A brief analysis of the factors affecting the sedimentation state in Dowler’s clarification tanks

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This post was last edited by sunjl1981 on 2013-1-6 at 21:26. 1. The Ca2+/Mg2+ levels in crude salt: During the refining process, the Ca2+ and Mg2+ ions in crude salt undergo the following reactions: Ca2+ + CO32- → CaCO3↓; Mg2+ + 2OH- → Mg(OH)2↓. CaCO3 is a crystalline precipitate with large particles and high density, making it easy to settle, but the reaction rate is quite slow； Mg(OH)2 is a flocculent colloid that remains in a stable dispersed state; it has a low density and does not tend to aggregate or precipitate easily, but its reaction rate is very fast, with the reaction occurring almost instantly. The varying levels of calcium and magnesium in brine have different effects on the quality of the brine. This is mainly manifested as follows: a. When Ca2+/Mg2+ > 2.0, fewer Mg(OH)2 floccules are formed as a result of the reaction; consequently, fewer CaCO3 particles can be entrapped. The remaining CaCO3 particles do not grow easily, and their settling speed is slow. As a result, there are more suspended particles in the clarified brine, resulting in poor transparency ； b. When Ca2+/Mg2+ = 1.5–2.0, the Mg(OH)2 flocs formed can completely encapsulate CaCO3 particles, resulting in the rapid formation of larger-sized particles that are prone to sinking. As a result, there are few suspended particles in the clarified brine, giving it good transparency ； c. When Ca2+/Mg2+ = 1.0–1.5, the formed Mg(OH)2 flocs can completely entrain CaCO3 particles; however, Mg(OH)2 accounts for a relatively high proportion, the particle density is relatively low, the settling speed is slow, and thus the production capacity of the clarification tank is reduced ； d. When Ca2+/Mg2+ < 1.0, the formed Mg(OH)2 flocs can completely entrain CaCO3 particles; however, due to the high content of Mg(OH)2, the particle density is relatively low, resulting in a slow settling speed and reduced production capacity of the clarification tank. Furthermore, some Mg(OH)2 floccules that do not contain CaCO3 remain stably suspended in the saline solution; the longer this persists, the more of them accumulate, resulting in a layer of sludge floating at the bottom of the clarification tank. Since the settling velocity of particles is proportional to the square of the diameter of the suspended particles, and the Ca2+/Mg2+ content in the raw salt is one of the main factors affecting particle size, it is very important to select high-quality raw salt. Furthermore, when raw salt from different batches or from various sources is used alternately, turbidity may occur. It is recommended to mix new salt with old salt first, in order to reduce the impact of changes in the composition of the brine on the Dor barrel. 2. The temperature of brine affects the sedimentation rate of particles; it is proportional to the difference between the density of the suspended solids and that of the clear liquid, and inversely proportional to the viscosity of the suspension. A low Mg2+/Ca2+ ratio only increases the density of the suspended particles, while temperature reduces the density and viscosity of the clear liquid; therefore, higher temperatures facilitate the sinking of particles. Furthermore, a higher temperature of the brine can increase the dissolution rate of raw salt as well as the concentration of the brine, thereby enhancing the production capacity of the equipment ； At the same time, high temperatures can accelerate the refining reaction rate, shorten the reaction time, and facilitate the formation of larger particles. Conversely, when the temperature of the brine is low and its viscosity is high, the settling speed of the particles is slow, making it difficult for them to settle and thus causing the clarification tank to become cloudy again. 3. A stable temperature difference between the inlet and outlet of the clarification tank is conducive to its stable operation. If the temperature difference inside the clarification tank exceeds 3°C due to reasons such as poor insulation of the tank or large fluctuations in the water supply temperature, heat convection of the saline solution will occur as a result of this large temperature difference (this convection is particularly evident when the temperature at the inlet of the tank is higher than that at the outlet), which alters the flow pattern of the clear liquid and solid particles. This, in turn, leads to instability in the sludge layer at the bottom of the tank, causing the tank to become turbid again. 4. Saltwater concentration: The NaCl concentration is directly related to the density of the saltwater. Fluctuations in the NaCl concentration can cause changes in the density of the saltwater, resulting in a difference in density between the saltwater entering the clarification tank and the rest of the liquid. This difference leads to local convection of the saltwater, causing the solution in the clarification tank to become turbid again. 5. Brine flow rate: The processing capacity Q of the clarification tank is equal to the rise velocity v of the clear liquid multiplied by the cross-sectional area S of the clarification tank. If the flow rate of the brine is too high, the residence time of the brine in the clarification tank is reduced, and the rising speed of the clear liquid increases, which is not conducive to particle sedimentation and can easily lead to re-clouding. Furthermore, large fluctuations in the flow rate of brine can impact the sediment seal at the bottom of the tank, causing the sludge to rise and leading to re-clouding. Therefore, it is also essential to maintain an appropriate and stable flow rate of the brine feedwater. 6. Excess alkali amount and pH: During the saltwater purification process, CaCO3 and NaOH are added to remove Ca2+ and Mg2+; the complete reaction time is proportional to the amount of these purifying agents added. That is, at a certain temperature, when the excess alkali in the brine is low, the reaction time is long ； Conversely, when the alkalinity of the brine is high, the reaction time is short. When the amount of base is too low or the amount of refining agent added is less than the theoretical amount, the refining reaction does not proceed completely. On the one hand, Ca2+ and Mg2+ cannot be completely removed ； On the other hand, CaCO3 has difficulty sinking together with the Mg(OH)2 flocs. Of course, it’s not the case that the more refined substance is added, the better ； When the alkali content is too high, the pH of the brine exceeds 12; in this case, the Mg(OH)2 colloid remains stable and is not easily disrupted, making it difficult for it to form large particles with CaCO3. This reduces the settling speed of the particles and can lead to re-clouding of the solution. 7. Preparation and dosage of sedimentation aids: During the operation in the clarification tank, in order to increase the particle size of the CaCO3 and Mg(OH)2 precipitates, it is usually necessary to add flocculants as sedimentation aids. The sedimentation aid used in our factory is solid sodium polyacrylate. This substance is a polymer compound; the electrostatic repulsion between the carboxyl groups on its molecular chains causes the twisted polymer chains to stretch, allowing the functional groups with adsorption properties to come into contact with the surface and form active sites that can adsorb particles, thereby accelerating the sedimentation of suspended particles. When the raw salt meets the required standards, the dosage is 0.5 kg per 100 m3 of brine; it must be spread evenly during addition to avoid clumping, otherwise the amount of coagulant used will be insufficient. Furthermore, coagulants are polymer compounds, and the mixing temperature must be controlled according to their properties; it must not exceed the allowable range, otherwise the coagulation effect will be lost. When the amount of coagulant added is insufficient, the salt-water separation in the central reaction chamber of the clarification tank is not clear, and there are many small particles in the salt water after clarification ； When an excessive amount of flocculant is added, the surplus flocculant will form a stable structure around the precipitates, preventing them from adsorbing other sedimenting particles and forming larger precipitate particles; this increases the viscosity of the saline solution and affects the efficiency of sedimentation. 8. Stirring speed: A stirrer is installed in the clarification tank; its function is to improve the uniformity of the brine, accelerate the reaction rate, and ensure a more complete reaction ； It also increases the probability and intensity of collisions between calcium and magnesium precipitates and the flocculant, facilitating the formation of larger particles, which helps to improve the sedimentation rate and ensures the proper operation of the clarification tank. If the mixer stops operating for some reason, mixing will not take place, which is detrimental to the stable operation of the clarification tank ； Conversely, if the stirring speed of the stirrer is too high, it will change the overall flow direction of the saltwater, causing some disturbance in the saltwater; as a result, the clear liquid carries up some solid particles, which hinders sedimentation. 9. Sludge discharge volume: Excessive or insufficient sludge discharge can affect the proper operation of the clarification tank. The sludge discharge cycle is short; discharging sludge too quickly or in excessive amounts can disrupt the stability of the sludge layer at the bottom of the clarification tank, causing this layer to become thinner and rise to the surface, as well as leading to the emergence of salty sludge and a decrease in the transparency of the saline water ； The sludge discharge cycle is long and the amount of sludge discharged is insufficient; as particles in the brine continue to settle, more and more sludge accumulates at the bottom of the clarification tank, forming a thicker layer of sludge. This reduces the space between the central reaction chamber and this sludge layer, and even slight changes in the flow rate or temperature of the brine can cause the sludge to rise to the surface. However, it should be noted that the salt sludge at the bottom of the tank cannot be emptied; otherwise, the large particles in the brine coming out of the central reaction chamber will sink downward due to inertia and gravity, and they will not come into contact with the sludge layer, so they won’t be captured by it immediately. Instead, they will be carried along with the brine to the clear liquid area, affecting the clarity of the clear brine. # + + . hcbbs