Poly Aluminium Chloride - PAC

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These compounds have the general formula (Aln(OH)mCl(3n-m))x and have a polymeric structure, totally soluble in water. The length of the polymerised chain, molecular weight and number of ionic charges is determined by the degree of polymerisation.
On hydrolysis, various mono- and polymeric species are formed, with Al13O4(OH)247+ being a particularly important cation.


These highly polymerised coagulants include the following:

  • polyaluminium chloride (PACl, n=2 and m=3),
  • aluminium chlorohydrate (ACH, n=2 and m=5), and
  • polyaluminium chlorohydrate (PACH): similar to ACH.

 

An important property of polyaluminium coagulants is their basicity.
The highly charged Al in the products is optimized for best performance for the different basicities of PAC. The low to medium basicity products (25% up to 45%) are excellent at reducing the turbidity while maintaining good phosphorus removal properties.
High basicity PACl products ( 45% up to 70%) have been optimized for particle removal by controlling the formation of Al species in the coagulant.
In practice the choice of which type of coagulant use in a water treatment applications is carried out in situ in order to determinate the best properties that must have the coagulant so as to obtain the best performance – Tailor-made coagulant.

 

Main advantages of poyaluminum chloride coagulants

  • The polyaluminium coagulants in general consume considerably less alkalinity than alum. They are effective over a broader pH range compared to alum and experience shows that PACl works satisfactorily over a pH range of 5.0 to 8.0;
  • PACl and ACH work extremely well at low raw water temperatures. Flocs formed from alum at low temperatures settle very slowly, whereas flocs formed from polyaluminium coagulants tend to settle equally well at low and at normal water temperatures;
  • less sludge is produced compared to alum at an equivalent dose;
  • lower doses are required to give equivalent results to alum. For example, a dose of 12 mg/L PACl was required for treatment of a coloured, low turbidity water (Rio de la Plata) compared to similar performance obtained when using an alum dose of 55 mg/L, and the increase in chloride in the treated water is much lower than the sulphate increase from alum, resulting in lower overall increases in the TDS of the treated water;
  • Polyaluminium chloride solution is stable for 5 to 6 months when stored at less than 50°C and is so ideal for bulk storage and dosing installations.

The growth rate of polyaluminium chloride has been very impressive. In many areas where PAC has been marketed for a reasonable time period (3/5 years) it has replaced over 85 % of the total alum demand. In Argentina, where PAC is not used at all as flocculants in potable water treatment, Contec has built two PAC plants and in 5 years almost all the drinking water is treated with PAC.

 

The process

Poly Aluminium Chloride process has been fine tuned by Contec during these years.
Contec selected process condition in order to obtain the maximization of most active oligomeric forms of PAC which are the most susceptible to give rise to high cationic poly-nuclear hydroxyl-aluminium complexes.
The result of this day by day work is an improved product stability and performance.

The main raw material for the PAC production is hydrochloric acid (about 700 kg per ton of PAC) ; Very often the producers of caustic soda have a surplus of chlorine and although the chlorine is used as an oxidizing agent in several organic processes and gaseous HCl is often released as a by-product. It is not always possible to find an use for the acid and the discharge to waste treatment is not a satisfactory solution.

A PAC plant is an investment opportunity with an attractive return in a growing market especially where the Hydrochloric acid is surplus product from other plant.

 

Contec produces a wide range of polyaluminum chlorides diversified by the value of the basicity and the aluminium content.

The process can be divided in two steps:

The reaction
Where we can produce, in the glass line reactor, polyaluminum chloride coagulants with a low and medium basicity.
The most important is PAC 18 a liquid coagulant potable grade approved and it is an effective coagulant for treatment of both potable and waste water treatment.
The relationship between the basicity and the alumina content is extremely important because from this value derives the stability of the products.

m Formula MW Basicity
1.5 Al2(OH)1.5Cl4.5 238.8 25.0%
2 Al2(OH)2Cl4 229.84 33.3%
2.7 Al2(OH)2.7Cl3.3 216.92 45.0%

 

The Neutralization
Where with Contec technology is possible to produce many different polyaluminum chloride coagulants having a basicitiy ranging from 45 up to 70% according to market request.
With our process technology a tailor-made coagulants products can be designed in order to have maximum efficiency with the specific water to be treated.

m Formula MW Basicity
3 Al2(OH)3Cl3 211.38 50.0%
3.6 Al2(OH)3.6Cl2.4 200.30 60.0%
4.2 Al2(OH)4.2Cl1.8 189.23 70.0%
5* Al2(OH)5Cl 174.5 80.0%

* Polyaluminum Chlorohydrate PACH