As an efficient flocculant, polyaluminium chloride (PAC) has been widely used in water treatment for a long time. In recent years, PAC has been increasingly used in papermaking industry. In the early 1980s, Europe took the lead in the development of distributed rosin gum (DRS)/ polyaluminum chloride (PAC) neutral sizing system, using polyaluminum chloride instead of aluminum sulfate successfully realized the rosin neutral sizing system. At the same time, PAC can also play a good role of retention and filtration in the wet part of paper making.
Polyaluminium Chloride (Polyaluminium Chloride), hereinafter referred to as the PAC, often also referred to as basic aluminium Chloride, poly hydroxy aluminium, etc., it is between aluminium Chloride and aluminum hydroxide two substances of a water-soluble inorganic polymer, usually in general formula Aln (OH) mCl3n - m, where n represents the degree of polymerization, it depends on various factors in the process of preparation, m said some Polyaluminium Chloride products of neutralization degree, that is, how much hydroxyl ions replace the Chloride ion. It can be seen that PAC does not represent a specific inorganic compound, but a series of inorganic polymers.
Polyaluminium chloride (PAC) is a prefabricated hydrolytic product, which is an intermediate product in the process of hydrolyzation-polymerization-precipitation of aluminium salts under certain controlled conditions. It has a tendency to replace traditional aluminium salts (such as aluminium sulfate) no matter as an efficient water treatment agent or as a precipitator for neutral sizing of wet part rosin in papermaking. Since the 1960s, polyaluminum chloride (PAC), as a new and efficient flocculant, has gradually developed into the main agent in the process of coagulation in water treatment. It is a variety with mature industrial production technology, high efficiency and wide application.
At present, polyaluminium chloride in China, Japan, the former Soviet union and the east, Western Europe has a considerable scale of production and application, but in general, application research and production more than basic theory research, and also stay in basic theory research on the mechanism of traditional aluminum salt effect, for this new type of flocculant why has obvious effectiveness is still a lack of depth of scientific validation and theoretical basis, to a certain extent, affected the high quality products to further improve the performance of and application. A large number of practices show that without in-depth basic research, it is difficult to produce highly efficient products.
Europe in the early eighty s was the first to put forward by polyaluminium chloride as rosin sizing precipitant neutral sizing, and success in production, and so far, about its superior to the traditional acid sizing precipitant (such as aluminum sulfate) the cause and the mechanism is not fully clear, among the various published literature, has not been on the new sizing precipitant to do system research, even the application research of the system. This will inevitably affect the further application of PAC, and some related theoretical studies are still in the traditional study of aluminum sulfate and other aluminum hydrolysate characteristics, which is not conducive to the greater use of PAC role.
Whether used as a flocculant for water treatment or used in wet parts of paper making, PAC, as a new, highly efficient and prefabricated inorganic polymer hydrolytic product, has a wide range of applications and remarkable efficiency that traditional aluminum salts cannot compare with.
Polyaluminum chloride in the paper industry in addition to part of the sewage and water treatment (paper), its main use is used for rosin neutral sizing precipitant and retention, filtration aid, in addition, it can also be used to control resin barriers and as anionic impurities capture agent and so on.
1) Polyaluminium chloride is used as a neutral sizing precipitant for rosin
Due to the worldwide shortage of resources, the papermaking with the raw material is becoming more and more precious, use cheaper CaCO3 filler to as much as possible alternative fiber becomes more practical, so Europe in the mid - eighty - s pioneered in neutral rosin sizing technology neutral paper production, the production process of CaCO3 available as filler, the technology is one of the key to the success of polyaluminium chloride instead of the traditional alum as sizing precipitant. Because of polyaluminium chloride is within the scope of the neutral and alkaline remained higher electropositive, unlike Al2 (SO4) 3 as soon precipitation formation of Al (OH) 3, and as a result of polyaluminium chloride hydrolysis, it is not like Al2 (SO4) 3 that reduce the pH of the system is very low, so the polyaluminium chloride are ideal for neutral sizing of new source of aluminum, its sizing pH value can reach 7.5. After Europe, paper mills in North America and many other regions have adopted this technology to convert acidic paper to neutral paper. Using this method for neutral sizing not only greatly reduces the production cost, but also overcomes some unavoidable shortcomings (such as skidding, sizing degree is difficult to control, etc.) when using synthetic sizing materials (such as AKD) for neutral sizing. Even so, because PAC is a complex inorganic polymer in a metastable state, there are still many problems in the process of use, such as whether reasonable selection and stability of the product itself, which need to be further solved in the future research and development.
2) Polyaluminium chloride is used as retention aid and filter aid
Wet side chemistry is the study of the law of surface and colloid chemistry of the interaction between various fine components, additives and fibers in paper during dehydration and forming. In the past decade or so, the main development trend of wet end chemistry has been the transition from acidic to alkaline environment in papermaking process. In recent years, another development trend has been the extensive use of various chemical additives and addition systems in wet end papermaking. Papermaking industry in China is given priority to with non-wood fiber raw material, which mainly gramineous plant fiber raw materials, due to the straw fibre itself exists, miscellaneous cell content high, cause the paper strength is poor, filter the water in the process of writing is difficult, therefore, in research in the writing and use a variety of additives to improve basic conditions keep, filtering performance is suitable for China's national conditions of wet end chemistry research direction.
A. Retention AIDS
At present, many theories believe that the retention of fine fibers and fillers in paper pages is mainly achieved through two ways, namely mechanical interception and colloidal flocculation. With the further research on retention mechanism, the latter is considered to be more important. Mechanical interjection mainly refers to the mechanical retention of particles in the gradually formed page structure. This mechanism is similar to the filtering effect. It is only applicable to the longer fiber part of the paper material, and cannot explain the reason for the high retention rate of small components, nor can it explain the retention effect of retention AIDS.
At present, more and more studies focus on the retention mechanism of fine fibers and fillers. Many different concepts and models have been established in these studies, which provide a theoretical basis for explaining and developing the retention mechanism and optimizing the technological process. Colloid flocculation includes coagulation and flocculation. Coagulation refers to the coagulation effect that the colloidal suspension loses stability through the use of inorganic salts or polymers with small molecular weight and high charge density. The coagulants produced are fine and tightly aggregated precipitates, which are mainly realized through charge neutralization, heterogeneous coagulation and patching.
Flocculation refers to the destabilization of a colloidal suspension by combining colloids together with a long chain of polymers. The resulting flocculation is large, loose and porous clumps. The flocculation is mainly realized in two forms: bridge flocculation and network flocculation.
B. Filtration aid
Because the dehydration process of paper material will directly affect the quality of paper and the running performance of paper machine, and the improvement of water filtration performance can improve the productivity, improve the paper forming, reduce the energy consumption of drying section, so improve the water filtration performance of paper material is one of the important contents of wet section chemistry research. Under normal circumstances, almost all retention AIDS and charge neutralizers are also helpful for filtration, so aluminum sulfate, polyaluminum chloride, anionic starch, polyelectrolyte and particle retention systems can play a role in filtration.
3) Other uses of polyaluminum chloride
With the paper making process gradually from acid to medium, alkaline range, white water sealing degree continues to improve, high yield pulp and coated damaged paper quantity increase, the problem of anionic impurities in pulp is more and more prominent, if not treated in time, it will seriously affect the paper machine operation and paper quality. Practice has proved that itself with a charge of polyaluminium chloride is a good material to capture agent, it can combine with the anion impurities and deposit to the fiber, thus not only control the interference effect of anion impurities in the pulp, also improved the filtering performance of paper pulp, because even a small amount of anionic fiber degradation products will greatly affect the filtering effect of paper material.
At the same time, PAC can be easily adsorbed to the surface of the fiber, but also can adsorb a negative charge of the filler and small fibers, so that they remain more inside the page; The synergistic effect with other organic polymers can effectively increase the retention rate of fine materials and improve the water filtration performance of slurry.
In addition, PAC can effectively control the resin barrier by the same principle as the removal of anionic impurities. Compared with Al2(SO4)3, PAC can fix the resin to the fiber more effectively, and the use of PAC can greatly reduce the deposition of insoluble substances such as BaSO4.
