WO2016005996A2 - A method and a system for recovery of anhydrous sodium sulfate from reject stream of sulfate removal system - Google Patents

Abstract

The present invention comprises teaches an innovative method and a system to recover anhydrous sodium sulphate from the SRS reject stream in the caustic chlorine industry by selective crystallization of sodium sulphate hydrate followed by vaporative/ non-evaporative/ combination dehydration of Sodium Sulphate Hydrate. The anhydrous sodium sulphate can then be sold and employed for other purposes.

Description

A METHOD AND A SYSTEM FOR RECOVERY OF ANHYDROUS SODIUM SULFATE FROM REJECT STREAM OF SULFATE REMOVAL SYSTEM FIELD OF THE INVENTION

The present invention relates to the field of caustic chlorine industry and more particularly to recovery of sodium sulphate from the reject stream obtained from the Sulphate Recovery System in the caustic chlorine industry.

BACKGROUND OF THE INVENTION

Sodium sulphate is introduced in the manufacturing process from the Common Salt in the caustic chlorine Industry. The common salt is dissolved in water and fed to electrolysis system. The depleted brine stream obtained after electrolysis is fed to a Sulphate Removal System (SRS) which is a membrane filtration based system. The Sulphate Removal System generates a permeate stream which is low in sodium sulphate content and a reject stream which is high in sodium sulphate content. The permeate stream thus obtained can be recycled back to the parent process as it contains sodium sulphate at a very low concentration. Owing to the high sodium sulphate content in the reject stream, it cannot be recycled to the original process. Generally, the reject stream has calcium chloride mixed in it. Calcium chloride reacts with sodium sulphate to give sodium chloride & calcium sulphate (gypsum). Calcium sulphate remains in the precipitated form and sodium chloride remains in dissolved form. The precipitate of Gypsum is filtered from the slurry and the filtrate is recycled to the main process of caustic chlorine production and Gypsum wet cake is disposed in the landfill. Alternatively barium chloride is mixed with the reject stream. Barium Chloride reacts with Sodium Sulphate to give Barium Sulphate & Sodium Chloride. The precipitate of barium sulphate is filtered & sent for landfill. The existing state of the art incurs costs because of raw materials and utilities which are required for removal of Sodium Sulphate from reject stream and generates waste in the form of Gypsum /barium sulphate which does not have value as such and hence is highly cost and time ineffective.

SUMMARY

The present invention seeks to redress the problems faced in the existing state of the art. It teaches an innovative method and a system to recover anhydrous sodium sulphate from the SRS reject stream in the caustic chlorine industry by selective crystallization of sodium sulphate hydrate followed by evaporative/non-evaporative/combination dehydration of sodium sulphate hydrate. The anhydrous sodium sulphate can then be sold and employed for other purposes. The hydrate of sodium sulphate is selectively crystallized out from the reject brine stream by changing the operating parameters such as pressure, temperature, mixing etc. The hydrate is the subjected to evaporative/non-evaporative/combination dehydration process for recovery of anhydrous sodium sulphate. The advantages of the present invention comprises eliminating use of raw materials such as calcium chloride/barium chloride, eliminating the need for solid disposal to landfill site and recovery of anhydrous, white & crystalline sodium sulphate.

BRIEF DESCRIPTION OF THE DRAWINGS

The detailed description is described with reference to the accompanying figures. In the figures the left-most digit of a reference number identifies the figure in which the reference number first appears. The same numbers are used throughout the drawings to reference like features and components

Figure lillustrates the process of selective crystallization; evaporation/non evaporation/combination dehydration to obtain recovered anhydrous sodium sulphate.

DETAILED DESCRIPTION:

Exemplary embodiments now will be described with reference to the accompanying drawings. The disclosure may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey its scope to those skilled in the art. The terminology used in the detailed description of the particular exemplary embodiments illustrated in the accompanying drawings is not intended to be limiting. In the drawings, like numbers refer to like elements.

The specification may refer to "an", "one" or "some" embodiment(s) in several locations. This does not necessarily imply that each such reference is to the same embodiment(s), or that the feature only applies to a single embodiment. Single features of different embodiments may also be combined to provide other embodiments.

As used herein, the singular forms "a", "an" and "the" are intended to include the plural forms as well, unless expressly stated otherwise. It will be further understood that the terms "includes", "comprises", "including" and/or "comprising" when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. It will be understood that when an element is referred to as being "connected" or "coupled" to another element, it can be directly connected or coupled to the other element or intervening elements may be present. Furthermore, "connected" or "coupled" as used herein may include wirelessly connected or coupled. As used herein, the term "and/or" includes any and all combinations and arrangements of one or more of the associated listed items. Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this disclosure pertains. It will be further understood that terms, such as those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the relevant art and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein.

The figures depict a simplified structure only showing some elements and functional entities, all being logical units whose implementation may differ from what is shown. The connections shown are logical connections; the actual physical connections may be different.

In addition, all logical units described and depicted in the figures include the software and/or hardware components required for the unit to function. Further, each unit may comprise within itself one or more components, which are implicitly understood. These components may be operatively coupled to each other and be configured to communicate with each other to perform the function of the said unit.

According to a non limiting embodiment of the present invention, the reject stream is subjected to selective crystallization. On changing the parameters such as pressure, temperature etc., the hydrate of sodium sulphate is selectively crystallized from the reject brine stream. The hydrate can be Hepta/Deca/ ixed hydrate of sodium sulphate. The crystallization can be carious out in a conventional crystallizer such as stirred vessel or special crystallizers such' as Oslo type crystallizer. The hydrate of sodium sulphate which is in precipitated form is filtered from the slurry. The filtration can be carried out using commercial available Solid Liquid Separators working in batch/continuous mode. The filtrate obtained which is now depleted in sodium sulphate is fed back to the SRS as input. The solid hydrate of sodium sulphate is then subjected to evaporative/non evaporative/combination dehydration to remove water of crystallization. The dehydration slurry thus obtained is filtered to get wet cake of sodium sulphate. The dehydration filtrate is recycled back to the selective crystallization system. The wet cake of sodium sulphate is dried to obtain anhydrous sodium sulphate.

Figure 1 illustrates selective crystallization, evaporative /non evaporative /combination dehydration to obtain recovered anhydrous sodium sulphate. The depleted brine obtained after electrolysis is subjected Sulphate Removal System (SRS) wherein a permeate stream and a reject stream is obtained. The permeate stream is low in the sodium sulphate content and the reject stream is high in sodium sulphate content and hence the permeate stream can be reused in the parent process while the reject stream cannot be. The hydrate of the sodium sulphate is selectively crystallized out from the reject stream by changing the temperature, pressure etc. The filtrate obtained is fed back to the SRS. The solid hydrate of sodium sulphate is then subjected to evaporative/non-evaporative /combination dehydration process to obtain dehydration slurry. The dehydration slurry is filtered to get wet cake of sodium sulphate which if finally dried to obtain anhydrous sodium sulphate.

In a non limiting embodiment, the reject brine contains lOOgm of sodium sulphate which is subject to sodium sulphate crystallization. Said crystallization can be carried out in the pressure range of 0.1 bar to 10 bars and the temperature range of 100 degrees C to -40 degrees C. The crystallization slurry is filtered to obtain 150 gm of sodium sulphate hydrate which gives a crystallization filtrate having a low sodium sulphate concentration. The hydrate thus obtained is further subjected to dehydration and is heated to obtain dehydration slurry which renders 80 gm of sodium sulphate wet cake after filtration. The wet cake of sodium sulphate is dried to get anhydrous sodium sulphate which is white and crystalline

As will be appreciated by one of skill in the art, the present invention may be embodied as a method, system, or computer program product. Accordingly, the present invention may take the form of an entirely hardware embodiment, a software embodiment or an embodiment combining software and hardware aspects. Furthermore, the present invention may take the form of a computer program product on a computer-usable storage medium having computer-usable program code embodied in the medium.

Furthermore, the present invention was described in part above with reference to flowchart illustrations and/or block diagrams of methods, apparatus [systems , and computer program products according to embodiments of the invention.

It will be understood that each block of the flowchart illustrations and/or block diagrams, and combinations of blocks in the flowchart illustrations and/or block diagrams, can be implemented by computer program instructions. These computer^"program instructions may be provided to a processor of a general purpose computer, special purpose computer, or other programmable data processing apparatus to produce a machine, such that the instructions, which execute via the processor of the computer or other programmable data processing apparatus, create means for implementing the functions/acts specified in the flowchart and/or block diagram block or blocks.

These computer program instructions may also be stored in a computer- readable memory that can direct a computer or other programmable data processing apparatus to function in a particular manner, such that the instructions stored in the computer-readable memory produce an article of manufacture including instruction means which implement the function/act specified in the flowchart and/or block diagram block or blocks.

The computer program instructions may also be loaded onto a computer or other programmable data processing apparatus like a scanner/check scanner to cause a series of operational steps to be performed on the computer or other programmable apparatus to produce a computer implemented process such that the instructions which execute on the computer or other programmable apparatus provide steps for implementing the functions/acts specified in the flowchart and/or block diagram block or blocks.

The flowchart and schematic diagrams illustrate the architecture, functionality, and operations of some embodiments of methods, systems, and computer program products for managing security associations over a communication network. In this regard, each block may represent a module, segment, or portion of code, which comprises one or more executable instructions for implementing the specified logical function(s). It should also be noted that in other implementations, the function(s] noted in the blocks may occur out of the order noted in the figures. For example, two blocks shown in succession may, in fact, be executed substantially concurrently or the blocks may sometimes be executed in the reverse order, depending on the functionality involved.

In the drawings and specification, there have been disclosed exemplary embodiments of the invention. Although specific terms are employed, they are used in a generic and descriptive sense only and not for purposes of limitation, the scope of the invention being defined by the following claims.

Claims

We claim

1. A method for recovering sodium sulphate from the reject stream of a Sulphate Recovery System (SRS), the method comprising

selective crystallization of hydrate of sodium sulphate;

filtration of the hydrate of sodium sulphate; and

dehydration of the hydrate of sodium sulphate

2. The method as claimed in claim 1, wherein the step of selective crystallization of hydrate of sodium sulphate is carried out from the reject stream by changing operating parameters including but not limited to pressure, temperature and mixing.

3. The method as claimed in claim 2, wherein the pressure shift is in the range of 0.1- 10 bars from and the temperature shift is in the range of -40-100 degrees

4. The method as claimed in claim 1, wherein the step of selective crystallization is carried out in a conventional crystallizer or a special crystallizer

5. The method as claimed in claim 1, wherein the hydrate can be a hepta/deca/mixed hydrate of sodium sulphate.

6. The method as claimed in claim 1, wherein step of filtration is carried out using Solid Liquid Separators working in a batch mode or a continuous mode.

7. The method as claimed in 6, wherein the filtrate obtained is depleted sodium sulphate which is fed to the SRS.

8. The method as claimed in claim 1, wherein the step of dehydration removes water of crystallization

9. The method as claimed in claim 8, wherein the step of dehydration can be evaporative and/or non-evaporative

10. The method as claimed in claim 9, wherein the dehydration slurry is filtered & wet cake is dried to obtain anhydrous Sodium Sulphate.

11. The method as claimed in claim 9, wherein the dehydration filtrate is subjected to the step of selective crystallization.