High CaSO4 Research Articles

Reverse Osmosis with Intermediate Chemical Demineralization: Scale Inhibitor Selection, Degradation, and Seeded Precipitation.

Two-stage reverse osmosis (RO) processes with intermediate concentrate demineralization (ICD) provide an efficient strategy to treat brines with high CaSO4 contents and reduce concentrate discharge. In this paper, an SRO concentrate is treated using ICD to remove CaSO4 and then mixed with a PRO concentrate for further desalination in SRO, thereby reducing the discharge of the concentrate. We investigate the selection and degradation of scale inhibitors, as well as seeded precipitation in the two-stage RO process with ICD, to achieve a high water recovery rate. A scale inhibitor is added to restrain CaSO4 crystallization on the membrane surface, and the optimized scale inhibitor, RO-400, is found to inhibit calcium sulfate scaling effectively across a wide range of the saturation index of gypsum (SIg) from 2.3 to 6. Under the optimized parameters of 40 W UV light and 70 mg/L H2O2, UV/H2O2 can degrade RO-400 completely in 15 min to destroy the scale inhibitor in the SRO concentrate. After scale inhibitor degradation, the SRO concentrate is desaturated by seeded precipitation, and the reaction degree of CaSO4 reaches 97.12%, leading to a concentrate with a low SIg (1.07) for cyclic desalination. Three UVD-GSP cycle tests show that the reused gypsum seeds can also ensure the effect of the CaSO4 precipitation process. This paper provides a combined UVD-GSP strategy in two-stage RO processes to improve the water recovery rate for CaSO4-contained concentrate.

Cleaning mechanism of gypsum scaling in hydrophobic porous membranes

Mineral scaling is one of the major challenges for membrane distillation in concentrating high-salinity brine. This work first investigated the characteristic of CaSO4 scaling, then explored different cleaning reagents to evaluate their efficiencies and mechanisms. By analyzing the SEM and EDX mapping images, the CaSO4 scaling process could be divided into surface and pore scaling stages. Three cleaning solutions, i.e., commercial detergent (20 %), EDTA-4Na (0.05 M), and MgCl2 (0.8 M), were chosen for their high CaSO4 solubility (0.7 g CaSO4 per 100 g H2O). At the surface scaling stage, three cleaning solutions exhibited excellent removal of CaSO4 scaling with approximately similar 96 % cleaning efficiencies. However, at the pore scaling stage, only the commercial detergent showed high efficiency (89 %). Such high cleaning efficiency was mainly attributed to the detergent's high total dissolved solids (salinity), low pH, and high diffusion rate (5.46 μm cm−1 min−1) across the membrane. Moreover, periodic cleaning at the surface scaling stage with the commercial detergent followed by drying throughout a 43.5 h continuous concentration test on an actual RO brine could restore the PTFE membrane performance completely, giving 90 % of the total water recovery. This work provides new insights into the cleaning mechanism for gypsum scaling in a hydrophobic porous membrane.

PRODUCT CHARACTERISTICS IN SIMULTANEOUS CRYSTALLIZATION OF NaCl AND CaSO4 FROM AQUEOUS SOLUTION WITH SEEDING

Zero liquid discharge strategies for industrial wastewater treatment have become prominent in recent years. When evaporative simultaneous crystallization is applied, knowledge about the particle crystallization mechanisms and morphology are important to ease solids downstream handling and for recovery of their valuable components. In this work, batch simultaneous crystallization of sodium chloride (NaCl) and calcium sulphate hemihydrate (CaSO4.0.5H2O) from aqueous solution was studied. It was found that CaSO4.0.5H2O is not an effective substrate for NaCl heterogeneous nucleation, but agglomerates with NaCl particles instead. CaSO4.0.5H2O particles on the surface of NaCl crystals sterically hamper agglomeration of NaCl particles with each other and, for a sufficiently high CaSO4.0.5H2O seed load, cover the NaCl crystals so that NaCl supersaturation rises, inducing NaCl primary nucleation. Simultaneous crystallization of CaSO4.0.5H2O and NaCl yields a satisfactory product for downstream handling even for high CaSO4.0.5H2O content in the crystallizer. However, seeding with CaSO4.0.5H2O is not recommended as it reduces the mean size and increases product size dispersion. NaCl seeding may be considered if it is desirable to separate CaSO4.0.5H2O from NaCl downstream the crystallizer by size classification, as it favors CaSO4.0.5H2O to build up in product sizes of 200 μm and below.

Novel Dry-Desulfurization Process Using Ca(OH)2/Fly Ash Sorbent in a Circulating Fluidized Bed

A dry-desulfurization process using Ca(OH)2/fly ash sorbent and a circulating fluidized bed (CFB) was developed. Its aim was to achieve high SO2 removal efficiency without humidification and production of CaSO4 as the main byproduct. The CaSO4 produced could be used to treat alkalized soil. An 83% SO2 removal rate was demonstrated, and a byproduct with a high CaSO4 content was produced through baghouse ash. These results indicated that this process could remove SO2 in flue gas with a high efficiency under dry conditions and simultaneously produce soil amendment. It was shown that NO and NO2 enhanced the SO2 removal rate markedly and that NO2 increased the amount of CaSO4 in the final product more than NO. These results confirmed that the significant effects of NO and NO2 on the SO2 removal rate were due to chain reactions that occurred under favorable conditions. The amount of baghouse ash produced increased as the reaction progressed, indicating that discharge of unreacted Ca(OH)2 from the reactor was suppressed. Hence, unreacted Ca(OH)2 had a long residence time in the CFB, resulting in a high SO2 removal rate. It was also found that 350 degrees C is the optimum reaction temperature for dry desulfurization in the range tested (320-380 degrees C).

Effects of CaSO4 and NaCl on growth and nitrogen fixation ofLeucaena leucocephala

Effects and interactions of varying CaSO4 and NaCl levels on growth and nitrogen fixation ofLeucaena leucocephala K8 were examined. Leucaena was grown in nutrient solution at four levels of CaSO4 (0.5, 1.0, 2.5 and 5.0 mM) and NaCl (1, 25, 50 and 100 mM) in randomized blocks with five replications. While NaCl significantly reduced plant growth, additions of CaSO4 increased plant height, leaf number, and biomass of salt treated plants. For the nonsaline treatments, high CaSO4 levels slightly depressed growth, which contradicts suggestions that Leucaena has a high calcium requirement. A significant calcium/sodium interaction was not seen for nodule number or weight. Nodule number was significantly depressed by 100 mM NaCl and nodule weight of the salt stressed plants significantly increased as CaSO4 concentration increased from 0.5 to 2.5 mM. Effects of NaCl and CaSO4 on nitrogen content of plant parts were inconclusive. The promotion of Leucaena salinity tolerance by addition of CaSO4 may be attributed to the effect of calcium in maintaing the selective permeability of membranes.

Charged Membrane Ultrafiltration of Multisalt Systems: Application to Acid Mine Waters

Abstract Ultrafiltration of dissolved and precipitated inorganic salts from synthetic and actual acid mine waters is investigated experimentally in a bench-scale, continuous-flow unit, utilizing negatively charged, noncellulosic membranes. The process is evaluated in terms of the simultaneous achievement of good water flux without membrane fouling and of adequate ultrafiltrate quality at high water recovery for water reuse operation. At a transmembrane pressure of 5.6 X 105 N/m2 and a channel Reynolds number of 6,000, the flux drop is less than 30%, even with a concentrated acid mine water containing 16,000 mg/1 total solids and a high CaSO4 concentration. At 90% ultrafiltrate recovery, depending on the type of noncellulosic membrane utilized, 98% iron removal, 85% aluminum removal, and 50 to 73% calcium and manganese removals can be obtained at an average water flux from 6.4X10−4 to 13.2X10−4 cm/sec. The overall ultrafiltrate quality is considerably better than that observed with a lime precipitation pro...