Is named aluminum polychloride to a class of water-soluble inorganic aluminum products, formed by the partial reaction of aluminum chloride AlCl3 with a base. It is a white to yellow solid. Its general formula is often expressed as Aln(OH)mCl(3n-m). They are also known as PAC or also PACl (acronym of English PolyAluminum Chloride).
PACs are formulated so that they contain highly cationic polymers (assemblies of several molecules with many positive charges) made up of aluminum ions (Al3+), chloride ions (Cl-), hydroxyl ions (OH)- and water molecules (HtwoOR).
The most important cationic polymer of these species is called Al13 o Keggin-Al13 which is very effective in water treatment and in the pulp and paper manufacturing industry.
In these applications, the PACs adhere to the surface of the particles causing them to bind together and can settle, that is, fall to the bottom and can be filtered..
It has also been successfully tested to improve the properties of Portland cement, as it modifies or changes its structure at a micro level and this makes the cement more resistant..
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PAC or PACl is made up of a series of species ranging from monomers (a single molecule), dimers (two molecules joined together), oligomers (three to five molecules joined together) to polymers (many molecules joined together).
Its general formula is Aln(OH)mCl(3n-m). When dissolved in water these species contain Al ions3+, hydroxyl ions OH-, chloride ion Cl- and water molecules HtwoOR.
In aqueous solution its general formula is Alx(OH)Y(HtwoOR)n(3x-y)+ or also AlxORz(OH)Y(HtwoOR)n(3x-y-2z)+.
The most useful of these polymers is called Al13 or Keggin-Al13 whose formula is AlO4To the12(OH)24(HtwoOR)127+. The species Al13 has a three-dimensional shape.
It is estimated that the precursor of this polycation is Al (OH)4-, which presents a tetrahedral conformation and is located in the center of the structure.
- Aluminum polychloride
- PAC Poly Aluminum Chloride)
- PACl (acronym of English Poly Aluminum Chloride)
- Polyaluminum Chloride
- Aluminum polyhydroxychloride
- Aluminum hydrochloride or ACH (acronym of English Aluminum ChlorHydrate).
White to yellow solid (powder) that is also obtained in the form of aqueous solutions of different concentrations.
Soluble in water.
The various PACs differ from each other mainly by two things:
- Its strength, expressed as% alumina AltwoOR3.
- Its basicity, which indicates the amount of polymeric material in the PAC, and can vary between 10% (low basicity), 50% (medium basicity), 70% (high basicity) and 83% (highest basicity, which corresponds to aluminum hydrochloride or ACH).
PAC is a kind of water soluble aluminum products. Its general formula is often expressed as Aln(OH)mCl(3n-m).
Since they are produced by reacting aluminum chloride (AlCl3) with a base, the basicity of this type of product depends on the relative amount of OH ions- compared to the amount of aluminum (Al).
According to the formula Aln(OH)mCl(3n-m), basicity is defined as m / 3n.
It is a flocculant. It has properties such as ease of adsorption on other particles of opposite charge (it adheres to the surface of these), coagulation (union of several particles on which it has been adsorbed) and precipitation of these groups of united particles.
PACs can be unstable as they depend on pH. They can be corrosive.
When dissolving the PAC in water and depending on the pH, various species of aluminum-hydroxyl (Al-OH) are formed.
Hydrolyzes or reacts with water to form monomers (unitary molecules), oligomers (3 to 6 molecules linked) and polymers (more than 6 linked molecules).
The most important species is a polymer with 13 aluminum atoms, which is called Keggin-Al13.
The Keggin-Al13 polymer adsorbs on the particles present in the water, that is, it sticks on the surface of these, and causes them to add to each other forming flocs.
Flocs are groups of very small particles agglutinated or united to form larger structures that can sediment, that is, go to the bottom of the aqueous solution..
After forming flocs, when they are large enough they go to the bottom and the aqueous solution is clean.
PAC or PACl solutions are generally obtained by adding a base or alkaline solution to a solution of aluminum chloride (AlCl3).
To obtain a high amount of Al polymers13 the base or alkali added must not provide OH ions- too fast not too slow.
Studies indicate that it is difficult to produce a stable high concentration of Al13 using NaOH because it releases OH ions- too fast in the water.
For this reason, basic calcium (Ca) compounds are preferred, which have low solubility in water and thus release OH ions.- slowly. One of these basic calcium compounds is calcium oxide CaO.
Here are the steps that occur for the formation of the PAC.
When the aluminum salts (iii) dissolve in water, a spontaneous hydrolysis reaction occurs in which the aluminum cation Al3+ takes hydroxyl ions OH- of water and binds to them, leaving protons H+ free:
To the3+ + HtwoO → Al (OH)two+ + H+
To the3+ + 2 htwoO → Al (OH)two+ + 2 h+
This is favored by adding an alkali, that is, OH ions.-. Aluminum Al ion3+ increasingly binds to OH anions-:
To the3+ → Al (OH)two+ → Al (OH)two+ → Al (OH)30 → Al (OH)4-
In addition, species such as Al (HtwoOR)63+, that is, an aluminum ion bound or coordinated with six water molecules.
Then bonds are formed between these species, forming dimers (sets of 2 molecules) and trimers (sets of 3 molecules) that are transformed into oligomers (sets of 3 to 5 molecules) and polymers (sets of many joined molecules).
Al (OH)two+ → Altwo(OH)two4+ → Al3(OH)54+ → Al6(OH)126+ → Al13(OH)327+
This type of species is linked by OH bridges with each other and with the Al (HtwoOR)63+ forming sets of molecules that are called hydroxy complexes or polycations or hydroxypolymers.
The general formula of these cationic polymers is Alx(OH)Y(HtwoOR)n(3x-y)+ or also AlxORz(OH)Y(HtwoOR)n(3x-y-2z)+.
The most useful of these polymers is thought to be called Al13 whose formula is AlO4To the12(OH)24(HtwoOR)127+, and it is also known as Keggin-Al13.
It is a species with 7 positive charges (that is, a heptavalent cation) with 13 aluminum atoms, 24 OH units, 4 oxygen atoms and 12 H water units.twoOR.
PACl is a commercial product to treat water and make it drinkable (clean and drinkable). It also allows treating waste and industrial water.
It is used as a coagulation agent in water improvement processes. It is more effective than aluminum sulfate. Its performance or behavior depends on the species present, which depends on the pH.
PACl allows organic material and mineral particles to be coagulated. Coagulate means that the compounds to be eliminated go from being dissolved to being solid. This is achieved through the interactions of its positive charges with the negative ones of the materials to be coagulated..
The species Al13, as it has so many positive charges (+7), it is the most effective in neutralizing charges. Then there is the formation of bridges between the particles which agglomerate and form flocs.
These flocs, being very heavy, tend to precipitate or settle, that is, to go to the bottom of the container that contains the water that is being treated. In this way they can be removed by filtration.
PAC is better than aluminum sulfate because it has better low temperature performance, leaves less aluminum residue, produces less sludge volume, less effect on water pH, and faster and larger flocs are formed. All of which facilitates sedimentation for later filtration..
PAC is especially effective in modifying colloidal fillers in papermaking. Colloidal charges are the charges of the suspended solids in the mixtures to make paper pulp..
It allows to accelerate the speed of drainage (elimination of water) especially in neutral and alkaline conditions, and helps in the retention of solids. The solids are those that then, when drying, form the paper.
In this application PAC with low (0-17%) and medium (17-50%) basicities are used.
Recently (2019) adding PACl to Portland cement has been tested. It was determined that the presence of chloride ions Cl- and of the polymeric groups of aluminum changes the structure of the cement. It is estimated that complex salts of formula 3CaO are formed.twoOR3.CaCltwo.10HtwoOR.
The results indicate that the PACl improves the properties of the cement, decreases the number of micropores (very small holes) and the matrix becomes more dense and compact, therefore the resistance to compression increases..
The effect increases with increasing content of PACl. The study confirms that by adding PACl to Portland cement, a mixture with superior mechanical and microstructural properties is obtained.
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