Improving settleability and dewaterability of Friedel's salt for chloride removal from saline wastewater

Highlights

• Settleability and dewaterability of Friedel's salt precipitate require improvement.

• Anionic PAM with high hydrolysis degree and molecular weight improved settleability.

• Friedel's salt decreased layers spacing at 40 °C, and was decomposed at >60 °C.

• Rising temperature improved settleability and dewaterability of Friedel's salt.

• Pilot test showed dosing PAM at 37 °C was optimal for separation of Friedel's salt.

Abstract

As an effective method for chloride removal from saline wastewater, Friedel's salt (FS) precipitation generates high concentration of precipitate requiring efficient solid-liquid separation. Effects of temperature, reaction time, polyacrylamides (PAM) type and dosage on settleability and dewaterability of FS precipitate were investigated to explore improving strategies for separation by using actual flue gas desulfurization wastewater. Raw FS precipitate was difficult to separate by gravity settling and mechanical dewatering owing to their small particle size and high solid concentration. Anionic PAM with higher hydrolysis degree and molecular weight greatly improved settleability and enlarged particle size of FS. The composition and crystal structure of FS varied with reaction temperature. Increasing temperature to 40 °C obtained the highest chloride removal, and improved settleability because layers spacing of FS decreased. FS precipitate was gradually broken down to Ca₃Al₂(OH)₁₂ and NaCl at temperature above 60 °C. Prolonging reaction time enhanced chloride removal by adsorption effects, and improved settleability by facilitating crystals growth and aging. The 0.5 m³/h pilot-scale experiment confirmed that the FS precipitation process was operated stably with improved settleability and dewaterability of precipitate by dosing PAM and elevating temperature. Techno-economic analysis showed that 37 °C was optimal for chloride removal and solid-liquid separation.

Introduction

Saline wastewater usually originates from many industrial processes, such as chemical industry, textile, inland seawater desalination, flue gas desulfurization (FGD), etc. [1,2] with chloride (Cl⁻) as common inorganic anions [3]. Recently, step utilization and reuse of water resources in plants have further accumulated salt in wastewater, and increased flow rate of saline wastewater [4]. Zero liquid discharge (ZLD), a wastewater management strategy accomplishing no wastewater discharge and full recovery of water and salts, was put forward in recent years and attracted attentions of both developed countries and emerging economies [5]. Stricter regulations, higher process expenses, and increasing value of freshwater have driven ZLD to become a beneficial or even a necessary option for wastewater management [6]. Separating dissolved salt, especially Cl⁻, from saline wastewater is the major challenge of ZLD process. Currently, membrane concentration (reverse osmosis, forward osmosis and electrodialysis [7]) and evaporation crystallization have been combined to get rid of Cl⁻ removal from saline wastewater [8]. Electrochlorination was also proposed for concentrated saline wastewater to enhance additional value of final products [9]. However, these ZLD processes have been criticized for their long process, unstable performance, high upfront investment, high operating cost, and complex maintenance [8,10]. It is necessary to develop new ZLD processes with short configuration, stable performance, simplified maintenance and reduced upfront investment.

Chemical precipitation by addition of copper slag or silver salt is a simple, rapid and efficient dechlorination process, but these two precipitants are too expensive to be adopted for industrial application [11]. Friedel's salt (FS), an insoluble layered double hydroxide (LDH) salt [Ca₂Al(OH)₆Cl·2H₂O] with a high adhesion for Cl⁻, was applied for saline wastewater desalination by dosing relatively cheap lime and aluminate in recent years, and considered as a promising alternative to membrane concentration and evaporation [12]. LDHs have been widely applied as heavy metal adsorbents [13], catalyst [14] and sludge dewatering agents [15]; therefore, the precipitated FS can be recycled as valuable materials. Abdel-Wahab and Batchelor [16] successfully applied FS precipitation for Cl⁻ removal from the recycled cooling water, and found that Cl⁻ removal efficiency was higher at a high initial Cl⁻ concentration. Lee, et al. [17] recommended a two-stage process by removing carbonate ions before precipitating Cl⁻, and achieved 56% Cl⁻ removal. Our previous study [10] also proposed a two-stage process for ZLD of FGD wastewater by FS precipitation. In the two-stage process, Mg²⁺ and part of SO₄²⁻ were removed by adding lime in Stage I, and the remaining SO₄²⁻ and 48.1% of Cl⁻ were precipitated as ettringite and FS in Stage II, achieving an alkaline and clear effluent recycled for desulfurization. Zhang, et al. [11] enhanced FS precipitation by ultrasound with cavitation and mechanical disturbance effect, and developed a two-stage de-chlorination process that effectively dislodged Cl⁻ from rare earth smelting wastewater. By combining the FS precipitation system with the effluent recirculation to the corresponding production unit, such as the FGD tower (see Fig. 1) or boilers, a compact and efficient ZLD process can be constructed to replace conventional expensive membrane concentration and evaporation processes.

Those intensive efforts mentioned above are very helpful to understand mechanisms, reaction conditions and competitive ions of FS precipitation, but most of the reported results were lab-scale and even batch test with synthetic wastewater [18,19]. More detailed research is needed for actual wastewater at a large pilot scale to validate the applicability of FS precipitation for wastewater desalination. On the other hand, it is noteworthy that FS precipitation will produce with yield of 7.9 g FS/g Cl⁻ removed, and the sludge concentration can be as high as 10–30 g/L for the treatment of saline wastewater with Cl⁻ concentration usually in the range of 5–30 g/L [20] considering side reactions, such as Ca₃Al₂OH₁₂ and Ca₄Al₂OH₁₄ [21]. It is reported that ettringite, another LDHs, usually had poor settleability because of its small particle size and columnar structure [22]. Therefore, solid-liquid separation of the formed FS precipitate, and the afterward sludge treatment and disposal are major concerns of FS precipitation; however, to our best knowledge, no work has been reported in literatures. More detailed research is needed to improve settleability and dewaterability of FS precipitate in the wastewater desalination process.

The objective of this work is to investigate settleability and dewaterability of precipitate when FS precipitation is applied for Cl⁻ removal of FGD wastewater. Eight different types of polyacrylamides (PAMs) were applied for coagulation of FS precipitate to determine the optimal type and dosage of PAMs. The effects of temperature and reaction time on the settling curve, particle size, composition and morphology of FS precipitate were further investigated. The dewaterability and drying performance of settled FS precipitate were also analyzed. Reliability and cost of FS precipitation under conditions optimized for improved settleability and dewaterability were evaluated in a pilot-scale system to treat actual FGD wastewater. The results are expected to provide an insight into the coagulation and conditioning of FS precipitate to improve their solid-liquid separation and dewatering performance in the desalination process of saline wastewater.