Chloride (Cl) and potassium (K) are the most problematic non-process elements in the kraft recovery cycle. Since the salts of these elements are very soluble, they build up in the recovery cycle, to an extent which depends on the amount of input, the degree of mill closure and the liquor sulphidity. Cl and K enter the mill with contaminated wood, chemical make-up and water. Depending on the type of wood, the K content can be high. As mills reduce water consumption, increase spill recovery or recycle bleach plant effluent to the recovery cycle, the level of Cl and K increases in the recovery cycle.

There are several methods employed in mills to combat this problem. Cl and K enrich in the electrostatic precipitator (ESP) dust, and most methods for removing them involve the treatment of the ESP dust. The simplest method is to discharge a portion of the dust and use relatively pure saltcake and caustic soda to replace the lost Na₂SO₄ and Na₂CO₃. This method incurs an operating cost, and may be problematic if the mill has limits on the effluent conductivity and flow. Other methods to treat ESP dust include: leaching, evaporation crystallization, cooling crystallization and ion exchange. The first three methods operate on the basis of differences in the solubility of alkali sulphate and alkali chloride salts. The ion exchange process uses ion retardation to separate chloride from sulphate.

Here are some examples of systems available:

Leaching: In this system the ESP dust is mixed with water to form a slurry. More Cl salts dissolve compared to sulphate salts and a filter or centrifuge is used to separate the solid sulphate. A leaching system was installed in a Brazilian mill in 2002 [1]; The Cl and K removal efficiencies were reported to be around 70%, with 80% recovery of Na and 85% recovery of S.

Evaporation Crystallization: This type of system (Chloride Removal Process or CRP) is offered by three manufacturers, each using different evaporator designs. As of 2004, there were six CRPs operating: three in North America, two in South America and one in Australia. Published data from one mill shows 95% Cl removal and 80% sulphate recovery. K removal varied between 50-85%, depending on the amount of K in the ESP dust and the balance between K removal, and Na and S recovery [1].

Cooling Crystallization: In this method a slurry of ESP dust is cooled to about 15 C, when Na₂SO₄ in solution re-crystallizes as Na₂SO₄.10 H₂O. The crystals are then separated by decanting. The removal efficiency was reported as 90% for Cl and 75% for K, with 70% recovery for Na. There are six cooling crystallization systems installed in Japanese mills [1].

Ion Exchange: This system uses an amphoteric ion exchange resin to separate Cl from sulphate in an ESP dust solution. Pilot plant data provide a Cl removal efficiency of 97%, with minimal loss of Na, S and carbonate. However, the K removal efficiency is low (5%). One difference between ion exchange and other methods is that the purified sulphate is returned to the liquor cycle in the form of a solution and not as a slurry or as crystals. This requires extra capacity in black liquor evaporation. The addition point of sulphate solution to the evaporators is also critical, as a high concentration of sulphate could promote burkeite (2Na₂SO₄.Na₂CO₃) formation and scaling of the evaporators.

1. Tran, H. and Earl, P., Chloride and potassium removal processes for kraft pulp mills: a technical review, Inter. Chem. Rec. Conf., 2004.