In this article, we want to get acquainted with the production methods of polyaluminum chloride (PAC). Click here to learn about this material .
There are several methods, including thermal decomposition, electrolysis, membrane reactor, and dissolving acid and base to produce polyaluminum chloride. Polyaluminum chloride production line generally includes 1- raw material storage and transfer system, 2- main acid and aluminum reactor, 3- polymer processing and growth tank, 4- product filter and refining unit, 5- spray dryer, 6- It is a weighing and packaging unit. The figure below shows an overview of the production line of this product.
In the following, we will examine the three most widely used methods in industrial production processes.
The first method – production with bauxite ore
In this method, bauxite, hydrochloric acid and calcium aluminate are used as raw materials to produce polyaluminum chloride. This production method usually has three stages. In the first stage, bauxite containing 30-40% alumina and 1-5% iron oxide is dissolved in high concentration acid in the form of fine particles with a size smaller than 150 mesh. In this reaction, about 20% of the acid evaporates and finally a solution with a high percentage of alumina is obtained. The reaction temperature with hot steam should reach 100°C for a period of 3-4 hours.
In the next step, the alkalinity is adjusted by calcium aluminate. Aluminum content and alkalinity are the main quality parameters of the final product. Normally, the use of calcium aluminate leads to the production of a stable product with a long shelf life. Adding calcium aluminate to a low alkalinity solution can increase the alkalinity by up to 90%. At this stage, the solution should be stirred for 2 to 4 hours.
The third step is the removal of insoluble substances and ions of other metals. In this stage, various materials such as sodium sulfide and polymer sulfhydryl are used along with heavy sedimentation and filter press to remove heavy metals and other metal ions and unwanted materials. Finally, a product is produced that has more than 10% aluminum oxide and alkalinity between 45-95%.
The second method – production with aluminum hydroxide
In this method, aluminum hydroxide and hydrochloric acid are the main raw materials that enter the reaction container in the form of a suspension. The reaction temperature should be around 180-220°C for 10-12 minutes, then it is raised to 130°C for 10 minutes, and then the temperature is adjusted to 100°C for 3 hours. Next, the alkalinity should be controlled and adjusted as in the previous method.
The third method – production with aluminum powder or granules
This method consists of three steps, in the first step, aluminum-rich solution is produced by dissolving aluminum granules or powder in hydrochloric acid. To improve the acid reaction, it should be preheated at about 65-70 degrees Celsius, and during the reaction, no heat is needed, but adding aluminum should be done slowly. In the second step, sodium aluminate solution is prepared by dissolving aluminum granules or powder in 50-10% caustic soda solution. These steps require about 2 hours each to completely dissolve aluminum and produce Al13.
In the third step, the concentrated sodium aluminate solution is diluted with a little water and slowly added to the initial solution under stirring. And finally, a product with aluminum oxide purity of about 29% and alkalinity of 45% is obtained.
This article is written with a view to the article “Technical and economic review of polyaluminum chloride production by introducing an innovative method of on-site production” .
In the next part, we will compare these production methods.
In the first part of this article , we got acquainted with the various methods of producing polyaluminum chloride or pack .
In this section, we want to compare these methods. To learn more about this material, you can see here .
We have said that there are three main and widely used methods in the industry to produce this material. Now we want to compare these methods from a technical point of view:
In the first method or production with bauxite ore , the raw materials have a low price and purity, but a lot of money and energy must be spent on refining the product, and finally, the waste of the production line, which is rich in impurities and heavy metals, will become an environmental problem. Due to the amount of raw materials required, the production line equipment in this method is bulky and expensive. But due to the absence of high temperature and pressure, the production equipment will be simple, refining units and removal of waste materials are among the additional parts required for this method.
In the second method or production with aluminum hydroxide , raw materials with reasonable price and good quality can be used, but the production process requires more resistant and therefore more expensive equipment due to the need for high temperature and pressure. This method will be more. This method has little environmental risks and the produced product will have an acceptable quality.
In the third method or production with aluminum powder or granules, raw materials with high purity and high price are used.
The need for energy in the production process is much less than other methods, and the equipment of the production line is also more limited. However, due to the production of hydrogen gas in the material mixing reactor, the material of the reactor must be explosion-proof.
It should be noted that in general, the production of this product has few environmental effects, but the comparison of methods shows the difference in these effects. In all methods, as a result of the heat to combine the acid with aluminum, a part of the existing acid evaporates, which must be absorbed in the absorption tower, and depending on the efficiency of the absorption tower used, some acid vapors enter the air as a pollutant. Based on the experiments, about 15-20% of the acid content evaporates, assuming the possibility of recovering half of this amount, eventually 7-10% of the acid vapor will enter the air. This section is the same for all methods.
In addition to the above, due to the use of impure materials (bauxite 28%), the first method requires several stages of purification and the use of polyelectrolytes to settle waste materials, therefore, in each stage of purification, a volume of sludge and sediments is removed, which Often it should be disposed of as waste, so far no complete study has been done on the nature of these wastes, but we know that disposing of these wastes will cause soil pollution and disruption of its natural composition.
In the phase of aluminum dissolution in the third method, in addition to acid evaporation, a volume of hydrogen gas is also produced, which, in case of non-storage, will increase the hydrogen concentration in the environment in addition to the risk of explosion.
The second method without producing waste and with minimal production of acid vapors is the best option from an environmental point of view.
All three methods finally turn the liquid product into powder in the spray dryer process.