1、 Characteristics and application of PAC
Polyaluminum chloride is a kind of high efficient flocculant, water purifying agent and phosphorus removing agent. Due to its outstanding characteristics and wide application range, its dosage can be reduced by more than 30% compared with the traditional water purification agent, and the cost can be saved by more than 40%. In addition, polyaluminum chloride can also be used to purify drinking water and tap water, such as removing iron, cadmium, fluorine, radioactive pollutants and floating oil.
PAC (polyaluminum chloride) features:
Polyaluminum chloride is between AlCl3 and alncl6-nlm], where M represents the degree of polymerization and N is the neutral degree of PAC products. Polyaluminum chloride (PAC) is also called polyaluminum chloride (PAC) or coagulant. The color of PAC is decorative color or light color, dark brown, dark gray Resin solid. The product has strong bridging adsorption performance. In the hydrolysis process, physical and chemical processes such as condensation, adsorption and precipitation occur.
Application of PAC (polyaluminum chloride)
flocculation
Polyaluminum chloride is mainly used for urban water supply and drainage purification: River water, reservoir water and groundwater; industrial water supply purification, urban Sewage Treatment, recovery of useful substances in industrial wastewater and waste residue, promotion of coal dust settlement in coal washing wastewater, and starch recovery in starch manufacturing industry; Polyaluminum chloride can purify all kinds of industrial wastewater, such as printing and dyeing wastewater, leather wastewater, fluorine-containing wastewater, heavy metal wastewater, oil-bearing wastewater, papermaking wastewater, coal washing wastewater, mine wastewater, brewing wastewater, metallurgical wastewater, meat processing wastewater, etc; Polyaluminum chloride for wastewater treatment: paper sizing, sugar refining, casting molding, cloth wrinkle resistance, catalyst carrier, pharmaceutical refined cement quick setting, cosmetics raw materials.
Dephosphorization
Chemical agents are added to the sewage to make the phosphate ion in the water form insoluble salt, which is separated from the water after the formation of flocs, so as to remove the phosphorus in the water. In the specific reaction process, there are two main reaction processes. The first is that the trivalent aluminum ion reacts with phosphate and precipitates, and the precipitated compound is AlPO4.
Al3++PO43-→AlPO4 ↓
Secondly, the trivalent aluminum ions can be hydrolyzed. In this process, there will be positive charges and mononuclear hydroxyl complexes and polynuclear hydroxyl complexes. After a series of actions, such as van der Waals force and net trapping, the ideal precipitation effect can be achieved, thus meeting the requirements of chemical phosphorus removal.
2、 Quality index of PAC
Quality index of PAC
What are the three most important quality indexes of PAC (polyaluminum chloride), which are the basicity, pH value and alumina content that determine the quality of PAC.
1. Basicity.
The degree of hydroxylation or alkalization of a certain form in PAC is called basicity or alkalinity. It is generally expressed by the molar ratio of hydroxyl aluminum B = [OH] / [al]. Basicity is one of the most important indexes of polyaluminum chloride, which is closely related to flocculation effect. The higher the concentration and salinity of raw water, the better flocculation effect. In conclusion, the best basicity of polyaluminum chloride is 409 ~ 853 in the range of raw water turbidity of 86 ~ 10000mg / L, and many other properties of polyaluminum chloride are related to the basicity.
2. PH value.
The pH of PAC solution is also an important index. It represents the amount of OH - free in the solution. The pH value of polyaluminum chloride generally increases with the increase of basicity, but there is no corresponding relationship between pH value and basicity degree for different liquid compositions. The pH value of the liquid with the same basicity concentration is different when the concentration is different.
3. Alumina content.
The content of alumina in PAC (polyaluminum chloride) is a measure of the effective components of the product. It has a certain relationship with the relative density of the solution. Generally speaking, the higher the relative density, the higher the alumina content. The viscosity of polyaluminum chloride is related to the content of alumina, and increases with the increase of alumina content. Under the same conditions and the same concentration of alumina, the viscosity of polyaluminum chloride is lower than that of aluminum sulfate, which is more conducive to transportation and use.
3、 The difference of PAC color
Due to the different production process and raw materials, the color of PAC produced is also different. Generally, there are six colors of PAC: white, light yellow, golden yellow, brown, brown and coffee.
1. White polyaluminum chloride
A new type of water purification material is newly developed, which is specialized in food, drinking water, urban water supply, water purification for precision manufacturing, papermaking industry, medicine, sugar refining, cosmetic additives, daily chemical industry and so on. It has very high purity and the highest price.
2. Light yellow polyaluminum chloride
Second only to white polyaluminum chloride, the medium and high series products are mainly used for drinking Water Treatment, among which the content of heavy metals is strictly limited. Especially for the drinking water grade polyaluminum chloride products, the treated water is clear without precipitation, and the content of Al2O3 is about 30 (± 0.5), with fine powder, uniform particles, good flocculation effect, high purification efficiency and stability, and small dosage With low cost, it is a special flocculant for water treatment of major waterworks with long-term cooperation.
3. Gold polyaluminum chloride
At present, flocculant and chlorination are widely used in industrial wastewater treatment.
4. Brown, brown and brown polyaluminum chloride
It is a water treatment product produced according to the special requirements of individual customers for water treatment, in which the iron content is higher than other polyaluminum chloride product series, so the color is darker than golden yellow. For low temperature, low turbidity, high algae sewage has strong effect, mainly used in drinking water, urban water supply, industrial water purification and other aspects.
4、 Preparation guide of PAC
According to the different water quality of raw water, a small test should be conducted before use to obtain the optimal dosage. Before use, the solid polyaluminum chloride PAC should be dissolved into a concentration of 5 ~ 10% in order to play an effective role quickly, When dissolving, first use normal temperature tap water to dissolve PAC. Increasing temperature can promote the dissolution slightly, but the water temperature should not exceed 50 degrees. If the water temperature is too high, it will cause thermal degradation of PAC, thus affecting the use effect. Add water to half of the volume of the dissolving tank. Turn on the stirrer to stir. While stirring the water in the dissolving tank, slowly add the solid to make it evenly dispersed in the water. Do not let it form pimples, which is easy to block the pipe and cause waste.
PAC needs sufficient stirring time to dissolve the powder. Insufficient mixing or agglomeration of PAC may affect other performance, and even deposit, block pipeline and dosing pump. Add water to the designated position and adjust to a specific concentration. Continue stirring until PAM is completely dissolved. Since the performance of the mixer will be reduced if it is stirred strongly for a long time, it is recommended that the mixer should be dissolved at the speed of about 20-200 rpm. If the prepared water solution is placed for a long time, its performance will gradually decrease depending on the water quality, so please use it within two days after the water solution is prepared.
Pay attention to the observation and adjustment during the operation. If there is little alum flower, large residual turbidity or too low TP in the sedimentation tank, the dosage is too small; if the alum flower in the sedimentation tank is large and turned up, the residual turbidity is high, or the TP is close to or exceeds the emission index, the dosage is too large and should be adjusted appropriately.
5、 Factors affecting the use of flocculant
1. PH value of water
The pH value of water has a great influence on the use effect of inorganic flocculant, and the value of pH value is related to the type of flocculant, dosage and coagulation sedimentation effect. H + and oh - in water participate in the hydrolysis reaction of flocculants. Therefore, pH value strongly affects the hydrolysis rate of flocculants, the existing forms and properties of hydrolysates.
For example, when the pH value is less than 4, Al3 + can not be hydrolyzed to Al (OH) 3 in a large amount, which mainly exists in the form of Al3 + ions, and the coagulation effect is very poor. When the pH value is between 6.5 and 7.5, Al3 + is hydrolyzed and polymerized to form a neutral colloid with high degree of polymerization. When the pH value is higher than 8, Al3 + is hydrolyzed into AlO2 -, and the coagulation effect becomes very poor.
The alkalinity of water has buffer effect on pH value. When alkalinity is not enough, lime and other chemicals should be added to supplement. When the pH value of water is high, it is necessary to add ACID to adjust the pH value to neutral. In contrast, the effect of pH value on polymer flocculant is small.
2. Water temperature
Water temperature affects the hydrolysis rate of flocculant and the formation rate and structure of alum flocculent. When the water temperature is low, the hydrolysis rate is slow and incomplete.
At low temperature, the viscosity of water is high, Brownian motion is weakened, the collision times of flocculant colloidal particles and impurity particles in water are reduced, and the shear force of water increases, which hinders the mutual adhesion of flocs. Therefore, although the dosage of flocculant is increased, the formation of flocs is still very slow, and the structure is loose and the particles are small, which is difficult to remove.
Low temperature has little effect on polymer flocculant. However, it should be noted that when using organic polymer flocculant, the water temperature should not be too high. The high temperature will easily cause the organic polymer flocculant to age or even decompose into insoluble substances, thus reducing the coagulation effect.
3. Impurities in water
The uneven size of impurity particles in water is beneficial to coagulation, and fine and uniform particles will lead to poor coagulation effect. If the concentration of impurity particles is too low, the coagulation effect can be improved by reflux sedimentation or adding coagulant aid. When the impurity particles in the water contain a large amount of organic matter, the coagulation effect will be poor, so it is necessary to increase the dosage or add oxidant and other coagulants. Calcium and magnesium ions, sulfides and phosphates in water are generally favorable for coagulation, while some anions and surface active substances have adverse effects on coagulation.
4. Types of flocculant
The selection of flocculant mainly depends on the properties and concentration of colloid and suspended solids in water. If the pollutants in the water are mainly in colloidal state, inorganic flocculant should be preferred to make it unstable and coagulate. If the floc is small, it needs to add high molecular flocculant or use activated silica gel and other coagulant aids.
In many cases, the combination of inorganic flocculant and polymer flocculant can obviously improve the coagulation effect and expand the application range. For polymers, the greater the charge amount and the higher the charge density of chain molecules, the more fully extended the chain, the larger the scope of adsorption bridging, and the better coagulation effect.
5. Dosage of flocculant
In the treatment of any wastewater by coagulation, there are the best flocculant and the best dosage, which are usually determined by experiments. If the dosage is too large, the colloid will be stabilized again. The dosage range of ordinary iron salt and aluminum salt is 10-100mg / L, that of polymerized salt is 1 / 2-1 / 3 of that of common salt, and that of organic polymer flocculant is 1-5mg / L.
6. Dosing sequence of flocculant
When a variety of flocculants are used, it is necessary to determine the optimal dosing sequence through experiments. Generally speaking, when inorganic flocculant and organic flocculant are used together, inorganic flocculant should be added first and then organic flocculant.
When the particle size of impurities is more than 50 μ m, organic flocculant is often added to adsorb and bridge, and then inorganic flocculant is added to compress the electric double layer to make the colloid unstability.
7. Hydraulic conditions
In the mixing stage, the flocculant and water should be mixed quickly and evenly. In the reaction stage, not only should sufficient collision opportunities and good adsorption conditions be created, but also the small flocs generated should be prevented from being broken. Therefore, the mixing strength should be gradually reduced and the reaction time should be long enough.
6、 Influencing factors of PAC as phosphorus removal agent
1. Mixing time of wastewater
After the phosphorus removal agent is added into the wastewater, the phosphorus in the wastewater is adsorbed by precipitation and flocculation reaction. Metal ions and phosphate are coagulated to remove phosphorus. Therefore, when the phosphorus removal agent is added, high-intensity mixing and stirring must be carried out immediately, otherwise, the mixing strength will be insufficient, which will hinder the precipitation reaction of phosphate. On the contrary, if the stirring time is too long, the floc growth will be affected.
2. Alkalinity of wastewater
In the process of chemical phosphorus removal, a large amount of calcium carbonate alkalinity in wastewater must be consumed whether aluminum or iron salts are added. When too much alkalinity is consumed and the alkalinity of wastewater is too low, the reaction effect of phosphorus removal agent will be affected. Therefore, alkali should be added immediately once this situation occurs.
3. PH range of wastewater
Whether aluminum salt or iron salt is used as phosphorus removal agent, the pH value range required for the best phosphorus removal effect is 6.5-7.0. In this pH range, aluminum salt and iron salt can play the largest role in precipitation and phosphorus removal. The pH value range of enhanced phosphorus removal agent developed by Changlong science and Technology Institute is 4-6. Within this range, good phosphorus removal effect can be achieved. However, if it is lower than this range, the effect will be affected, and the solubility of some aluminum salt phosphorus removers will be deviated, which will affect the phosphorus removal effect.
4. Suspended solids (SS) and dissolved solids (TDS) in Wastewater
In chemical phosphorus removal, the role of phosphorus removal agent is to coagulate the solid particles in the wastewater, so if there is too much SS in the wastewater, a large amount of phosphorus removal flocculant will be consumed. Therefore, many chemical phosphorus removal processes occur in the case of low SS and low ds.
5. What is the dosage of phosphorus removal agent
In the process of chemical precipitation phosphorus removal, the dosage of phosphorus removal agent has a reverse parabola shape to the phosphorus removal effect. When we test the dosage of the best phosphorus removal effect through experiment, more or less addition will reduce the phosphorus removal effect. In addition, the dosage of phosphorus removal agent is directly affected by the different dosing points.
7、 PAC operation and storage precautions
1. In operation, the water purification process of polyaluminum chloride is generally divided into three stages. The three stages are coagulation stage, flocculation stage and sedimentation stage. In the coagulation stage, when the liquid medicine is injected into the coagulation vessel and coagulates rapidly with the raw water, tiny alum will be formed in a very short time. At this time, the water becomes more turbid, which requires the water flow to produce intense turbulence. Then polyaluminum chloride enters the flocculation stage, which is a process of alum bloom growth and coarsening, which requires appropriate turbulence degree and sufficient residence time (10-15min). In the later stage, a large number of alum flowers can be observed to accumulate and sink slowly, forming a clear surface layer.
When the flocculant is in the sedimentation stage, it is the flocculation sedimentation process in the sedimentation tank, which requires slow water flow. In order to improve the efficiency, inclined tube or plate settler is generally used. A large number of coarse alum flowers are blocked by the inclined pipe (plate) wall and deposited at the bottom of the tank. The upper water is clear water. The remaining alum flowers with small particle size and low density drop slowly while continuing to collide with each other, In the later stage, the turbidity remained unchanged.
2. Polyaluminum chloride must be stored in a dry, moisture-proof and heat-proof place (< 80 ℃, do not damage the package, the product can be stored for a long time).
3. Polyaluminum chloride products must be dissolved before use. Corrosion resistant materials should be used for dissolving equipment and dosing facilities.
4. The effective storage period of the liquid product of polyaluminum chloride is half a year, and the solid product can still be used after being damped.
