BRINE AND SCALE CHEMISTRY IN MSF DISTILLERS

The two mechanisms accounting for alkaline scale formation inside MSF units are reviewed in short. Previous studies addressing the problem w ere primarily concerned with the mass or composition of the formed sca les. As the nature of the scale depends on the composition of the brin e out of which it separates, it is only reasonable to consider both ph ases simultaneously. Brine samples were extracted from the condensers, brine heater and flash chambers of two types of 18-stage, MSF units o perating in the Umm A1 Nar desalination plant (Abu Dhabi, UAE) over a period of 4 years. These were analyzed for Ca2+, Mg2+, CO3(-2), HCO3-, pH and the temperatures of the various cells recorded. During the coo ling cycle of the brine, the first five parameters remained constant i n cells (18-16) and (15-1). Between cells 16 and 15 the [Ca2+], [Mg2+] , [CO3(2-)] and pH increased whereas the [HCO3-] decreased. These chan ges are due to mixing of the concentrated brine with make-up water. Th is conclusion was confirmed by calculating the material balance of all ions. The fact that the product [Ca2+][CO3(2-)] of the brine is ca. 1 20-170 times larger than the solubility product of either aragonite or calcite, indicates delayed precipitation and excessive super-saturati on with CaCO3. The analysis of brine samples extracted from the flash chambers revealed a continuous, linear build-up of [Ca2+] and [Mg2+] a s one moves from cell 1 to cell 18. The pH value increased noticeably in the first 3-4 cells and remained constant thereafter. The [CO3(2-)] and [HCO3-] appeared to remain constant in all cells. The data were q uantitatively treated by calculating cells' concentration factors and computing the expected amounts of the various ions in the different ce lls. Differences between these values and the experimentally determine d quantities signified scaling. The results indicated the formation of Mg scales in cells 1-10 and of Ca scales in cells 1-16. The amounts o f disappearing [CO3(2-)] exceeded that needed for CaCO3 deposition. Th e rest, together with the loss of [HCO3-] were not enough to account f or Mg(OH)2 precipitation. This latter was assumed to involve the parti cipation of CaCO3 present in super-saturation. Scale samples were coll ected from some of the condenser tubes and the flash chambers of the d istillers. These were fully analyzed for Ca2+, Mg2+, CO3(2-), OH-, Cl- , SO4(2-), acid-insolubles, adsorbed humidity and organics. A scheme w as proposed to relate the results of analyses to compounds of definite compositions with agreement better than +/-3%. The results indicated that in one type of distillers (IHI) Ca-rich scales formed both in the condensers and flash chamber. In the second type of distillers (Itali mpianti) the scales were predominantly Mg-rich. This disparity in beha viour does not signify two scaling mechanisms. It is suggested that th e nature of scaling is decided by distiller top-temperature, volume, m aterial, deaeration, and depends on type of anti-scalant additive and ball cleaning. Attention is drawn to the need of basic studies on the effect of additives and metallic surfaces on the rate of HCO3- decompo sition, as well as on the conditions governing super-saturation by CaC O3 and Mg(OH)2 and the kinetics and morphology of their crystallizatio n.