Boric Acid Production Research Articles

Production of boric acid by bipolar membrane electrodialysis: Evaluation of commercial and polyelectrolyte multilayers-coated ion exchange membranes

Boron is an essential element for many industrial sectors, but the European Union (EU) is entirely dependent on imports for its boron supply. To ensure a sustainable supply of boron for the EU, developing recovery strategies from secondary sources is essential. Thus, boron recovery from seawater reverse osmosis (SWRO) desalination brines using bipolar membrane electrodialysis (BMED) as part of a multi-mineral brine mining process is proposed. BMED experiments were conducted with 35 triplets of commercial membranes (100 cm2), using sodium borate solution (92.5 mM) as feed, at three different pHs (2, 9 and 12), identifying pH 12 as the best option. Moreover, commercial ion exchange membranes from PCA GmbH and Mega a.s. were coated with polyelectrolyte multilayers using layer-by-layer technique. Characterization of the coated membranes included zeta potential, contact angle, ATR-FTIR, transport number and ion exchange capacity analyses. Subsequent BMED experiments with three triplets comparing virgin and modified membranes (100 cm2), using sodium borate solution (92.5 mM) as feed, demonstrated that the coating with 4 bilayers increased the selectivity of membranes. Although the coated RALEX membranes presented the highest selectivity (7.2 for B(OH)4-/Na+ in the acid compartment), uncoated PCA membranes were the optimal choice for H3BO3 and NaOH production due to higher Na and B removal (97 %), higher concentration factors (2.5 for the acid and 4.3 for the base), and higher current efficiency for H3BO3 production (78 %).

Dissolution behavior and kinetic investigation of in solution in the reaction of colemanite ore with propionic acid

Colemanite ore, which is one of the most significant commercially substantial boron minerals, is used to produce various boron compounds in the industry with its rich B_2 O_3 content. Calcium propionate, used in the food industry, is formed as a by-product in its production. In the production of boric acid from colemanite, the use of different solvent reagents is at the forefront to prevent the formation of Mg^(2+), Ca^(2+) and SO_4^(2-) impurities and borogypsum by-products. For this reason, in our study, the dissolution kinetics of colemanite ore in propionic acid solution in an aqueous medium were carried out in a batch reactor system. As dissolution parameters; reaction temperature, solid/liquid ratio, propionic acid (CH_3 CH_2 COOH) concentration, stirring speed and particle size were selected as. According to the experimental results, the amount of Mg^(2+) passed to the solution; was observed that the solution increased with the increase in reaction temperature with the decrease in solid-liquid ratio, grain size, and acid concentration. In addition, it was determined that the mixing speed was not effective. Obtained experimental data were analyzed according to homogeneous and heterogeneous reaction models using the Statistica 10.0 package program. It was determined that the dissolution kinetic of Mg^(2+) passing to solution conformed to the "Avrami model" and activation energy (E) was calculated as 8.18 kJ⁄mol.

RECOVERY OF BORON WASTES WITH INORGANIC ACID

Approximately 73-74% of boron mineral reserves all over the world are located in the Western Anatolian region of Turkey, in the provinces of Eskişehir, Kütahya, Balıkesir, and Bursa. The most intense minerals extracted from these fields are colemanite, ulexite, and tincal minerals, respectively. The general principle in the recovery of these minerals is to extract the raw ore and obtain it by size reduction processes of concentrates in high grade. In addition to concentrates, boric acid (H3BO3) production is also possible in Balıkesir and Kütahya. The production of this acid in question includes a series of processes from dissolution with sulfuric acid at high temperatures to crystallization. In this study, except for the aforementioned high concentrations, boron recovery from clay waste, which is high in boron content and is collected from plant waste ponds, is investigated. For this purpose, ulexite-colemanite-containing slime wastes of the Bigadiç (Balıkesir) Plant were dissolved in 7% solids at 60°C in different sulfuric acid concentrations (1-6% H2SO4) and were obtained with 83-97% B2O3 recovery. When the boron wastes are evaluated, issues such as the discovery and establishment of a new waste pond/dam will be solved, the valuable content in the wastes will be recovered, and the environmental problems of boron and other contents in clay will be eliminated. This article includes information about the characterization of the structure by considering the dissolution of boron and other compounds, as well as obtaining boric acid from inorganic acid and boron wastes.

Calcium silicate solid-state matrices from boric acid production waste for 60Co removal and immobilization by spark plasma sintering

A mesoporous adsorbent based on calcium silicate CaSiO3 for the removal and immobilization of cobalt 60Co radionuclides in durable ceramic CaCoSi2O6 matrices was synthesized by hydrothermal conversion of boric acid production waste. The obtained material had a high Co2+ ions adsorption capacity of 220.8 mg/g. Cobalt adsorption was carried out mainly by ion exchange, which led to the formation of CaCoSi2O6 precursor ceramic matrices. The use of spark plasma sintering (SPS) technology at an optimal temperature of 1000 °C allowed the safe immobilization of Co2+ ions in CaCoSi2O6 ceramic matrices characterized by density (3.33 g/cm3), compressive strength (481 MPa) and microhardness (∼9.81 GPa). Sintered CaCoSi2O6 ceramic samples were characterized by high hydrolytic stability (cobalt leaching rate RCo ∼10−7 g/(cm2 × day)) and complied with the requirements for cured highly active waste GOST R 50926 96/ANSI/ANS 16.1.

Dissolution behavior and kinetic investigation of 〖Ca〗^(2+) in the dissolution of colemanite in propionic acid presence saturated with synthetic flue gas

Colemanite (〖2CaO.3B〗_2 O_3.5H_2 O) is a calcium borate mineral and can be expressed as the primary material of industrial production of boric acid. Boric acid is one of the most essential raw materials obtained by solving colemanite ore in acidic solutions and gases. The dissolution behavior and kinetics of 〖Ca〗^(2+) in a solution of colemanite in a propionic acid solution saturated with synthetic flue gas were examined. In this context; the effects of particle size, reaction temperature, acid concentration, solid-liquid ratio and synthetic flue gas flow rate parameters were investigated. According to the experimental data, it was observed that the amount of 〖Ca〗^(2+) passing to the solution increased with the increase of the reaction temperature, acid concentration and gas flow rate, and with the decrease in solid-liquid ratio and grain size. Experimental results were analyzed by homogeneous and heterogeneous models using the Statistica package program, and it was determined that the dissolution of 〖Ca〗^(2+) in solution complies with the “Avrami” model. The activation energy (E) was found as 〖26.83 kJ.mol〗^(-1) and the Arrhenius constant (A) was found as 〖8.9*10〗^3.

Production of boric acid from colemanite ore in the semi-batch reactor: Investigation of product yield and impurity control

Boric acid is produced by the reaction between colemanite ore and sulfuric acid in industrial processes. Since colemanite ore is not pure, various side reactions, including partial clay decomposition, can occur in the reactor medium. These reactions also lead to the presence of impurities, specifically sulfate salts. In this context, semi-batch reactors (SBR) provide selectivity, and the rate of side reactions is reduced. However, batch reactors (BR) and continuous stirred tank reactors (CSTR) have been used for boric acid production both on an industrial scale and on a laboratory scale, and the semi-batch reactor has never been used for this area. This study used SBR to minimize the side reactions rate and impurities for boric acid production. In the experimental process, a specific concentration of sulfuric acid solution was fed to the reactor at varying flow rates. The yield in the SBR increased up to 99.6%, depending on the sulfuric acid feed rate. When the same conditions were applied in the BR experiment, the yield was calculated to be 90.8%. Compared with the batch reactor results, it was observed that the impurity concentration in the boric acid solution produced within the SBR decreased by 41.7% to 45.3%, depending on the feed rate. Consequently, the utilization of SBR demonstrated clear advantages over the BR in terms of both yield and impurity control. Furthermore, this study serves as a compelling illustration of the practical applications of solid–liquid heterogeneous phase reactions within the context of SBR technology.

Effect of poly(vinyl alcohol) (PVA) weight ratio on spray-dried apatite–PVA composites

Türkiye has the largest colemanite reserves in the world, mainly used in glass and boric acid production. To broaden the current applications of colemanite, it was previously transformed to apatite using a novel method by the same working group of the authors. Natural bone is an organic–inorganic composite where inorganic apatite crystals are accumulated on organic collagen fibers. With inspiration from nature, one further step was taken as an addition to the previous study: apatite, produced from colemanite, and poly(vinyl alcohol) (PVA) were mixed at different weight ratios and then dried using a spray dryer to investigate the effect of PVA content on the composite structure and to see the effect of using a spray dryer on the morphology of the composites. The weight ratios of apatite–PVA were maintained as 3:7, 5:5 and 8:2. The samples in each step were analyzed using X-ray diffraction, Fourier transform infrared spectroscopy and scanning electron microscopy. It was shown that spray-dried apatite–PVA composites were successfully synthesized and could be potential biomaterials. Using a spray dryer and the manipulation of the weight ratios of apatite–PVA were effective ways to tailor the particle size of the powder and the morphology of the particles, which could be beneficial for their biomedical usage.

Application of different treatment systems for boron removal from industrial wastewater with extremely high boron content

The aim of the study is purification and environmental management of boron containing industrial wastewater wastewaters comparing the efficiency of chemical precipitation and membrane filtration systems on the removal of boron at high concentrations from wastewater coming from boric acid production process and ground water. In order to eliminate adverse effects and find optimum treatment system, chemical precipitation, electrocoagulation and reverse osmosis processes have been studied. Different chemical precipitants such as poly aluminum chloride (PACl), lime, calcium chloride (CaCl2) and barium chloride (BaCl2) and hydrogen peroxide as oxidant were used. When solution pH was adjusted at 10, boron removal increased to 99.5 %. With application of electrocoagulation, 9 % of SO42-, 80 % of Si and 45 % of B were removed from spring water. At membrane-system, single and double pass BWRO/SWRO membrane system operations were carried out in order. Boron-removal increased 98 % by increasing pH up to 10, and DP-SWRO membranes system at 10-pH was found to be the best system. Obtaining a sufficient result for boron removal from spring water, a hybrid membrane process which contains an electrocoagulation process as a pre-treatment method and a double pass RO membrane system has been developed as an attractive technology.

A comprehensive analysis on radiation shielding characteristics of borogypsum (boron waste) by Phy-X/PSD code

In the present study, radiation shielding characteristics of the borogypsum, which is a waste generated during the boric acid production in Turkey, was analyzed. For this purpose, we used recently developed Phy-X/PSD software, which is provided to calculate shielding parameters such as mass attenuation coefficient, linear attenuation coefficient, half-value layer, tenth-value layer, effective atomic number, total atomic cross section, total electronic cross section, effective conductivity, effective electron number, buildup factors and fast neutron removal cross section in a wide photon energy range. Additionally, mass attenuation coefficients of borogypsum were compared with those of other radiation shielding materials (ordinary concrete, obsidian, pumice, clay) in order to give a significant evaluation about the radiation shielding capability of borogypsum. Half value layer and fast neutron removal cross section values are also evaluated by other materials. It is noticed that borogypsum has higher shielding potential than other reported shielding materials.

The citric acid leaching of boron process wastes

ABSTRACT With boron plants belonging to ETI Mine Operations concentrated in Western Anatolia, Turkey has the largest boron reserves in the world (73.6%). The annual wastes of boron plants have a higher B2O3 content than the raw material amount of many regions in the world. These wastes, increasing in abundance every year, cause other environmental problems, in addition to waste dam/pond storage problems. For this reason, it is essential to recover these wastes. In this study, optimisation tests were carried out regarding the production of boric acid from organic acid in the utilisation process of borate (Ca/Na-Ca) wastes (8–12% B2O3) from the Balıkesir/Bigadiç Boron Plant. For this purpose, the Yates technique (ANOVA) was used in the experiments at an 8–12 wt.% solids ratio, 40–80°C temperature, 20–60 min of leaching time, and 0.15–0.45 M citric acid concentration. According to the optimisation test results, recovery efficiencies of 90–95% were identified, and net acid consumption (NAC) values were determined as 22–86 kg/t. Characterisation studies such as FT-IR, SEM, XRD Rietveld's, and XRF were also carried out on the raw ore (primary waste) and treated waste samples.

Carbon fixation of CO2 via cyclic reactions with borane in gaseous atmosphere leading to formic acid (and metaboric acid); A potential energy surface (PES) study.

Carbon dioxide (CO2), a stable gaseous species, occupies the troposphere layer of the atmosphere. Following it, the environment gets warmer, and the ecosystem changes as a consequence of disrupting the natural order of our life. Due to this, in the present reasearch, the possibility of carbon fixation of CO2 by using borane was investigated. To conduct this, each of the probable reaction channels between borane and CO2 was investigated to find the fate of this species. The results indicate that among all the channels, the least energetic path for the reaction is reactant complex (RC) to TS (A-1) to Int (A-1) to TS (A-D) to formic acid (and further meta boric acid production from the transformation of boric acid). It shows that use of gaseous borane might lead to controlling these dangerous greenhouse gases which are threatening the present form of life on Earth, our beautiful, fragile home.

Production of boric acid added brake friction composite and the effect of heat treatment on braking characterization

Purpose The purpose of this study was to investigate the use of boric acid as a friction modifier material in brake friction composites and to determine the effect of heat treatment applied during production on braking performance. Design/methodology/approach The addition of five different amounts of boric acid was balanced with cashew, which is in the friction modifier material group. The samples were produced in the following order: dry mixing, preforming and hot-pressing. The effect of the heat treatment that can be applied after the hot-pressing process on the braking performance was investigated. The tribological and physical properties of the samples were determined using tests performed according to appropriate standards. The microstructures of the friction surfaces were investigated using scanning electron microscopy. Findings It was observed that the tribological properties of brake friction composites containing 20% by weight of boric acid were improved. It has also been observed that the heat treatment applied after hot pressing increased the friction coefficient of the samples by 7% on average and decreased the specific wear ratio of the samples. When the surface morphologies of the samples are examined, it is seen that the friction layers of the heat-treated samples are wider, and the microvoids and cracks are reduced. Originality/value This study showed that boric acid can be used as a friction modifier in brake friction composites. It also revealed the tribological and physical contribution of the applied heat treatment to the composite. Thus, it guides brake friction composite manufacturers in the industry and researchers working in this field.

Selective sulfate sorption from boric acid factory process liquor: Chitosan-bentonite biocomposite film synthesis as sorbent

Chitosan is a natural polymer that is renewable, biocompatible, and biodegradable. Bentonite is a natural mineral with low environmental effects. For this reason, chitosan- bentonite biocomposite film was synthesized as a sorbent and its characterized by FT-IR, SEM, EDX, CHNS elemental analysis and thermal analysis techniques. The sorbent was utilized for selective sulfate (SO42−) sorption from boric acid factory process liquor, because one of the most important problems in the boric acid production process is sulfate impurity. The sulfate sorption capacity in the acidic region (pH < 5) was determined as 308.38 mg/g and the equilibrium time was determined as 10 min. It has also been found that the sorbent is regenerative and reusable. Sulfate sorption took place by means of primary amine (-NH2) groups in the structure of the biocomposite. In addition, it was considered that the synthesized sorbent acts as a weakly basic anion exchanger resin. It was determined that the total sulfate salt concentration in the process liquor medium decreased by approximately 30.36% depending on the sulfate sorption. In this context, it is considered that the boric acid factory process liquor has been successfully purified.

Use of acetic acid from organic acids in the production of boric acid from Boron wastes

Turkey's boric acid production has the highest production share among refined boron products. This production is takes place at the Bandırma (Balıkesir) and Emet (Kütahya) Boric Acid Plants of ETI Maden Operations. Concentrated ore (by physical processes) and high-grade wastes accumulated in waste ponds/dams are obtained from boron production plants. In this study, slime size (d80=52.70μm) wastes belonging to the Bigadiç field with 9-12% B2O3 (ulexite-colemanite) content have been evaluated. Due to the small size of feeding without the need for grinding, the sample was subjected to leaching of acetic acid at 60°C for 1 hour, at a mixing speed of 1500 rpm, at 7% solids. In addition to the recovery of 90% H3BO3 (at 4% acid concentration) after the leaching process, different compounds such as Na2O, and CaO were also obtained. Similar peaks were found in the FT-IR analyzes of boric acid and the product obtained. This study is an important output in terms of evaluating the % B2O3 content in the wastes of both this plant and other boron plants.

Boron recovery from desalination seawater brines by selective ion exchange resins

The European Union (EU) depends on third markets to supply many important raw materials. Increasing the circularity of critical raw materials within the EU is important not only from an environmental perspective, but also as a competitive advantage for the EU economy. In the case of boron, the EU's import dependency is about 100%. This work aims to evaluate the boron recovery from seawater desalination plants (SWDP) brines using ion-exchange resins in a circular economy approach. Commercial boron selective resins Purolite S108, DIAION CRB03 and CRB05 were tested and compared on batch and dynamic experiments. Thermodynamic and kinetic experiments were performed, and results were fitted by linear and non-linear models. After a comparison, results showed a good fit to the Langmuir isotherm and the pseudo-second order model, respectively, for all the commercial resins tested. The DIAION CRB03 resin presented higher sorption capacity and percentage of boron sorbed than the other resins and was selected as the best option for boron recovery from SWDP brine. Dynamic experiments in fixed bed column using DIAION CRB03 resulted in a sorption capacity of 13 mg/g of resin, a boron recovery of 98% and a concentration factor of 30, for an initial boron concentration of 50 mg/L. In addition, an economic analysis was carried out as a preliminary estimate of the revenues obtained from the production of boric acid from the brine produced by El Prat desalination plant.

The Energy and Exergy Analysis of the Reactor Unit of Boric Acid Production Process with ChemCAD Simulation

Energy and exergy analysis of systems are of great importance to enhance the energy and exergy efficiency of industrial production facilities. With the energy and exergy analyses performed, the energy dependency of the production facilities and their energy consumption can be reduced, the price of the product can decrease, and the profit margin can increase. Additionally, it is ensured that the energy produced based on fossil fuels is used in a controlled way. In the present study, the analysis of energy and exergy has been performed for the production reactor unit of the Boric Acid from Colemanite. The first law of thermodynamics and ChemCAD simulation program was used for energy analysis calculations, and the calculations of exergy analysis were carried out by using the second law of thermodynamics. The total energy loss of the reactor unit and the calculated energy loss per 100 kcal input steam were calculated as 110880 kcal/h and 3.724%, and the losses of total exergy in the reactor units and the losses of exergy calculated per 100 kcal input steam were calculated as 225058.86 kcal/h and 30.095%, respectively. Exergy efficiency for the reactor unit has been determined as 3.3 %. Some suggestions were given for the reactor units of boric acid production plants to minimize system losses.

Tracing Ancient Carbon Dioxide Emission in the Larderello Area by Means of Historical Boric Acid Production Data

The impact of natural CO2 emissions in the development of geothermal areas is presently gaining more attention than ever before. In Italian geothermal fields, a reduction in the natural CO2 emissions has been observed. This paper reviews and provides an analysis of the historical production data of boric acid from 1818 to 1867 used to calculate the natural emissions of CO2 associated with boric acid production that pre-dates the use of geothermal resources for power production, which started in 1913. Boric acid was already being extracted from the natural geothermal fluids in geysers and natural ponds emitting steam and gases. After 1827 the ‘lagone coperto’ (covered lake) equipment optimized production, and the drilling of shallow wells (20–30 m) starting in 1836, which further increased the quantity of its extraction. The first geothermal reservoir was developed at the turn of the century and the Larderello geothermal field began to grow. The use of deep wells, keeping pace with the power production, led to the gradual disappearance of the natural ponds and the ‘lagoni’ (lakes) in the historical area, so the residual natural emission of CO2 is presently restricted to diffuse soil emission. Comparisons of the ancient CO2 emissions with those of the Geothermal Power Plant (GPP) in the Larderello area show that both amounts are in the same order of magnitude, suggesting a balance between the depletion of natural emissions and geothermal activity.

The Association between Hair Levels of Boron and Disease Incidence in the Population of a Boron Geochemical Province: A Cross-Sectional Study

Background: A stable natural and technogenic boron geochemical province characterized by high environmental levels of boron has formed in the Aktobe Region of West Kazakhstan. The main local source of boron is sludge collectors of the boric acid production of the now destroyed chemical plant, built without impervious screens in the former old channels of the Ilek River, as well as in the river floodplain on alluvial quaternary sand and gravel deposits with high permeability. The objective of the study was to map hair levels of boron in residents of the Aktobe Region and to establish their relationship with disease incidence of the adult population. Materials and methods: The study included 340 permanent residents of the Aktobe Region, Republic of Kazakhstan, aged 18–60. Hair boron levels were measured by inductively coupled plasma mass spectrometry using a Nexion 300D spectrometer (PerkinElmer Inc., USA) coupled with an ESI SC-2 DX4 sampler (Elemental Scientific Inc., USA). Mapping was performed using the QGIS cross-platform geographic information system (QGIS 3.18). Results: The map of the boron status of the study participants visually demonstrated the uneven distribution of hair boron concentrations in the inhabitants of the region. Hair levels of boron were significantly higher in men than in women (p = 0.000). We established a strong direct correlation between hair levels of boron and the incidence of congenital anomalies and chromosomal abnormalities (r = 0.886, p = 0.019), diseases of the genitourinary system (r = 0.829, p = 0.042), respiratory diseases (r = 0.943, p = 0.005), diseases of the digestive system (r = 0.878, p = 0.021), diseases of the blood, hematopoietic organs and the immune system (r = 0.880, p = 0.017). Conclusion: The mapping of hair levels of boron in the residents of the Aktobe Region of West Kazakhstan gave a visual demonstration of the uneven distribution of environmental pollution with boron in the study area. The revealed relationships between boron concentrations in hair tissue and disease incidence in the boric geochemical province confirm the assumption that the ecological features of the environment can affect the health of the population, which requires further in-depth research.

Techno-economic Analysis of Boric Acid Production from Colemanite Mineral and Sulfuric Acid

ABSTRACT In the present study, a sustainable production of boric acid from colemanite mineral and sulfuric acid was investigated, considering waste treatment and colemanite mineral processing costs. The purchasing price of sulfuric acid and the selling price of boric acid obtained from the market were used as $50/t and $900/t, respectively, to perform the simulation via professional SuperPro designer (v.9.0). The effect of the colemanite mineral price was investigated between $50 and $250/t for a 100 kt.y−1 plant capacity. The effect of a waste treatment cost was investigated by applying values between $10 and $50/ton for a 100 kt.y−1 plant capacity. The annual waste treatment cost was calculated using the value of $50/ton and was found to be $9.3 M, which contributes to the operating cost by 13.4%. The most significant components that influenced the boric acid production costs were determined to be the facility-dependence (41.9%) and cost of the raw materials (30.3%). The fixed capital investment and annual operating costs of the boric acid plant are calculated as $170.7 M and $70.1 M/y, respectively, for a 100 kt.y−1 capacity. The unit production cost of boric acid having an impurity of <100 ppm was calculated as $0.70 kg−1. Therefore, a payback time of 6.3 years was estimated from the economic analysis of a commercial boric acid production process including environmental impacts.

Динамика сорбции борной кислоты борселективным анионитом типа СБ

Investigation and choice of an effective sorbent are significant for protection of water bodies against boron The boron high concentration in natural waters produces toxic impact on aquatic biota due to its high biological activity. This paper studied the dynamics of boric acid sorption from aqueous solutions by polycondensation anionite SB-1 synthesized on the base of monoethanolamine and epichlorohydrin. The anion exchanger contained aminooxyethyl and alcohol functional groups in its structure. Methods. It has been established the influences of the ion form of the anionite on the dynamic characteristics of boron sorption from both a model boric acid aqueous solution (0.43 mol/dm3) and an industrial wastewater effluent after boric acid production (0.47 mol/dm3), other things being equal (pH = 5.0; the volume rate of the solution passing through the anionite layer = 1.5 ml/min∙cm2; t = 20 0C). Structural studies to determine the form of boron in the anionite phase were performed using the IR spectroscopic method. Dynamic characteristics of boron sorption (the cation and overall ion-exchange capacity values, period of the protecting effect of the filter) were determined graphically from the sorption curves. Results. It has been found that the hydroxyl form of the anion exchanger SB-1, in comparison with the chlorine form, performs superequivalent boron sorption while boron sorption from both a model and real production solutions. For the first time we have demonstrated that at the initial stage, boric acid sorption is carried out owing to the chemisorption, resulting to the formation of stable hydroxoborate forms [B(OH)4]– in the ionite phase. As the anionite is being saturated and the concentration of free functional hydroxyl groups is decreasing, the formation of polynuclear forms of boric acid [B4O5(OH)4]2– occurs in the anionite phase by sorption of boric acid molecules on free tetrahydroxoborate anions and, further, the complex formation of polynuclear forms of boron by the nitrogen atom of aminooxyethyl functional groups. It has been stated that desorption of boron from anionite is considered to be effective when carried out with a 2 n. NaOH solution with a regeneration degree of 0.995.