Alkaline scale formation in boiling sea water brines

A laboratory method for the study of alkaline scale is described. Evaporator conditions are easily simulated in experiments of relatively short duration (5 hours). Reproducible results have been achieved by chemical analysis for total scale which consisted of mixtures of calcium carbonate and magnesium hydroxide. Experiments with natural sea water show the amount and composition of alkaline scale to be a function of temperature, brine concentration, bicarbonate ion concentration, and flow conditions through the evaporator. The transition between calcium carbonate and magnesium hydroxide was shown to be influenced by factors other than temperature. A new mechanism for alkaline scale formation is proposed in this paper. The first step involves a unimolecular breakdown of bicarbonate ion to form hydroxide ion. This concept is in disagreement with the generally accepted mechanism described in the literature.     

Gypsum Scale Formation in White Water of a Closed‐Cycled Paper Mill

The accumulation of calcium and sulfate ions in a process stream of a closed papermaking system may cause calcium sulfate scale formation. This study presents measurements of calcium sulfate precipitation conditions and a thermodynamics‐based model for the prediction of such precipitation conditions in the papermaking white water at 50°C. The precipitates were found to have the structure of gypsum (CaSO₄·2H₂O). The model calculations were compared with experimental data and found to be in good agreement.     

Use of gypsum/brucite mixed precipitate instead of gypsum in Portland cement

The possibility of replacing the natural gypsum used in cement production by a chemical precipitate consisting of gypsum (CaSO₄ ·2H₂O) and brucite (Mg(OH)₂), was investigated. This precipitate is a by‐product of a new hydrometallurgical process, which was developed in order to treat economically low‐grade nickel oxide ores. More specifically, it is obtained by hydrolytic precipitation of magnesium at temperatures not exceeding 80 °C, from sulfate solutions which result from heap leaching of nickel oxide ores with dilute sulfuric acid at ambient temperature, using calcium hydroxide as a neutralizing agent. The mixture generally consists of 20–30% non‐fibrous magnesium hydroxide, 60–75% gypsum and any excess of calcium hydroxide, depending on the precipitation conditions. In the present work, a mixture was produced by hydrolytic precipitation at 25 °C, using 1.1 times the stoichiometric quantity of Ca(OH)₂ required to precipitate all of the magnesium. The possibility of using the above precipitate as a substitute for gypsum in cement was examined by testing four different cement mixtures, one reference sample, containing 4.5% gypsum and 0.5% anhydrite ((PC)_Ref) and another three with 4.1%, 5.2% and 6.3% of gypsum/brucite mixed precipitate ((PC)_B/G), in the place of gypsum. All samples were tested by determining the grindability, setting time, expansion and compressive strength. The results of the physico‐mechanical tests showed that the replacement of natural gypsum by the above precipitate did not affect negatively the quality of the produced cements. Copyright © 2005 Society of Chemical Industry     

Effect of adding magnesium to lithium on the performance of discharge and hydrogen evolution of the lithium anode

The present study evaluates the effect of magnesium as an inhibitor on the performance of discharge and hydrogen evolution of lithium anode in alkaline electrolyte with additives. The electrochemical behaviors of lithium and lithium–magnesium alloy are assessed by hydrogen evolution rate, discharge current density, anodic potential, and potentiodynamic polarization. For these conditions, the results show that addition of magnesium to lithium enhances the current efficiency. Addition of 0.07 wt% Mg to lithium has minor effect on discharge current and anodic potential of lithium anode. The chemical composition and the morphology of the anode surfaces were evaluated by X-ray diffraction and scanning electron microscopy. The results show that the slow dissolution of lithium–magnesium alloy generates the formation of LiOH, LiOH·H₂O, and Mg(OH)₂. After discharge in saturated alkaline electrolyte with additives, the lithium–magnesium surface is less porous than lithium surface. Hydrogen evolution decrease, prompted by adding magnesium to lithium, is related to surface integrity enhanced by Mg(OH)₂.     

Influence of inorganic and organic additives on the composition of the precipitate of synthetic hydroxylapatite and calcium oxalate monohydrate

Synthetic hydroxylapatites are prepared with additives, such as Mg²⁺, CO ₃ ^2? , and C₂O ₄ ^2? . An increase in the concentration of magnesium leads to the formation of struvite. In the Ca(NO₃)₂-(NH₄)HPO₄-Na₂CO₃-NH₄OH-H₂O system, an excess of carbonate ions leads to the formation of calcite. When the synthesis is performed using oxalate ions as additives, calcium oxalate does not form the inherent phase. Calcium oxalate monohydrate is synthesized with additives, such as CO ₃ ^2? , HPO ₄ ^2? , and SO ₄ ^2? ions and urea, glycine, and glutamic acid. X-ray powder diffraction analysis has revealed that the composition of the CaC₂O₄ · H₂O precipitate remains unchanged under these conditions and in the presence of the aforementioned additives.     

Electrochemical preparation of cuprous oxide powder: Part I. Basic electrochemistry

The preferred process for the production of cuprous oxide powder is via the anodic dissolution of copper in alkaline solution of sodium chloride. The principal reactions are as follows: $$\begin{gathered} Cu + nCl^ - = CuCl_n^{1 - n} (n = 2, 3) \hfill \\ 2H_2 O + 2e = H_2 \uparrow + 2OH^ - \hfill \\ 2CuCl_n^{1 - n} + 2OH^ - = Cu_2 O \downarrow + 2nCl^ - + H_2 O \hfill \\ \end{gathered} $$ In the present investigation the basic electrode processes were studied systematically under a broad range of conditions using linear sweep voltammetry. Variables studied include the concentration of sodium chloride and sodium hydroxide (i.e., alkalinity), temperature of the solution, two categories of additives (an inhibitor for preventing the deposition of spongy metallic copper powder on the cathodes, and a chemical reducing agent for reducing the cupric ions to the cuprous state), and the effect of carbonate ions (resulting from the spontaneous absorption of carbon dioxide from the air by sodium hydroxide). Useful guidelines concerning the electrolysis conditions, additives, and the concentration limit of carbonate ions have been established. The proper operating conditions can be considered to be as follows: 80–85°C, NaCl 240–260 gl^?1, NaOH below 1 gl^?1. Conditions pertaining to the use of additives are the following: calcium gluconate 0–5 gl^?1, Na₂CrO₄ below 0.5 gl^?1, Na₂Cr₂O₇ below 0.25 gl^?1, NH₂OH·HCl below 2.5 gl^?1, N₂H₄·H₂O below 2.5 gl^?1, sucrose 0–5 gl^?1. Special attention must be given to eliminate or reduce the presence of carbonate ions in the electrolyte below 0.25 gl^?1 Na₂CO₃.     

Boron removal by means of chemical precipitation with calcium hydroxide and calcium borate formation

Boron removal was investigated by chemical precipitation from aqueous solutions containing boron using calcium hydroxide. pH, initial boron concentration, amount of Ca(OH)₂, stirring speed and solution temperature were selected as operational parameters in a batch system. The highest boron removal efficiency was reached at pH 1.0. Increasing initial boron concentration and amount of calcium hydroxide raised to boron removal efficiency. Boron removal efficiency was highest at a stirring speed of 150 rpm. The most important parameter affecting boron removal efficiency was solution temperature. Increasing solution temperature increased importantly boron removal. XRD analysis showed that CaB₃O₃(OH)₅·4H₂O, which is a borate mineral called inyoite, occurred between Ca(OH)₂ and borate ions. As a result of the obtained experimental data, when the optimum operational conditions were selected, over 96% of boron removal efficiency was reached by this method.     

An improved method for evaluating detergent builders for water hardness control

Commercial detergent additives to control water hardness may act through sequestration, crystal growth inhibition, precipitation, or ion exchange. These builders lower the free hardness (Ca⁺⁺, Mg⁺⁺ concentration by different mechanisms. A full factorially designed experiment has been developed to evaluate builders functioning by the sequestration or crystal growth inhibition of calcium carbonate or magnesium hydroxide. The builder’s performance is determined by its ability to prevent precipitation while in the presence of carbonate and hardness ions. The tests are based on incubation followed by filtration and determination of calcium and magnesium in the filtrate by Inductively Coupled Plasma (ICP). Variables in the design include builder concentration, temperature, pH and time. Regression equations and response surfaces for tripolyphosphate and several polyacrylates and phosphonates are included.     

Effect of particle size of hydromagnesite on properties of calcium aluminate cement bonded corundum based castables

Calcium aluminate cement was premixed with hydromagnesite having different particle sizes, as a calcium magnesium aluminate cement precursor, to investigate the influence of particle size of hydromagnesite on the volume stability and thermo-mechanical properties of corundum based castables after firing at 1550 °C. The impact of particle size of hydromagnesite on the phase composition and microstructure evolution of fired castables matrix were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM), respectively. The results demonstrate that hydromagnesite with smaller particle size has a better volumetric stability and thermo-mechanical properties of castables, because of the more homogeneously distributed micro-pores, and smaller size MA and CA₆ resulted from the more uniformly distributed hydromagnesite in castables.     

Basic magnesium carbonate flame retardants for polypropylene

Magnesium hydroxide and magnesium carbonates have attracted attention as endothermic flame retardants that are sufficiently stable to be incorporated into thermoplastics without decomposition. In this survey, a basic form of magnesium carbonate, magnesium carbonate hydroxide pentahydrate [(MgCO₃)₄ · Mg(OH)₂ · 5H₂O] was evaluated as a flame retardant for polypropylene. This filler (MCHP) has a thermal stability intermediate between that of alumina trihydrate (ATH) and magnesium hydroxide, which is sufficient to allow incorporation into polypropylene without decomposition. The MCHP is most effective at high filler concentrations near 60% where it was found to impart a Limiting Oxygen Index of 28.2 with a V-O rating (no dripping). This is slightly more effective than the flammability ratings for ATH and Mg(OH)₂ under the same conditions. The effectiveness of MCHP was attributed to the large endothermic loss of water of hydration, which also dilutes the combustion gases. This action was further aided by the formation of an intumescent char on the burning surface, which eventually extinguished the flame. Various combinations of magnesium oxide, magnesium hydroxide, magnesium carbonate, and MCHP were evaluated in order to clarify the mechanism of the flame retardant and improve the efficiency of the protective action. However, no synergism was evident, and the flame retardant results were found to be additive. The mechanical properties and processabilities of these highly filled compounds are very sensitive to the type of surface treatment. The method of Savides was employed to compare the burning temperature of the test specimens and to measure the rates of combustion.     

Influence of phosphate and iron on the extent of calcium carbonate precipitation during anaerobic digestion

The extent to which calcium carbonate deposition in an anaerobic reactor can be reduced by adding inhibitors (phosphate and iron) of calcium carbonate crystal growth was investigated. At several concentrations of the additive, the extent of precipitation was assessed in continuous experiments with laboratory‐scale reactors. In the reactor, phosphate concentrations as low as 0.5–5 mg total‐P dm⁻³ were found to severely inhibit CaCO₃ precipitation. However, iron did not inhibit the deposition of CaCO₃, which was found to be due to the fact that iron, in contrast to phosphate, only inhibits the growth of calcite and not the formation of aragonite. The results led to the conclusion that only additives which inhibit the formation of both aragonite and calcite can be used as effective inhibitors during anaerobic digestion. A chemical equilibrium model was developed and shown to be a useful tool to calculate the extent of calcium carbonate deposition during anaerobic digestion provided the proper apparent solubility product of calcium carbonate can be estimated. © 1999 Society of Chemical Industry     

Synthesis of magnesium sulfate from seawater using alkaline industrial wastes,sulfuric acid,and organic solvents

ABSTRACT

In this study, we used three processes to synthesize magnesium sulfate from seawater. First, alkaline industrial wastes, cement kiln dust and paper sludge ash, were injected to the seawater to precipitate magnesium in the form of magnesium hydroxide (Mg(OH)₂). Then, magnesium was eluted with a small amount of H₂SO₄ to make a high concentration magnesium solution. Finally, an organic solvent was added to precipitate magnesium sulfate (MgSO₄). Over 90% of magnesium was recovered through the three processes. It is expected that 11.3 kg of magnesium sulfate (based on MgSO₄ · 6H₂O) can be synthesized from 1 ton of seawater.     

Development of the electrochemical scale removal technique for desalination applications

The possibility of alkaline scale precipitation and removal by electrolytic devices has long been recognized. The scale removal principle of the electrochemical technique is based on the creation of a high pH environment around the cathode by water and oxygen reduction reactions which release hydroxyl ions. The alkaline environment induces precipitation of the calcium hardness in the form of CaCO₃ and of the magnesium hardness, in the form of Mg(OH)₂. Despite the commercial availability of such equipment, the use of electrochemical scale control methods is quite limited. Currently, the main field of application of electrolytic devices is for reducing the hardness of water recirculating in cooling towers. The lack of authoritative technical information on electrochemical scale removal reflects the paucity of research and development efforts in a technology which holds considerable promise for expanding the rather limited scope of viable scale control techniques. The objectives of this research project are to evaluate the potential of the electrochemical technique for RO desalination processes in general and for increasing water recovery levels in particular. The paper summarizes results of the first phase of the research. Models describing cell resistance in the absence and in the presence of a deposit on the cathode are presented. The effects of several parameters on the deposition rate and on the electric energy consumption are investigated. Results show that the higher the water hardness, the higher the scale precipitation rate and the lower the specific energy consumption. An increase in the flow velocity augments the scale deposition rate. Analysis of the velocity effect data indicates that the scale precipitation reaction is mass transfer controlled. The main optimization parameter is the current density. As may be anticipated, a low specific electrical energy is consumed when the electrolyzed solution is exposed to a large electrode surface and a high specific energy is consumed when the solution is exposed to a small electrode surface. The energy consumption can be rather low. For instance, in the electrolysis of a typical concentrate stream of a brackish desalination plant at a current density of 25 A/m², the energy consumption is of the order of 4 kWh per kg of precipitated CaCO₃ and the scale precipitation rate is of the order of 25 g CaCO₃/h m². Finally, a flow scheme is presented indicating the possibility of beneficial increase of the water recovery level in brackish water RO desalination, by partial recycle of the concentrate after electrochemical precipitation of the scale forming ions held in solution by the anti-scalant.     

Recovery of Lithium Hydroxide from Spent Lithium Carbonate using Crystallizations

Abstract

Recovery of LiOH from the spent Li₂CO₃ used as absorbent for carbon dioxide in breathing apparatus was successfully explored by precipitation and crystallization. A lithium hydroxide solution was prepared by precipitation of calcium carbonate using reaction of spent Li₂CO₃ and calcium hydroxide. The effects of the operating conditions on the reaction were investigated. Conversion of calcium carbonate was about 95%. Lithium hydroxide monohydrate from lithium hydroxide solution was obtained in batch evaporative crystallization. The effect of the evaporation rate on crystal morphology was investigated. The evaporation rates were affected to control size and yield of crystals. Eventually, the purity of crystals was above 99 wt% and yield was about 80%.     

The crystallization of magnesium hydroxide

The thermodynamic association constant for the formation of the ion-pair MgOH⁺ in solutions of magnesium hydroxide has been studied by potentiometric and conductometric methods.The growth of magnesium hydroxide seed crystals in supersaturated solutions over a range of concentrations of magnesium and hydroxide ions and in the presence of phosphonate additives has been investigated by a precision conductance method. After a rapid initial surge of growth, an equation first order with respect to supersaturation can be used to explain the experimental data. A surface diffusion process is proposed as the controlling step in the crystallization process.     

Review of the hydrolysis of iron(III) and the crystallization of amorphous iron(III) hydroxide hydrate

Hydrolysis of ferric solutions leads initially to mono- and dinuclear species which interact to produce further species of higher nuclearity. These polynuclear species age eventually to either crystalline compounds or to an amorphous precipitate (amorphous iron(III) hydroxide hydrate). Amorphous iron(III) hydroxide hydrate is thermodynamically unstable and gradually transforms to α-FeO(OH) and α-Fe₂O₃. These crystalline products form by competing mechanisms and the proportion of each in the final product depends on the relative rates of formation. The master variable governing the rates at which these compounds form is pH. Other important factors are temperature and the presence of additives. Most additives retard the transformation and by suppressing formation of α-FeO(OH) lead to an increase in the amount of α-Fe₂O₃ in the product; some additives also directly promote formation of the latter compound. Metal ions can oftxen replace a proportion of Fe in the α-FeO(OH) and α-Fe₂O₃ lattices. At high enough concentrations they can induce formation of additional phases. Additives may also modify the morphology of the crystalline products.     

Kinetics, energy characteristics, and intensification of crystallization processes in chemical precipitation of hardness ions

An analysis of the literature data on the solubility and kinetics of the chemical (reagent) precipitation of calcium carbonate and magnesium hydroxide is performed. The possible causes of the significant discrepancy in the available data are considered. The kinetics of the homogeneous and heterogeneous crystallization of calcium carbonate and magnesium hydroxide in reagent precipitation is studied using different methods. It is shown that the use of heterogeneous crystallization on seed particles pretreated in an ultrasonic field raises the rate of crystallization by an order of magnitude. The kinetics of nucleation is studied; the effect of supersaturation and temperature on the induction period in the homogeneous and heterogeneous nucleation of CaCO₃ and Mg(OH)₂ crystals is investigated. The energy characteristics of nucleation, namely, the interfacial tension (surface energy) and activation energy, are determined. The use of fine-dispersed particles activated by ultrasound makes it possible to considerably reduce the energy barrier for nucleation.     

Liquid phase oxidation of 3,4-dihydroxybenzoic acid and the effect of the products on trihydroxy aluminium precipitation yields

Organic compounds are known to interfere with the precipitation of trihydroxy aluminium (gibbsite) in the Bayer process. Studies have been carried out of the effect of 3,4-dihydroxybenzoic acid on precipitation and of the beneficial effect that can be achieved by oxidising the acid. Measurements of precipitate yields both in the presence of the acid and of products obtained by oxidising the acid were made. Oxidation in the presence of various metal ions was also examined. The yield of precipitate was found to decrease in the order Cu²⁺ (1·9 mM) > Fe³⁺ (2·2 mM) > blank (no added organics) > Mn⁴⁺ (44·3 and 2·3 mM) > Mn²⁺ (2·3 mM) > Mn²⁺ (44·3 mM) ≈ uncatalyset oxidation > Fe³⁺ (43·5 mM) > Cu²⁺ (38·2 mM) > untreated sample. This relative ordering was generally unaffected by reaction temperature or oxygen partial pressure. Soluble manganese salts were formed by partial dissolution of MnO₂ in the alkaline solutions. These salts were required for oxidation of the 3,4-dihydroxybenzoic acid to products which, collectively, did not poison the trihydroxy aluminium precipitation process.     

Cyclic voltammetric study of zinc and zinc oxide electrodes in 5.3 M KOH

The behaviour of zinc and zinc oxide in 5.3 M KOH in the presence of alkaline earth oxides, SnO, Ni(OH)₂ and Co(OH)₂ was examined by cyclic voltammetry. The influence of the alkaline earth oxides was compared with additives of established effects (Bi₂O₃, LiOH, Na₂CO₃ and CdO). The alkaline earth oxide each exhibits a distinct behaviour towards zincate. Whereas, a single process of interaction with zincate was shown by CaO; two modes of reaction were obtained with SrO and BaO. Solid solution formation was noticed with BeO and MgO. The other additives forming solid solution with ZnO were CdO, SnO. The ionic sizes of Ni(OH)₂ and Co(OH)₂ allow solid solution formation with Zn(OH)₂. Both Bi₂O₃ and Na₂CO₃ enter into complexation with zincate. LiOH forms two distinct zincates, of which one is an oxo zincate leaching the `hydroxyl' functionality. Cyclic voltammetry revealed the deposition of the oxide/hydroxide additives as metal prior to the onset of zinc deposition and the potential range for this additive metal deposition is almost the same for different additives (SnO, CdO, Ni(OH)₂). The beneficial action of these additives to zinc alkaline cells is associated with a substrate effect. The implication of this electrocatalytic deposition of metals on a zinc oxide electrode is also discussed.