Mineral Scale Management Part II

PEER-REVIEWED SCALE Mineral Scale Management Part II. Fundamental Chemistry ALAN W. RUDIE AND PETER W. HART ABSTRACT: The mineral scale that deposits in digesters and bleach plants is formed by a chemical precipitation process.As such, it is accurately modeled using the solubility product equilibrium constant. Although solubility product identifies the primary conditions that must be met for a scale problem to exist, the acid-base equilibria of the scaling anions often control where in the process scale will occur. These equilibria are the primary control method to minimize or pre vent scale in a bleach plant. Both the acid-base equilibria and solubility product equilibria are influenced by temper ature and ionic strength. This paper reviews the chemistry of precipitation and acid-base equilibria. The Gibbs free energy expression is presented as a method to estimate equilibrium constants at temperatures in the bleach plant.The effect of ionic strength on ion activity and its influence on these processes are also discussed. Application: Preventing scale problems requires an understanding of the solubility product equilibrium, the acid-base equilibrium of the scaling anion, and the effects of temperature and ion activity on these equilibria. ome pulp mills have always experi trations in the wood supply. Those conditions that barely exceed the Senced mineral scale buildup, but cases required more detailed equilibri saturation concentrations, it can this problem has increased with um modeling and will be handled in take hours or longer for an obvi efforts to comply with environmental the third paper of the series on non- ous precipitate to form. requirements.The recycling of washer process elements in the paper industry. filtrates to reduce effluent volume has • Some salts can exceed the solubil increased the concentrations of both OVERVIEW OF CHEMICAL cations and anions and with it the ity by an order of magnitude or PRECIPITATION more without precipitating [4]. potential for mineral scale deposits. Mineral scale formation is a chemical The switch from chlorine-initiated precipitation process. Although the This supersaturation condition bleach sequences to the use of chlo chemical phenomenon of precipita can be manipulated. One method rine dioxide in the first stage has tion can identify where and when of suppressing scale is to add decreased the ability of the bleach scale deposition is possible, it cannot chemicals that interfere with crys plant to remove calcium and barium determine that scale will definitely tal growth or interfere with crys in the first stage.The result has been a occur.The reasons are many; a few are listed here: tal nucleation [5,6]. Organic frag significant increase in the frequency ments may occur in bleach plant of calcium oxalate and barium sulfate filtrates that behave in these ways scale deposits in the first bleach stage • In locations where precipitation to suppress precipitation. and an increase in calcium carbonate is general and not directed deposits in the first extraction stage. toward a specific surface, fiber In particularly severe cases, the forma represents the largest surface • Numerous organic fragments can tion of calcium oxalate has also area in the process. Precipitates form complexes with trace met- occurred in later bleach stages [1,2]. that deposit on fiber are general- als.These complexes are in equi This paper is the second in a series ly-and at least temporarily-harm- librium with the free-solvated on mineral scale management.A collec less.These precipitates tend to trace metal and the organic frag- tion of case studies was presented in stay with the fiber and carry ment.The net effect of these the first paper [3]. Most of these scale trace elements further into the equilibrium processes is to problems were resolved using the fun bleaching process, where they reduce the concentration of free damental chemistry principles to be often cause problems. cations. For example, nearly all presented here. Several cases were carboxylic acids have a weak more difficult, usually because of • Precipitation is time and concen attraction for calcium and bari exceptionally high trace metal concen tration dependent [4]. Under um. Fragments with two acid VOL. 5: NO. 7 TAPPI JOURNAL 17 SCALE groups form much more stable • Cation—A molecule containing PRECIPITATION complexes, but complexes with a positive charge.The trace More detailed explanations of precipi one or two appropriately placed metal nonprocess elements are tation and acid base equilibria can be found in any good analytical chem alcohol groups can also form all cations.The common cations istry textbook[4].The scale that forms stronger complexes. At present, in bleach plant scales are calci in digesters and bleach plants is basi we can not catalog all the possi um (Ca2+) and barium (Ba2+). cally a localized chemical precipita ble complexes and determine • Monovalent and divalent—Terms tion process. As such, it is usually their formation constants. described using the solubility product that refer to the charge of an convention. For the generalized chem Fortunately, in the bleach plant, ion. Monovalent anions and ical reaction, organic fragments that can form cations contain a single negative strong complexes are usually at or positive charge; divalent mA + nB pC + qD low concentrations, and frag anions and cations contain two ments that are common tend to The chemical equilibrium is usually negative or two positive charges. written as form weak complexes. It Typically, salts of monovalent appears that both can largely be (1) ignored in bleach plant environ anions and cations are quite sol ments. Precipitation models that uble. Salts containing a divalent ignore these effects, however, anion or cation, and two mono For solubility product, there is typical ly just one product. Following the represent a worst case analysis. valent ions of the opposite charge, are also typically very equilibrium expression convention, the numerator is the concentration of • Chemistry is often defined under soluble. But salts where both the the product. But, what is the concen unrealistic conditions. For exam anion and cation are divalent are tration of a precipitate? By convention, ple, what does “infinitely dilute” typically sparingly soluble or the concentration or, rather, activity of insoluble.This trend cannot be the precipitate is defined as 1.0 and mean, and where can this condi the equilibrium expression is inverted: tion be found in a pulp mill? The extended to trivalent ions (ions obvious answer is that whatever containing three charges). mA + nC AmCn infinitely dilute means, it does Because the charge density of not apply anywhere in a pulp trivalent ions is so high, they The solubility product expression is mill or bleach plant.We need to rarely exist in the 3+ or 3- state in (2) understand that the approxima solution and often form charged tions that allow us to extend molecules that are soluble. For the common precipitates encoun chemical principles to the condi • Molarity (M) and molality (m)— tered in a bleach plant, the chemical tions in a mill are just approxi equation and corresponding equilibri Molarity (M) is the molar concen um equations are as follows: mations. Because of this, results tration of a dissolved substance are not absolutes and are subject relative to a volume of 1 L. For calcium carbonate (lime, calcite or to some interpretation. Molality (m) is the molar concen aragonite), Ca2+ + CO 2- CaCO tration relative to 1 kg of total 3 3 TERMS AND GENERAL mass. Activity (a) is ion mobility (3) PRINCIPLES and is related to both the concen Some common terms in aqueous inor tration of the substance and the ganic chemistry are essential for For calcium oxalate, concentration of other dissolved Ca2+ + C O 2- CaC O understanding the chemical process 2 4 2 4 es involved in scale formation: ions. Ion activity is usually consid (4) ered to be the product of con centration (either molarity or • Anion—A molecule containing a and for barium sulfate (barite), negative charge.The typical molality) and the appropriate ion 2+ 2- Ba + SO4 BaSO4 scale-forming anions in the activity coefficient (f). At “infi bleach plant are carbonate nite” dilution and 25°C, molarity, (5) (CO 2- ), oxalate (C O 2-), and sul molality, and activity of a sub 3 2 4 Assuming equal molar concentrations 2- stance are all the same. fate (SO4 ). of anion and cation, all three precipi 18 TAPPI JOURNAL JULY 2006 SCALE tate at about a 10-4 molar concentration or about 4 or 5 parts per million.The typical pulp mill bleach plant receives over a ton of calcium a day in the unbleached pulp, and all of this must be removed to avoid scale problems. ACID-BASE EQUILIBRIA A key issue in precipitation is that the anions that partici pate in the precipitation process are the divalent anions, - - - not HCO3,HC2O 4 , or HSO4 . This means that the conditions for scale formation are strongly influenced by the process pH. The general chemical equation for an acid equilibrium is written as HA ➝H+ + A-. The resulting equilibrium expression is shown in Eq. 6: (6) Figure 1. The pH dependence of carbonic acid, bicar + + bonate, and carbonate. It is common to express H as H3O , recognizing that the proton is heavily solvated and does not exist independent of the surrounding water. This convention has not been adopted here for simplicity.The choice as to how to repre sent this cation does not affect the equilibria or calculation to be discussed in this paper. For all three cases contributing to scale, there are two acid dissociations to consider, but only the second of the two is important to the precipitation process. For carbon ic acid, ➝+ - H2CO 3 H + HCO3 - ➝+ 2 HCO3 H + CO3 The corresponding equilibrium equations are presented in Eq.

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