Development of Efficient and Scalable Production Process of Analytical Grade Sodium Chloride at Laboratory Scale

    Analytical grade Sodium chloride (NaCl) is one type of salt with a purity of > 99.50 % NaCl content, including other impurities. In laboratories, salt is used as a reagent and a standard solution. This research carried out the salt production process through purification, acidification, neutralization, crystallization, and drying methods. Compared to existing technologies, the advantages of this technology are its simplicity and ability to produce accompanying products. In the first stage of the purification process, the precipitation of sulfate (SO₄^2-) and magnesium (Mg²⁺) ions was examined. In this step, 20 % w/v barium chloride (BaCl₂) and 20 % w/v sodium hydroxide (NaOH) solution were consecutively used, and after 12 hours of sitting, the filtering process separated the mixture. The second stage of purification will investigate the precipitation of calcium (Ca²⁺) ions using 20 % w/v sodium carbonate (Na₂CO₃) solution, and the mixture was again separated by filtering after 12 hours. All stages of the purification process have shown that the ions concentration decreases by adding BaCl₂, NaOH, and Na₂CO₃ solution. Furthermore, the acidification, neutralization, and crystallization processes can control pH and potassium (K⁺) content. The results demonstrated that the technology produced 30% analytical grade NaCl with purity > 99.50% and high brine solution as a by-product, which can be further processed for producing pharmaceutical and food-grade salts.

Analytical grade; Development; Production process; Purification; Sodium chloride

Sodium Chloride (NaCl) or salt is an essential commodity. It can be used as a raw material or a mixture in various fields, including chemical, agricultural, animal husbandry, pharmaceutical, health, cosmetic, food, and beverage industries. In the chemical industry, salt is the primary raw material for the production of chlorine gas (Cl₂), hydrochloric acid (HCl), sodium hydroxide (NaOH), sodium sulfate (Na₂SO₄), sodium carbonate (Na₂CO₃), and sodium bicarbonate (NaHCO₃) (Ono, and Erhard, 2012; Sedivy, 2010). Salt is widely used in leather tanning, fish preservation, and oil drilling. Moreover, it is also used in weather modification through seeding and cloud formation to form artificial rain. Types of salt suitable for use include analytical, pharmaceutical, industrial, food, and miscellaneous grade salts. Food-grade salt is known for making salted fish-tanning skin (Ono, and Erhard, 2012; Sedivy, 2009).

Analytical grade NaCl is a high-purity salt with a NaCl content of 99.50 - 100.50% and a low content of other compounds. The most common use of salt is in laboratory chemicals.  Its common uses include chemical reagents, standard sodium ion in absorption atomic spectrophotometers, standardization of AgNO₃ solutions in argentometry volumetric, microbiological media, material synthesis, cell microbiology, and ion chromatography.

NaCl is also used for special purposes in the synthesis of material. The effect of hydrothermal reaction time and NaCl addition on nano rosette TiO₂ crystal growth characteristics during hydrothermal reaction has been examined. With no addition of NaCl, the nano rosette forms at full growth, indicated by the high intensity of the crystal structure indexed to rutile P42/mnm with lattice parameters of a = 4.557(6) Å and c = 2.940(5) Å. The cross-sectional rosette petal grew to 250 nm. On the contrary, the crystal growth during hydrothermal reaction could be controlled with the addition of NaCl. In this work, with the addition of 2.5% v/v NaCl, the cross-sectional rosette size was only about 50 nm, 80% smaller than that of the crystal grown with no NaCl addition (Sofyan et al., 2019).

Analytical grade NaCl specifications refer to Sigma Aldrich, Merck, and Fisher Scientific standards. The quality requirement of the analytical grade NaCl was showed in Table 1.

Table 1 The quality of requirements for analytical grade NaCl

This process produces analytical-grade salt. Therefore, the purification stages are necessary to increase NaCl content and reduce impurities of ions such as sulfate, calcium, magnesium, and potassium. Therefore, many studies have been conducted to improve the quality of salt by using several methods. Two of the common processes for purifying crystalline chemical compounds including salt is using the crystallization (Lu et al., 2017; Jones, 2002; Giulietti et al., 2001; Mullin, 2001) and precipitation methods (Mersmann, 2004).

These reported methods include re-crystallization (Rismana and Nizar, 2014; Rositawati, Taslim, and Soetrisnanto, 2013; Nurhidayati, 2007), hydro extraction (Kharismanto et al., 2021; Sumada, Dewati, and Suprihatin, 2018; Martina et al., 2016), coagulation (Astuti, 2021; Lukum et al., 2021 Pujiastuti et al., 2017; Rathnayaka et al., 2014; 2013; Sulistyaningsih, Sugiyo, and Sedyawati, 2010; Ihsan and Djaeni, 2002; Yang, Yun, and Yang, 2007), geomembrane (Hoiriyah, 2019), purification – evaporation (Luo et al., 2022; Schimdt, Meirhofer, and Schwaiger, 2009; Wae, Tjahjanto, and Mardiana, 2021), simultaneous crystallization (Penha, Zago, and Seckler, 2020; Zago, Penha, and Seckle,  2019; Penha et al., 2018), purification using porous material composite (Sulhadi, et al., 2017) and High-Density Polyethylene (HDPE) membrane (Rusiyanto Soesilowati, and Jumaeri 2013). However, these results are only able to produce industrial and food-grade salts.

The study of the process of purification of raw salt is evaluated by washing with a saturated solution of pure NaCl to remove the impurities. The results showed that washing the salt with brine can decrease contaminants such as magnesium, calcium, sulfate, and heavy metal (Ahmmed et al., 2018). Furthermore, research to improve NaCl quality has been performed by drying the salt using a hot air-drying system. As a result, the NaCl concentration increased but could not reduce the impurity content (Amir et al., 2021). Research for preparing analytical grade salt was investigated by re-dissolved, coagulation, ion exchange, and re-crystallization processes from rock salt (Rahman, Islam, and Farrukh, 2010). However, this studied process uses resin to reduce potassium levels, which is costly when developed on an industrial scale. The preparation of pharmaceutical-grade salt was also investigated using purification by precipitation and only one partially crystallized stage from crude salt as raw material (Rismana, 2016; Rismana and Srijanto, 2013). Nevertheless, the product still has a high potassium content which does not meet the NaCl analytical grade standard. 

The novelty of this study was to reduce calcium, magnesium, and sulfate impurities using two stages of precipitation. Three processes of partial re-crystallization were also carried out to decrease potassium content. In addition, this production process produces a high-quality salt solution, which can be used as raw material to make pharmaceutical or food salts. Therefore, developing the process production technology analytical grade NaCl in Indonesia is still necessary at a laboratory and bench scale. The benefit of this research is the availability of the technology process and accessibility that the domestic industry can easily apply.

2.1. Materials

      The materials are divided into raw, refining, and analysis groups. Raw and refining materials consist of crude salt, industrial-grade of Na₂CO₃ (PT. Brataco, Bandung, Indonesia), industrial-grade of NaOH and 32% HCl (PT Asahi Mas, Cilegon Indonesia), industrial-grade of BaCl₂ (Haihang Industry, Co. Ltd., Jinan,  China). While, the materials pro analysis grade for analysis include AgNO₃, NaCl K₂CrO₄, Na-EDTA, EBT indicator, murexide indicator, 36% w/v HCl, NaOH, blue thymol bromine indicator, NH₄OH, NH₄Cl, Na₂SO₄, BaCl₂, oxalic acid (Merck, Germany).

2.2. Purification and Production Processes of Analytical Grade NaCl

The purification stages for this process will be used as follows: (1) crude salt was dissolved with water to obtain a brine solution with NaCl content of about 27-30 % w/v, (2) the brine solution was then filtered to remove insoluble impurities, (3) analysis of NaCl, SO₄^2-, Mg²⁺, Ca²⁺, water-insoluble content, and water content is carried out on the sample of the initial brine solution, the brine solution after purification, after filtering stages, and the analytical grade NaCl product, (4) purification and filtering process, (5) acidification and neutralization, (6) crystallization, and (7) separation and drying process. Figure 1 shows a complete set of production processes.

The purification process was performed using the precipitation of SO₄^2- with BaCl₂, Mg²⁺ with NaOH, and Ca²⁺ with Na₂CO₃ using the two-stage method. The first process was examined by coagulating SO₄^2- and Mg²⁺ ions using 20 % w/v BaCl₂ and NaOH solution successively, and after 12 hours left, the mixture was separated by the filtering process. The second process was studied by precipitation of Ca²⁺ ion using 20 % w/v Na₂CO₃ solution, and after 12 hours, the mixture was separated by the filtering process again. The detailed procedure was as follows; the brine solution was reacted with a 20% w/v BaCl₂ solution until the SO₄^2- the content was < 0.001 % (10 ppm). Furthermore, a brine solution containing BaSO_4(s) was added 20% w/v NaOH to Mg²⁺ content of < 0.001 % (10 ppm). The next step was to separate BaSO_4(s) and Mg (OH)_2(s) by filtering.

Figure 1 Schematic diagram of process production analytical grade NaCl by two stages of precipitation

2.3. Product Characterizations and Analysis

        Product and intermediate NaCl were investigated according to the standard CoA of NaCl analytical grade. The NaCl content was calculated based on the chloride ion content determined by argentometry volumetric. While the calcium, magnesium, potassium, and heavy metal ions content were analyzed by Inductively Coupled Plasma Mass Spectrometry (ICP-MS) spectrophotometer. A pH solution was determined using a pH meter, and the Loss on Drying (LOD) apparatus measured water content. While sulfate content was determined using a turbidimetry spectrophotometer.

3.1.   Analysis of Salt Raw Materials and 27-30 % w/v Initial Brine Solution

The quality of salt raw material needs to be analyzed to determine whether it meets the specification requirements. Table 2 shows the results of the analysis of NaCl content and the main impurities of the salt raw material. The analysis results in Table 2 show that raw salt materials have 87.30 % NaCl content. This salt indicates that the raw material still has low NaCl content. However, the calcium, magnesium, and sulfate impurities content is standard and meets consumption salt specifications. The content of water-insoluble impurities is relatively low; it will facilitate the separation of contaminants during the filtering process to get a clear salt solution.

        The higher NaCl and lower impurities compound content in salt raw material will be advantageous for producing industrial-grade salt (Chlor Alkali Plant (CAP), pharmaceutical, and analytical grade) due to fewer materials needed for refining and higher-yield products. The salt is dissolved in water to obtain a brine solution with a NaCl concentration of about 27-30% w/v. Adding water to the 27-30% salt solution also causes lower levels of calcium, magnesium, and sulfate impurities. Table 3 shows the analysis results of several main parameters of the initial brine solution.

3.2. Production of Brine Solution after Purification and NaCl Analytical Grade

Analyzing impurity ions in the brine solution is essential to determine the number of chemicals needed during refining. Table 4 and Figure 2 show the results of the NaCl content analysis and the brine solution's main impurities after purification. In this research, the purification process was carried out in two stages: the first stage of precipitation using 20 % w/v BaCl₂ and NaOH to reduce the sulfate (SO₄^2-) and magnesium (Mg²⁺) ions content. In contrast, to reduce calcium (Ca²⁺) ion content, we studied the second coagulation stage by adding 20 % w/v Na₂CO₃ to reduce the calcium (Ca²⁺) ion content.

In the first stage, the amount of SO₄^2- and Mg²⁺ ions were decreased after precipitation with 20 % w/v BaCl₂ and NaOH at concentrations 0.0005-0.0006 % (5-6 ppm) and 0.0008-0.0012 % (8-12 ppm), respectively. The Ca²⁺ content did not significantly reduce because calcium did not react with NaOH and BaCl₂. After storage for 12 hours and filtration, the SO₄^2- and Mg²⁺ ions content remained relatively unchanged.

In the second precipitation stage, a reaction with 20 % w/v Na₂CO₃ reduced the concentration of Ca²⁺ ion to 0.0015 % (15 ppm). In contrast, SO₄^2- and Mg²⁺ ion content after 12 hours of storage and filtering showed a similar concentration. The NaCl concentration decreased from 26.30 % to 25.55 % after the first and second stage processes due to the dilution process of adding reagents. As presented below in Table 4, replicating three batches of the purification process showed good repeatability.

In the following process, HCl and NaOH solutions were added to control the pH brine solution in the acidification and neutralization stage. First, HCl solution is added, so the brine solution's pH becomes 2-3. Furthermore, the pH was raised again to pH 5-6 by adding a solution of NaOH. Adjusting the pH brine solution is essential to remove carbonate content so the product can meet the standard acidity specifications. Therefore, decreasing the pH to 2-3 in the salt solution was done to remove residual carbonate ions that should not be present in the NaCl pro analysis. While increasing the pH after that process to pH 5-6 is to return the pH of the solution to be neutral so that the final NaCl product can meet the pH requirements of 6-7 when testing the pH of the solution. challenging.

3.3.   Production of Analytical Grade NaCl Product

The process was validated by a triplicate. Table 5 presents the yield and analysis of product results. The average yield of the process production was about 30.33 %. Meanwhile, the quality of the product also met the requirements of the main parameters of the salt analytical grade specifications. In addition to calcium, magnesium, sulfate, and potassium impurities, barium must also be tested as impurities because it is a heavy metal and one of the reagents added as BaCl₂ to meet the requirements according to the standard.

One advantage of this process is that the outcome by-products of high-quality brine solution can be used as a raw material for producing pharmaceutical and food-grade salts. Table 6 shows the analysis of pharmaceutical-grade salt after crystallization and drying from high-quality residual brine. The pharmaceutical-grade salt product offers good quality with NaCl, Ca²⁺, SO₄^2- and Mg²⁺ contents and meets the requirements set by pharmacopeia.

        This research carried out the salt production process through purification, acidification, neutralization, crystallization, separation, and drying methods. This developed technology process of analytical grade salt production at the laboratory scale has produced salt according to the specifications required, with an average product yield of 30.33 % and a purity of NaCl of 100.15 %. Its simplicity and ability to produce accompanying products. Compared to existing technologies, the advantages of this production technology are its simplicity and ability to make multiple accompanying products. Moreover, by-products of high-quality brine in this technology can be used as raw material to produce pharmaceutical and food-grade salt. Therefore, this invention has excellent potential to accommodate domestic demands, substitute imported products, and increase the economic value of crude salt as raw material.

       The authors would like to thank the PT Karya Darya Syafarmasi for the research collaboration and application of the innovation at an industrial scale. 

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