ARTICLE IN PRESS JID: JTICE [m5G;July 13, 2017;5:17] Journal of the Taiwan Institute of Chemical Engineers 000 (2017) 1–8 Contents lists available at ScienceDirect Journal of the Taiwan Institute of Chemical Engineers journal homepage: www.elsevier.com/locate/jtice Removal of calcium hardness from solution by fluidized-bed homogeneous crystallization (FBHC) process Nicolaus N.N. Mahasti a , Yu-Jen Shih b,∗ , Xuan-Tung Vu a , Yao Hui Huang a,∗ a Department of Chemical Engineering, National Cheng Kung University, Tainan, Taiwan b Institute of Environmental Engineering, National Sun Yat-sen University, Kaohsiung, Taiwan a r t i c l e i n f o Article history: Received 15 May 2017 Revised 21 June 2017 Accepted 21 June 2017 Available online xxx Keywords: Calcium Carbonate Softening Calcite Aragonite Crystallization a b s t r a c t Calcium is one of the divalent ions contributing to the hardness level of the water. This work describes the removal of calcium ions from aqueous solution using carbonate salts as precipitants and the recov- ery of homogeneous calcium carbonate crystals via a fluidized-bed homogeneous crystallization (FBHC) process without a heterogeneous seed material. The considered parameters were effluent pH, initial mo- lar ratio of carbonate salt to Ca, up-flow velocity, and cross-section loading. The removal efficiency of Ca hardness reached 95% at the optimal pH of 10–11 and the corresponding crystallization ratio was 88% for initial concentrations of Ca of 50–330 ppm. The FBHC process was effective with a cross-section load- ing of calcium in the water of up to 4.5 kg/m 2 /h. The efficiency of Ca immobilization as the crystal grew on the fluidized pellets was greatly improved by adjusting the degree of supersaturation in the range 2–3, resulting in the crystallization ratio (CR) and total removal of Ca (TR) of 88% and 92%, respectively. XRD analysis revealed that the formed crystals comprised two calcium carbonate (CaCO 3 ) phases—calcite and aragonite. SEM images of the surface morphology revealed that calcium carbonate particles (around 1–2 mm) were formed by the aggregation of fine crystals (around 5 μm). © 2017 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved. 1. Introduction As an alkaline-earth metal, calcium is widely spread in natu- ral and industrial waters. Since greater than 97% of global runoff is influenced by the dissolution of limestone (i.e. CaCO 3 ), water hard- ness and alkalinity are typically correlated in aquatic ecosystems [1]. Although the calcium content in the tap water does not have any serious impact on the health problem, but it still is responsible for the scaling of the household appliances and reduction of clean- ing performance of the detergents and soaps. [2] During industrial operation, calcium cation easily precipitates with carbonate (or bi- carbonate) anion, resulting in the growth of solid CaCO 3 in the in- ner pipe wall. The growth of CaCO 3 in a heat-transfer pipe, such as heat exchanger tube, boiler tube, inhibits the heat transfer effi- ciency, since CaCO 3 has a lower thermal conductivity (2.9 W/m/K) as compared to that of the copper metal tube commonly used in the heat equipment pipe (401 W/m/K) [3]. The carbonate scaling also leads to a large energy loss because of an increased pressure drop in the fluid flow system [4]. The WHO classified the concen- tration of calcium carbonate at the level up to 60 mg/L as a soft ∗ Corresponding authors. E-mail addresses: yhhuang@mail.ncku.edu.tw (Y.-J. Shih), mcdyessjin@gmail.com (Y.H. Huang). water, 60–120 mg/L as a moderately hard water, 120–180 mg/L as a hard water, and more than 180 mg/L as a very hard water. The rec- ommended dietary calcium intake is about 1000 mg/day also set by WHO [5]. Ion exchange, adsorption, filtration (membrane), precipitation using lime-soda are common techniques for softening water [6– 7]. Lime (as hydroxide) and soda ash (as carbonate ion) introduced to the system stimulate the precipitation of dissolved calcium. The advantages of lime soda are the abundant source, high efficiency of removal, low cost, and easy operation. However, the process pro- duces large amounts of sludge with a high water content (70–98%) and requires long time to settle (about 1.5–3 h). Precipitative soft- ening is typically achieved through conventional rapid mix, floc- culation, and sedimentation, which apply sludge blanket clarifiers to collect sludge from basins [8]. A post sludge-dewatering pro- cess that combines multiple treatment units is main defect of us- ing lime soda. Sludge dewatering is of major interest in sludge vol- ume reduction, transportation and ultimate disposal [9]. The resid- ual sludge from the lime soda process varies from a fraction of a percent to as much as 6% of the inlet capacity which is resulting in the disposal of thousands cubic meters of sludge per day. The con- ventional approach to manage lime softening sludge are thicken- ing, conditioning and dewatering process followed by landfill dis- posal. Since the regulations for lime sludge disposal become more http://dx.doi.org/10.1016/j.jtice.2017.06.040 1876-1070/© 2017 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved. Please cite this article as: N.N.N. Mahasti et al., Removal of calcium hardness from solution by fluidized-bed homogeneous crystallization (FBHC) process, Journal of the Taiwan Institute of Chemical Engineers (2017), http://dx.doi.org/10.1016/j.jtice.2017.06.040