 956,784. In both methods, the slurry is fed to the top of the separation column and warm water, a temperature of about 30 ° to about 70 ° C, is fed to the bottom of the column in the opposite direction to the downstream flowing slurry. The water continuously washes the crystals in the downstream slurry, the washed aqueous slurry is removed from the lower end of the column, and the wash water containing washed chemicals from the slurry is fed to a generator.

The washing efficiency of the decantation washing method of the present invention is considerably higher than that of the latter countercurrent washing method.

Thus, each step of the decantation wash is at least 50% efficient, with the efficiency of each step of the prior patent method being n.

20%. Thus, in order to achieve at least the same total washing power as in the ten steps of the washer of the previous patent, only four steps are required in the method of the invention.

According to the method of the present invention, the concentration of the crystalline slurry to be washed can vary widely, but is generally about 1 to about.

30% by weight.

Prior to decantation washing of the slurry, it is first thickened to partially remove the associated aqueous phase, e.g. in a cyclone separator or in the uppermost, suitably designed chamber of the decatement washing device. Generally, this initial thickening results in a slurry having a consistency of about 50 to about 80% w / w. The product from the washing step has this sludge concentration.

The use of a cyclone thickening step is recommended because this is rapid and the wash water of the first wash step dilutes the aqueous phase of the thickened slurry, thereby reducing the concentration of reactants in the aqueous phase to a level that cannot sustain the formation of residual chlorine dioxide. Otherwise, such formation of residual chloride dioxide may interfere with the proper settling of the crystals in the decantation washing steps.

The thickened slurry is fed through the wash chambers of a multi-stage decanter scrubber while the wash water passing through the series of chambers is usually removed for recycling to the chlorine dioxide generation system. Fresh water is fed to the lowest chamber while the washed product is removed from this chamber.

The temperature of the wash water used in the wash can vary widely and generally from ambient temperature (about 20 ° C) to about 80 ° C depending on the solid phase of the sludge to be washed. When washing, for example, sodium chloride crystals, the temperature range of the wash water can be from ambient temperature to about 80 ° C. When washing anhydrous, neutral sodium sulfate, the temperature of the wash water may be from about 30 ° to about 80 ° C, preferably from about 38 ° to about 80 ° C. When washing hydrated, neutral sodium sulfate, the temperature of the wash water may be from ambient temperature to about 30 ° C, or preferably n.

20 ° to about 30 ° C.

In addition to being washed clean of the impurities that have entered, the various sodium sulfates can be converted to other products in the wash. For example, when the solid product of the generator is sodium hydrogen sulfate such as sodium sesquisulfate or sodium bisulfate, which are formed when the acidity of the reaction medium is above about 5, washing the crystals with water involves converting hydrogen sulfate to neutral sodium sulfate as follows: 2 NaHSO. -> Na_S0. + H ^ SO, 4 2 4 2 4

Thus, the acid is not wasted with the hydrogen sulfate, but is returned to the generator with the spent wash water.

The latter method is particularly advantageous because chlorine dioxide evolution methods using high acid normality are inherently highly efficient and no catalysts are required. Because at the same time the solid by-product of the process can be washed and converted to the neutral form when the excess acid returns to the generator, the economy does not suffer from the acid loss caused by the higher acidity of the system.

The temperature of the wash water used in the latter method varies depending on whether a hydrated or anhydrous form of neutral sodium sulfate is desired. If desired, the hydrated form will have a wash water temperature of about 20 ° to about 30 ° C, and if desired, the anhydrous form will have a wash water temperature of about 30 ° to about 80 ° C, preferably about 38 ° to about 80 ° C.

The washing method of the invention can also be used to obtain hydrated, neutral sodium sulfate from an anhydrous, neutral sodium sulfate-containing slurry using a wash water having a temperature of about 20 ° to about 30 ° C.

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The number of individual decantation-thickening steps used in the process of the invention can vary widely depending on the size of the individual chambers. The number is at least two. Usually, about four chambers are sufficient for efficient crystal washing.

The size of the decanter scrubber varies widely depending on the size of the chlorine dioxide generator associated with it and the number of washing and / or reactor steps. Typical dimensions are a diameter of about 0.31 to 0.92 m and a height of about 3.7 to 4.9 m.

Figure 1 shows a schematic flow diagram of an apparatus for implementing the scrubbing method of the invention and the associated specific chlorine dioxide generation system.

The chlorine dioxide generator 10 of Figure 1 contains an acid reaction medium with an acid having a total normality of about 2 to about 4.8 and is fed with sodium chlorate and sodium chloride reactants via line 12 and sulfuric acid via line 14. If desired, sodium chlorate and sodium chloride can be fed separately. The catalyst can also be fed to the generator 10.

The reaction medium is maintained at a boiling point above n.

30 ° C and below the temperature above which considerable decomposition of chlorine dioxide takes place and a vacuum is sucked into the generator.

A mixture of chlorine dioxide, chlorine and steam from the reaction medium is removed from the generator 10 via line 16 for treatment in the usual manner to give an aqueous solution of chlorine dioxide.

After start-up, the reaction medium is saturated with sodium sulfate, which crystallizes from the reaction medium. Under the reaction conditions described, sodium sulfate crystallizes as anhydrous and neutral sodium sulfate. This is removed from the generator 10 via line 18 as a slurry with the reaction medium.

The slurry is transferred to a cyclone separator 20 for thickening, such as from a slurry concentration of about 1 to about 30% by weight to a concentration of about 50 to about 80% by weight. The downstream of the thickened slurry of the cyclone is fed through line 22 to the decanter washer 24. In some cases, sodium sulfate crystals in excess of the amount processed in the washer 24 can be fed from the cyclone separator 20 via line 22, thus ensuring that washer 24 is always full of crystals. Excess crystals are recycled to generator 10 with overflow wash water from scrubber 24.

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The washing device 24 may be at a desired position with respect to the generator 10. For example, the washing device 24 may be immediately below the generator 10 or alternatively the washing device 24 may be on the side of the generator 10.

The washing device 24 is formed by a series of overlapping decantation-washing and thickening chambers 26, the connections of which are connected to each other as described below.

The thickened slurry in line 22 enters the uppermost chamber 26, to which at the same time wash water is fed mainly from the lower chamber 26 via line 28. The solid phase settles on a conical surface 30 which separates the upper chamber 26 mainly from the lower chamber 26 and the liquid phase is removed from the upper chamber 26 via line 32 for recirculation to generator 10 via line 34 with overflow liquid phase via transmission line 38.

In this way, gravity sludge transfer has been found to be successful and should be recommended. However, suitable flaps may be used to facilitate transfer.

The wash water enters the transfer tube 38 via its associated line 40, mixing with the slurry to be transferred. The wash water supplied in this way is decanted mainly at the top of the lower chamber 26 via line 40. Mixing, decanting and thickening are then repeated mainly in the lower chamber 26.

A direct connection between the line 40 and the transfer tube 38 for supplying the wash water to the corresponding chamber 26 has been found to work satisfactorily and is recommended. Other means of washing water supply can be used, e.g. a set of rings under the transfer tube 38 or several wash water inlets in the transfer line 38.

The steps of transfer, mixing, decanting and thickening are repeated in each chamber 26 below, until a finally washed and thickened sodium sulphate product is formed, which is removed from the lowest chamber 26 via an associated line 42 by means of a suitable removal device 44. Preferably, the discharge device 44 is a power-adjustable discharge device such as a reversing flap, a pressure valve or a Moyno pump. The product slurry of line 42 typically contains 50-80% by weight of solid, neutral and anhydrous sodium sulfate, 40-16,989,222% by weight of water, and 4-10% by weight of dissolved sodium sulfate.

The product sludge discharge rate in line 42 is almost the same as the sludge flow rate through column 24, so column 24 is constantly filled with sludge. As already mentioned above, intermittent variations in flow rate that may empty the column can be offset by feeding excess sludge to the uppermost chamber 26 and recirculating the excess crystals through lines 32 and 34.

New warm wash water is supplied via line 46 to a transfer pipe 38 which connects the lowest chamber 26 closest to the upper chamber 26. The wash water used in each successive upper chamber 26 consists of a continuously fouling liquid overflow mainly from the lower chamber 26.

The liquid discharged from the uppermost chamber 26 via line 32 has the same volume as the liquid fed to device 24 through lines 46 and 22. Less liquid is discharged with the product through the discharge pipe 42.

Thus, in each chamber 26 of the decantation washer 24, the thickened slurry coming through line 22 is mixed and diluted with a dilute wash solution that overflows, i.e., decanted from the next subsequent stage of the decantation wash. In all cases, after thickening, the crystals flow through the transfer tube 38 to the next washing step and the overflow is recovered and used for washing purposes in the earlier stage of the decantation wash.

In chamber 26, the non-settling, small-diameter crystals decant with the wash water and pass through line 40 to the next upper chamber. In line 40, the flow rate must be such that clogging by crystals is minimized. Generally, a flow velocity above about 0.92 m / s is used.

The flow of crystals from one chamber to another through the transfer tube 38 takes place by gravity. This preferred transfer mechanism has been shown to be effective in ensuring that fluid from the predominantly lower chamber 26 does not flow predominantly to the upper chamber 26 through the transfer tube 38.

It is recommended to design the individual chambers 26 to act as mass flow cones, whereby gravity thoroughly transfers the solids deposited on the conical surface 30 mainly into the lower chamber 10 69822 via a corresponding transfer line 38. To achieve this result, the conical surface 30 and the transfer tube 38 must be shaped and dimensioned appropriately. The transfer tube 38 may have a desired cross-sectional shape, e.g., round or oval.

But if desired, a precise flow control device can be used for this purpose. Suitable flow control devices include an overflow shut-off device, a fluid shut-off that alternately communicates with the upper and lower chambers, but never both, or a threaded screw device or the like.

In the preferred embodiment described, the decantation-washing device 24 operates without moving parts and thus has a simple structure and requires almost no maintenance.

Through the cyclone separator 20 to initially thicken the slurry in line 18, it is preferable because the thickening is rapid and the wash water fed to line 28 in the uppermost chamber 26 dilutes the aqueous phase of the thickened slurry to prevent the formation of residual chlorine dioxide.

But if desired, thickening can be performed in the uppermost chamber 26 of the decantation-wash column 24, to which the slurry in line 18 is fed directly.

The decantation-washing process in device 24 is described with reference to one specific chlorine dioxide generation system performed in generator 10. As mentioned above, the washing process can be carried out with each solid by-product of the chlorine dioxide generation process carried out either in one generator-evaporator-crystallizer vessel or by precipitating the solid product in a separate crystallization zone.

Successful decantation washing in column 24 involves several important variables. In each chamber 26, a two-stage step is required to achieve efficient washing, i. efficient mixing of the transferred sludge and decanted wash water and efficient settling and thickening of the crystals on the surface 30.

The mixing of the transferred slurry and the decanted wash water depends on several variables, such as the sizing of the transfer tube 38, the solids and wash water production in column 24, the rate of wash water encountered by the transferred sludge, and the number of wash water tables.

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It has been found that the dimensioning of the transfer tube 38 must be such that sludge fluidization in the tube 38 is prevented and laminar flow occurs in the tube. In this way, the washing water entering the pipe 40 is prevented from flowing upwards through the transfer pipe 38, mainly to the upper chamber 26.

The Froude number indicates the rate at which fluidization occurs, and in the present invention, the transfer tube 38 is generally dimensioned such that the Froude number is from about 0.05 to about 0.15. The Freude number is obtained from the equation

Np. ^

Fr - gd where Vc is the flow rate, d is the smallest diameter of the retaining particle and g is the acceleration due to gravity.

It has been found that a suitable cross-sectional area of the transfer tube 38 that achieves Froude figures and acceptable production rates in the desired range is about 9.3 cm / t solids / day, although a value of about 4.65 can be used. about 13.9 cm / t solids / day.

The Reynolds number indicates the rate at which laminar flow occurs. In general, the Reynolds number is less than about 2000 because there is a vortex flow above this. The Reynolds number is determined using the following equation:

N_ = DVP

Re - u where D is the diameter of the transfer tube 38, V is the flow rate, P is the density of the slurry and u is the viscosity of the slurry.

The successful settling and settling of the slurry depends on the overall width of the chamber 26 and is independent of the dimensioning of the transfer tube 38, provided that the constraint imposed by the Froude number is taken into account.

The unit area required for a given material is determined by batch settling experiments at the desired sludge concentration. This method is known and is first presented by Coe and Clevenger, Trans. Am. Inst. Mining Engineers, Vol. 55, p.

356 (1916) and a mathematical model and analysis for determining the unit areas of a thickener have been presented by Talmage and Fitch, 12 69822

Industrial and Engineering Chemistry, Vol. 47 (1), p. 38 (1955). The latter Matti gives the following equation

Unit area = _ ^ _ m2 / kg solids / s

Co Ho where tu is the time at the intersection of the tangent and the sludge settling curve, when the settling time is arbitrary and the sludge height is at a minimum maximum concentration, Co is the initial sludge concentration 2 (kg / m) and Ho is the initial sludge height.

The unit area can vary widely depending on the selected support value and is suitably in the range of about 65.0 to about 102.2 m / t solids / day, e.g. about 85.5 m / t sols / day.

The total height of the column 24 depends on the number of chambers 26 therein and the individual height of each chamber. The number of chambers 26 depends on the washing power of each individual step. The power of each individual step can be calculated from the equation.

„_ Solvent Inlet - Solvent Removal E- ------ x 1.25

Solvent Inlet where Solvent Inlet and Solvent Removal are solvent flows associated with solids flows into and out of the chamber.

2

When the equipment capacity is about 60%, the unit area is 85.5 m / tn / day and the planned total capacity is more than 96%, a column with five chambers is obtained 26.

The following example illustrates the invention.

Example

A thickening-washing column according to Figure 1 was set up with five bottoms 30, a diameter of 25.4 cm and a length of 3.05 m. The column was used at an ambient temperature of about 20 ° -25 ° C. Sodium chloride slurry was added to the top of the column at a concentration of 17% by weight, the slurry was thickened in the upper chamber, and the slurry containing about 65% by weight solids was removed from the lower end with a total removal efficiency of about 98%.

1 3 69822

The column was used continuously with a solids content corresponding to about 4 tons of chlorine dioxide per day with a wash water to salt ratio of about 1. The average washing efficiency of each stage of the column was about 67%.

Claims (13)

14 Claims 69822 A process for recovering a by-product salt from a suspension thereof in an aqueous acidic reaction medium used for the production of chlorine dioxide containing chlorate and chloride ions, comprising allowing a portion of the suspension to flow downward by gravity from several overlapping chambers filled with liquid and solid, the suspension is passed downstream of the wash water from one chamber to the next lower chamber, whereby the crystals settle to the bottom of the next lower chamber and are thickened before being transferred to the next lower chamber, and the water phase removed near the top of the chamber is passed near the next upper chamber top end, for use as downstream wash water. A method according to claim 1, characterized in that the solids content of the initial suspension is about 1-n. 30% w / w and the suspension is thickened to a solids content of about 50. 80 w / w% before multi-stage washing. Method according to Claim 2, characterized in that the first thickening is carried out in a cyclone separator. Process according to one of Claims 1 to 3, characterized in that the temperature of the wash water is from about 20 ° to about 80 ° C. Process according to Claim 4, characterized in that the temperature of the washing water is from about 30 ° to about 80 ° C. 6. A chlorine dioxide generating apparatus comprising at least one chlorine dioxide generating vessel and at least one scrubber for washing a suspension of solid by-product salt recovered from the reaction medium to reduce the concentration of the reaction medium therein, the scrubber consisting of a washing column with a series of washing chambers to remove the washed suspension from the column, a lower inlet to supply the wash water to the column and an upper outlet to remove the spent wash water from the column, characterized in that a series of suspension transfer tubes connect one chamber and the next lower chamber and a series of wash water transfer tubes connect the top of one chamber and the next upper chamber. 15 69822 Apparatus according to claim 6, characterized in that the decantation-washing column (24) is generally cylindrical and each decantation-washing chamber (26) is separated from the next lower chamber by a downwardly sloping frustoconical spacer (30) with a central opening connected to a short suspension transfer tube. (38). Device according to Claim 6 or 7, characterized in that a cyclone separator (20) is provided for thickening the by-product suspension before it is fed to the decantation-washing column (24). Apparatus according to claim 6 or 7, characterized in that the decantation-washing column (24) comprises a thickening chamber above and connected to the uppermost decantation-washing chamber (26). Device according to one of Claims 6 to 9, characterized in that the transfer tube (38) is open and there are no flow control devices between the successive chambers. A method according to claim 1, characterized in that the feeding of the thickened suspension from the chamber to the next lower chamber is performed without fluidization while maintaining the laminar flow. A method according to claim 11, characterized in that the thickened suspension is fed with a Froude value of about 0.05 to about 0.15. Apparatus according to claim 11, characterized in that the cross-sectional area of the transfer tube (38) corresponds to a unit surface area 2 of about 4.6 to about 13.9 m / t solids / day and the cross-sectional area of the column (24) corresponds to a unit area value of about 65.0 to about 2,102.2 m / t solids / day. 16 69822