The insolubles in soda ash water remain persistently high. What should be done?
Thread Content
In the combined soda process, the level of water-insoluble substances remains high, which directly affects market sales. How can this be resolved?
Excerpt from: “Technological verification of the content of calcium and magnesium impurities in the combined soda ash process”, Soda Ash Industry, published on December 15, 2000. (1) The quality of raw salt has a significant impact on the quality of both soda ash and ammonium chloride products. Therefore, to improve the product quality in the combined soda ash process, it is necessary to start with improving the quality of raw salt, primarily by reducing the water-soluble impurities contained therein. (2) Further improving the ability of the salt washing process to remove calcium and magnesium impurities is an effective way to enhance product quality, and operating the calcium and magnesium reactor to reduce the content of such impurities in the washed brine is undoubtedly the best approach. (3) Strengthening the operation and management of AⅡ clarification tanks can also improve product quality; at the very least, it can significantly reduce the calcium content in the product. (4) By taking other measures, including clarification and filtration of mother liquor II, it is also possible to reduce the impact of calcium and magnesium impurities on product quality.
Purifying the mother liquor in the soda ash process to improve the quality of soda ash products (reposted from China Chemical Industry Information). The main factors affecting the quality of soda ash products include the contents of Cl-, SO42-, Ca2+, Mg2+, Fe3+/Fe2+ in the product, as well as its loss on ignition. The loss on ignition can be controlled by the calcination process ； The content of Cl- is determined by the design of the production process and its operational control ； The amount of SO42- is primarily determined by the input volume of raw salt; thus, it can be controlled by regulating the SO42- index in the raw salt ； Fe3+/Fe2+ is mainly introduced due to equipment corrosion during the production process; however, it can be effectively controlled as long as the process parameters are properly managed. Ca2+ and Mg2+ originate from raw salt and some recycled wastewater. In the traditional production process, they are removed along with the product and AⅡ sludge. However, the clarification effect of the mother liquor often results in excessive levels of water-insoluble substances (Ca2+, Mg2+) in the product, thereby affecting the quality of the soda ash product. 1 Current status of the purification of ammonia-soda mother liquor: The ammonia-soda process for producing soda ash features a dedicated brine purification step, which removes calcium, magnesium, and other precipitates brought in by raw salt. This yields purified brine with a magnesium content of less than 6 mg/L and a calcium content of less than 20 mg/L; after ammonia absorption, soda ash is produced from this brine. In the combined soda process, however, calcium, magnesium, and other impurities present in the raw salt enter the system. The resulting calcium and magnesium precipitates remain within the system. After heat exchange with the mother liquor and ammonia absorption, the mixture is clarified and then discharged from the system via the AII sludge outlet. Prior to ammonia absorption, the mother liquor contains a large amount of suspended impurities such as calcium, magnesium, and sediment; these tend to adhere to and form scale on heat exchangers and pipelines. Consequently, these pieces of equipment must be cleaned periodically, which inevitably affects production ； For the mother liquor exchanger, only shell-and-tube heat exchangers with low heat transfer efficiency can be selected; scaling and sludge blockage also pose problems for the operation of efficient clarification equipment. Currently, this purification process is used in all soda ash production. Since the raw salt used in soda ash production is not refined, it is also impossible to refine it due to the influence of mother liquor equilibrium. After crude salt enters the system, while the salting-out reaction of ammonium chloride occurs, calcium and magnesium react with carbonate ions to form carbonate precipitates. Since the saturation levels of calcium and magnesium in the mother liquor increase with rising temperature, studies have shown that at 31°C, the saturation levels of Ca2+ and Mg2+ in the mother liquor are 3–4 times higher than at 19°C. Therefore, the calcium and magnesium precipitates formed at low temperatures (around 16°C) should be removed from Mother Liquor II, which has low solubility. They should not be allowed to partially dissolve during the subsequent heating of the mother liquor as part of the ammonia mother liquor II preparation process; otherwise, the precipitate particles would become smaller in size before they can be clarified. This is one of the unreasonable aspects of the current mother liquor purification process in soda ash production. Additionally, the purification of combined alkali mother liquor involves only single-stage clarification. The production capacity enhancement efforts have not been sufficient, as the clarification area is generally too small. Production fluctuations occur, and after clarification, the turbidity of Ammonia Mother Liquor II fluctuates and remains unstable. Coupled with a lack of emphasis on the clarification process for Ammonia II, these factors are currently responsible for the high levels of calcium and magnesium impurities in the resulting soda ash products. Therefore, improving the purification process and refining methods for combined alkali mother liquor is the primary way to enhance the quality of soda ash products at present. 2 Principles of mother liquor purification The principle of purifying Ammonia Mother Liquor II in the combined soda process is as follows: during the ammonia absorption process of the mother liquor, double salt precipitates of calcium carbonate and magnesium carbonate are formed; these are then removed from the ammonia mother liquor in a clarification tank to eliminate Ca2+ and Mg2+. Some have questioned this theory: are calcium and magnesium carbonates formed after ammonia absorption, or are they already formed during the salting-out process? The bicarbonates in Mother Liquor I are converted into Ammonia Mother Liquor I during ammonia absorption; when controlling the α value, they are transformed into carbonates. In salt-out semi-mother liquor II, there is a sufficient concentration of carbonate ions to undergo a chemical reaction with the calcium and magnesium ions introduced by the added raw salt, resulting in the formation of carbonate precipitates. At this time, the temperature is low; consequently, the solubility and equilibrium concentrations of Ca2+ and Mg2+ are also low. Therefore, the removal of calcium and magnesium can proceed to a considerable extent at low temperatures. Furthermore, since the combined soda process utilizes ammonia gas from the synthetic ammonia system for ammonia absorption—unlike the ammonia used in the ammonia-soda process, which originates from the ammonia stripping step—the ammonia gas contains a large amount of carbon dioxide distilled out from the mother liquor. Therefore, the reactions for removing calcium and magnesium in the combined soda process do not occur after the ammonia absorption step in Mother Liquor II. Consequently, such reactions cannot take place during this ammonia absorption process. Meanwhile, calcium and magnesium precipitates have already formed in the salting-out crystallizer. This provides a theoretical basis for changing the purification process of Ammonia Mother Liquor II to that of Mother Liquor II. Of course, after the ammonia absorption step in Mother Liquor II, the pH value increases further, which is more conducive to the precipitation of basic salts as well as additional carbonate precipitation. 3 Two optional purification processes for the mother liquor
3.1 Traditional purification process for ammonia mother liquor II
Currently, the main types of clarifiers used in the soda ash industry for treating combined soda mother liquor are: Dorr clarifiers (single- or multi-stage), honeycomb clarifiers, inclined plate clarifiers, and modified Dorr clarifiers. In selecting a clarifier tank, apart from the requirement of having a sufficient clarification area from a process standpoint, it should also possess characteristics such as operational stability, great flexibility, rapid rise rate of clarified liquid, and high capacity. Structurally, it should be easy to maintain and manage, require minimal cleaning and servicing, and have a long operating cycle. Although the single-layer Dorr barrel features a simple structure, is easy to operate, and operates stably and reliably, it has drawbacks such as large size, low efficiency, and high investment costs. Although the multi-layer Doyle tank occupies less space and requires lower investment, its layer height is constrained by process layout requirements. It suffers from unstable operation, difficulty in exhaust ventilation between layers, and tendency for scaling in inlet/outlet pipes; this leads to accumulation of calcium and magnesium sludge that must be periodically cleaned out ； Inclined plates (tubes) in honeycomb clarifiers have also experienced issues such as collapse or floating; thus, their maintenance is relatively difficult. This is highly undesirable in large-scale production. Although the improved Dowell tank is large in size, it has a simple and stable structure. The addition of forced flocculation promotes the growth of aggregates, resulting in the formation of dense flocs. Its inverted conical outer shape ensures that when the suspension enters the filter layer, the average upward velocity of Ammonia Mother Liquor II decreases as the cross-sectional area of the tank increases exponentially from bottom to top. This naturally forms a layer of suspended slurry, which functions as a filter for the suspension. The inverted cone shape also thickens this sludge layer, thereby enhancing the clarification efficiency. This is a single-layer barrel, which is easy to manage in production and has a long operating cycle. From the comparison of the above-mentioned clarifiers, it can be seen that the use of a modified Dorr clarifier for clarifying the soda ash process mother liquor yields better results. Its purification process is as follows: After mother liquor II absorbs ammonia, the ammoniated mother liquor II enters a separator, where any entrained inert gases are separated. It is then mixed with an added flocculant. From there, it is distributed to various clarification tanks via a distribution tank located at the bottom of the separator, according to the capacity of each tank. The clarified ammoniated mother liquor is directed to the storage tank for ammoniated mother liquor II. The calcium and magnesium sludge precipitated in the clarification tanks is discharged from the tank bottoms and sent to Ammonia II Sludge for filtration; the recovered clear liquid is returned to the distribution tank. The flocculant is added to the flocculant dissolution tank; after dissolution, it flows into the flocculant storage tank. Depending on the requirements for clarification in the clarifier, it is then added to the distribution tank as needed. 3.2 Advanced purification processes for mother liquor II Currently, the traditional purification process for ammonia mother liquor II is still being used in soda ash production. However, this process is not entirely reasonable; it is an inefficient and unclean method that gives rise to numerous problems in production management. It also hinders energy conservation efforts and further improvements in the quality of soda ash products. As a result, over time, numerous technical personnel involved in soda ash production have been exploring various new technologies for refining mother liquor. 3.2.1 Two-stage clarification and purification process for soda ash mother liquor: According to available data, the solubility product of calcium carbonate in concentrated brine decreases as temperature rises; the solubility of magnesium increases significantly with rising temperature; and temperature has a considerable effect on the sedimentation rates of calcium and magnesium. Based on these principles, the pH value after ammonia absorption by water and Mother Liquor II further increases. Can a two-stage clarification and purification process be considered for the soda ash mother liquor? Salting-out overflow mother liquor II is fed into the Mother Liquor II clarification tank, where low-temperature conditions are used to remove the magnesium carbonate, which has a low solubility in Mother Liquor II. After that, Mother Liquor II is heat-exchanged with Ammonia Mother Liquor I, followed by ammonia absorption. After the temperature is increased, secondary clarification of Ammonia Mother Liquor II is carried out under a low impurity load. This process is not entirely a clean process, but it represents an improvement over the original purification process for ammonia mother liquor II, and is more conducive to the precipitation of magnesium from the mother liquor. The key to using this process is the rising speed of the clear liquid resulting from the clarification of Mother Liquor II. In the mother liquor of the ammonia-soda process, the sedimentation of solid particles follows Stokes’ law. For free sedimentation with a constant liquid composition and particle size, the sedimentation velocity depends on the viscosity of the liquid; a higher viscosity results in a slower sedimentation rate. The viscosity of the soda ash process mother liquor is primarily a function of temperature; it is higher at lower temperatures. At 15°C, the viscosity of the mother liquor is twice that at 40°C. Therefore, a low mother liquor temperature is detrimental to clarification. In production practice, there have been precedents for the clarification of Mother Liquor II. For instance, in Asahi Glass Co., Ltd.’s Shin-Ashi process for soda ash production in Japan, a low-temperature condition is required in the external cooling tower; thus, a clarification process using Mother Liquor II at 20°C was employed. This has proven that the clarification process for Mother Liquor II is feasible under 20°C conditions. However, there are no precedents for industrial production in China yet. When mother liquor II is clarified at 20°C, determining the rate at which the clear liquid rises is a technical issue that needs to be resolved in order to adopt this technological route; naturally, it also involves economic considerations such as investment. 3.2.2 Consider using a mother liquor filtration method to purify Mother Liquor II. Employing this filtration method for Mother Liquor II can replace large-scale clarification equipment with smaller-sized filtration equipment, thus reducing the required floor space. At the same time, it makes the purification process a clean one. Currently, this filtration method is generally used for mother liquors with very low impurity content or for the secondary fine filtration of such liquors ； It is used for the primary filtration of mother liquors with high impurity content; however, situations involving very large processing volumes are relatively rare. In particular, treating mother liquors that contain extremely fine precipitate particles and are difficult to filter still poses certain technical challenges. The soda ash industry has done a great deal of work in this regard; it has attempted to filter mother liquor II and obtained some data. However, there are still several problems preventing its application in engineering practice—namely, extremely high investment costs and relatively complex management requirements. Filtering of caustic soda mother liquor is not only a problem due to small sediment particles and large amounts of mother liquor, but also an issue of scarring. This causes the filtration rate of the filter medium to decline rapidly, thus shortening its service life. The small-sized precipitated particles make it a difficult-to-filter medium. The large volume of mother liquor and low filtration rate result in an excessively large filtration area, rapid decline in filtration speed, short operating cycles, frequent cleaning requirements, and extremely complex operations. Although a great deal of work has been done in this area and a significant amount of data has been collected, selecting appropriate equipment that can be used continuously and evenly in engineering applications is still a process that requires further work and testing; moreover, a detailed technical and economic evaluation is needed from an economic perspective. 3.2.3 Current progress in technology for mother liquor filtration. The use of Gore membranes for filtering Mother Liquor II was unsuccessful. This is because Gore membranes operate on a surface filtration principle, utilizing the membrane’s micropores for high-precision liquid filtration; their fatal drawback is that they cannot handle viscous substances. When using PE microporous filters to filter natural soda liquor, the filtration intensity reaches only 0.014 m³/(m²·h). Moreover, as the impurity content increases, the filtration intensity decreases. After being used for some time, although it can be regenerated with dilute acid, it can still only be used for about one year ； Tests were conducted on fiber bundle filters, and the filtration effect was satisfactory; however, further testing could not be carried out due to the bonding of the fiber bundles. Although Gore membranes and fiber filters have been applied to the purification of caustic soda brine, no significant progress has been made in the purification of ammonium bicarbonate mother liquor. Tests were conducted on the secondary brine using the fiber ball filtration ammonia-alkali method; when the suspended solids concentration at the inlet of the secondary brine was 68.5 mg/L, it dropped to 5.45 mg/L at the outlet, representing a reduction of 90.82%. The filtration rate reached 33 m/h, and the production capacity was 31.74 m3/(m2·h). However, this technology is still only in the experimental stage. Fiber balls are polyester filaments, and fiber ball filtration technology is a fine solid-liquid separation technique that has developed within the field of deep filtration. Based on the filtration tests of the secondary brine in the ammonia-alkali process, it can be seen that the purification efficiency is high and the filtration speed is fast; however, the behavior of the secondary brine under varying conditions as well as its service life still need to be tested. Filtration tests of Soda Ash Combined Process ammonia mother liquor II and caustic soda brine were conducted using a planting membrane. The planting film is a composite material of POLYMER/PV that is planted during the weaving process. At the beginning of filtration, solid particles can be completely trapped. The turbidity of the filtrate can
3.1 Traditional purification process for ammonia mother liquor II
Currently, the main types of clarifiers used in the soda ash industry for treating combined soda mother liquor are: Dorr clarifiers (single- or multi-stage), honeycomb clarifiers, inclined plate clarifiers, and modified Dorr clarifiers. In selecting a clarifier tank, apart from the requirement of having a sufficient clarification area from a process standpoint, it should also possess characteristics such as operational stability, great flexibility, rapid rise rate of clarified liquid, and high capacity. Structurally, it should be easy to maintain and manage, require minimal cleaning and servicing, and have a long operating cycle. Although the single-layer Dorr barrel features a simple structure, is easy to operate, and operates stably and reliably, it has drawbacks such as large size, low efficiency, and high investment costs. Although the multi-layer Doyle tank occupies less space and requires lower investment, its layer height is constrained by process layout requirements. It suffers from unstable operation, difficulty in exhaust ventilation between layers, and tendency for scaling in inlet/outlet pipes; this leads to accumulation of calcium and magnesium sludge that must be periodically cleaned out ； Inclined plates (tubes) in honeycomb clarifiers have also experienced issues such as collapse or floating; thus, their maintenance is relatively difficult. This is highly undesirable in large-scale production. Although the improved Dowell tank is large in size, it has a simple and stable structure. The addition of forced flocculation promotes the growth of aggregates, resulting in the formation of dense flocs. Its inverted conical outer shape ensures that when the suspension enters the filter layer, the average upward velocity of Ammonia Mother Liquor II decreases as the cross-sectional area of the tank increases exponentially from bottom to top. This naturally forms a layer of suspended slurry, which functions as a filter for the suspension. The inverted cone shape also thickens this sludge layer, thereby enhancing the clarification efficiency. This is a single-layer barrel, which is easy to manage in production and has a long operating cycle. From the comparison of the above-mentioned clarifiers, it can be seen that the use of a modified Dorr clarifier for clarifying the soda ash process mother liquor yields better results. Its purification process is as follows: After mother liquor II absorbs ammonia, the ammoniated mother liquor II enters a separator, where any entrained inert gases are separated. It is then mixed with an added flocculant. From there, it is distributed to various clarification tanks via a distribution tank located at the bottom of the separator, according to the capacity of each tank. The clarified ammoniated mother liquor is directed to the storage tank for ammoniated mother liquor II. The calcium and magnesium sludge precipitated in the clarification tanks is discharged from the tank bottoms and sent to Ammonia II Sludge for filtration; the recovered clear liquid is returned to the distribution tank. The flocculant is added to the flocculant dissolution tank; after dissolution, it flows into the flocculant storage tank. Depending on the requirements for clarification in the clarifier, it is then added to the distribution tank as needed. 3.2 Advanced purification processes for mother liquor II Currently, the traditional purification process for ammonia mother liquor II is still being used in soda ash production. However, this process is not entirely reasonable; it is an inefficient and unclean method that gives rise to numerous problems in production management. It also hinders energy conservation efforts and further improvements in the quality of soda ash products. As a result, over time, numerous technical personnel involved in soda ash production have been exploring various new technologies for refining mother liquor. 3.2.1 Two-stage clarification and purification process for soda ash mother liquor: According to available data, the solubility product of calcium carbonate in concentrated brine decreases as temperature rises; the solubility of magnesium increases significantly with rising temperature; and temperature has a considerable effect on the sedimentation rates of calcium and magnesium. Based on these principles, the pH value after ammonia absorption by water and Mother Liquor II further increases. Can a two-stage clarification and purification process be considered for the soda ash mother liquor? Salting-out overflow mother liquor II is fed into the Mother Liquor II clarification tank, where low-temperature conditions are used to remove the magnesium carbonate, which has a low solubility in Mother Liquor II. After that, Mother Liquor II is heat-exchanged with Ammonia Mother Liquor I, followed by ammonia absorption. After the temperature is increased, secondary clarification of Ammonia Mother Liquor II is carried out under a low impurity load. This process is not entirely a clean process, but it represents an improvement over the original purification process for ammonia mother liquor II, and is more conducive to the precipitation of magnesium from the mother liquor. The key to using this process is the rising speed of the clear liquid resulting from the clarification of Mother Liquor II. In the mother liquor of the ammonia-soda process, the sedimentation of solid particles follows Stokes’ law. For free sedimentation with a constant liquid composition and particle size, the sedimentation velocity depends on the viscosity of the liquid; a higher viscosity results in a slower sedimentation rate. The viscosity of the soda ash process mother liquor is primarily a function of temperature; it is higher at lower temperatures. At 15°C, the viscosity of the mother liquor is twice that at 40°C. Therefore, a low mother liquor temperature is detrimental to clarification. In production practice, there have been precedents for the clarification of Mother Liquor II. For instance, in Asahi Glass Co., Ltd.’s Shin-Ashi process for soda ash production in Japan, a low-temperature condition is required in the external cooling tower; thus, a clarification process using Mother Liquor II at 20°C was employed. This has proven that the clarification process for Mother Liquor II is feasible under 20°C conditions. However, there are no precedents for industrial production in China yet. When mother liquor II is clarified at 20°C, determining the rate at which the clear liquid rises is a technical issue that needs to be resolved in order to adopt this technological route; naturally, it also involves economic considerations such as investment. 3.2.2 Consider using a mother liquor filtration method to purify Mother Liquor II. Employing this filtration method for Mother Liquor II can replace large-scale clarification equipment with smaller-sized filtration equipment, thus reducing the required floor space. At the same time, it makes the purification process a clean one. Currently, this filtration method is generally used for mother liquors with very low impurity content or for the secondary fine filtration of such liquors ； It is used for the primary filtration of mother liquors with high impurity content; however, situations involving very large processing volumes are relatively rare. In particular, treating mother liquors that contain extremely fine precipitate particles and are difficult to filter still poses certain technical challenges. The soda ash industry has done a great deal of work in this regard; it has attempted to filter mother liquor II and obtained some data. However, there are still several problems preventing its application in engineering practice—namely, extremely high investment costs and relatively complex management requirements. Filtering of caustic soda mother liquor is not only a problem due to small sediment particles and large amounts of mother liquor, but also an issue of scarring. This causes the filtration rate of the filter medium to decline rapidly, thus shortening its service life. The small-sized precipitated particles make it a difficult-to-filter medium. The large volume of mother liquor and low filtration rate result in an excessively large filtration area, rapid decline in filtration speed, short operating cycles, frequent cleaning requirements, and extremely complex operations. Although a great deal of work has been done in this area and a significant amount of data has been collected, selecting appropriate equipment that can be used continuously and evenly in engineering applications is still a process that requires further work and testing; moreover, a detailed technical and economic evaluation is needed from an economic perspective. 3.2.3 Current progress in technology for mother liquor filtration. The use of Gore membranes for filtering Mother Liquor II was unsuccessful. This is because Gore membranes operate on a surface filtration principle, utilizing the membrane’s micropores for high-precision liquid filtration; their fatal drawback is that they cannot handle viscous substances. When using PE microporous filters to filter natural soda liquor, the filtration intensity reaches only 0.014 m³/(m²·h). Moreover, as the impurity content increases, the filtration intensity decreases. After being used for some time, although it can be regenerated with dilute acid, it can still only be used for about one year ； Tests were conducted on fiber bundle filters, and the filtration effect was satisfactory; however, further testing could not be carried out due to the bonding of the fiber bundles. Although Gore membranes and fiber filters have been applied to the purification of caustic soda brine, no significant progress has been made in the purification of ammonium bicarbonate mother liquor. Tests were conducted on the secondary brine using the fiber ball filtration ammonia-alkali method; when the suspended solids concentration at the inlet of the secondary brine was 68.5 mg/L, it dropped to 5.45 mg/L at the outlet, representing a reduction of 90.82%. The filtration rate reached 33 m/h, and the production capacity was 31.74 m3/(m2·h). However, this technology is still only in the experimental stage. Fiber balls are polyester filaments, and fiber ball filtration technology is a fine solid-liquid separation technique that has developed within the field of deep filtration. Based on the filtration tests of the secondary brine in the ammonia-alkali process, it can be seen that the purification efficiency is high and the filtration speed is fast; however, the behavior of the secondary brine under varying conditions as well as its service life still need to be tested. Filtration tests of Soda Ash Combined Process ammonia mother liquor II and caustic soda brine were conducted using a planting membrane. The planting film is a composite material of POLYMER/PV that is planted during the weaving process. At the beginning of filtration, solid particles can be completely trapped. The turbidity of the filtrate can
Ultimately, it is due to insufficient clarification of the mother liquor; either the type of salt needs to be changed, or the desalination process for the salt needs to be improved, or else the capacity to clarify the mother liquor must be enhanced.
It might be due to washing; check the ammonia scrubber and calcination section
Water-insoluble stuff, huh? Can it be washed off with water?