ТЕХНОЛОГИЯ ПРОИЗВОДСТВА СОЛЕЙ МАГНИЯ ИЗ СОЛЕНЫХ ОЗЕРНЫХ РАССОЛЕЙ КАРАКАЛПАКСТАНА: БИСХОФИТ, ГИДРОКСИД/ОКСИД МАГНИЯ, СУЛЬФАТ НАТРИЯ И ИСПОЛЬЗОВАНИЕ ОТХОДОВ ПРОИЗВОДСТВА СОДА

УДК 661.831.1

Abstract

The aim of this study is to develop, optimize, and pilot-validate an integrated zero-waste technology for the complex processing of saline lake brines, dry mixed salts, and soda plant distillery liquid from Karakalpakstan, Uzbekistan. Experimental methods include vacuum evaporative crystallization, chemical precipitation, X-ray powder diffraction (DRON-3M), thermogravimetric/differential thermal analysis (TG/DTA), and pilot-scale trials at JSC Ferganaazot. This study presents a scientifically validated, pilot-tested, zero-waste integrated technology for the complex processing of saline lake brines and dry mixed salts (DMS) from Karaumbet and Barsakelmes lakes (Karakalpakstan, Uzbekistan), alongside utilization of distillery liquid (DL) — a large-tonnage solid waste of the Kungrad Soda Plant. The technologies enable co-production of bischofite (MgCl₂·6H₂O, ≥99.5%), premium anhydrous sodium sulfate (≥99%, GOST 6318-77), magnesium hydroxide (≥94% Mg(OH)₂), and magnesium oxide (≥94.6% MgO, GOST 844-79), together with chemical gypsum (CaSO₄·2H₂O) and precipitated chalk (CaCO₃ ≥97.9%). A two-stage evaporative crystallization process achieves simultaneous NaCl (≥98%) and bischofite separation. Magnesium hydroxide precipitation at pH 10.5–11 using NaOH yields 99% Mg(OH)₂; calcination at 700–800 °C for 120 min produces MgO meeting applicable standards. Decalcification of DL with sodium sulfate at 20–30 °C for 30 min attains 96.4% Ca²⁺ removal. Process flowsheets, material balances, six original figures, techno-economic calculations, and pilot-plant trial results are reported. Annual net profits of 4.16 and 2.38 billion UZS are projected for the DMS and DL routes respectively, confirming strong import-substitution potential for Uzbekistan's chemical industry.

Аннотация

В данной работе представлена научно обоснованная и апробированная в пилотных условиях безотходная интегрированная технология комплексной переработки рассолов солёных озёр и сухих смешанных солей (ССС) озёр Караумбет и Барсакельмес (Республика Каракалпакстан, Узбекистан), а также утилизации дистиллерной жидкости (ДЖ) — крупнотоннажного отхода Кунградского содового завода. Разработанные технологии обеспечивают совместное получение бишофита (MgCl₂·6H₂O, ≥99,5%), высококачественного безводного сульфата натрия (≥99%, ГОСТ 6318-77), гидроксида магния (≥94% Mg(OH)₂) и оксида магния (≥94,6% MgO, ГОСТ 844-79), а также химического гипса (CaSO₄·2H₂O) и осаждённого мела (CaCO₃ ≥97,9%).

Двухстадийный процесс выпарной кристаллизации обеспечивает одновременное выделение NaCl (≥98%) и бишофита. Осаждение гидроксида магния при pH 10,5–11 с использованием NaOH позволяет получать Mg(OH)₂ с выходом 99%; прокаливание при 700–800 °C в течение 120 минут обеспечивает получение MgO, соответствующего действующим стандартам. Декальцинация дистиллерной жидкости сульфатом натрия при 20–30 °C в течение 30 минут обеспечивает степень удаления ионов Ca²⁺ до 96,4%.

В работе представлены технологические схемы процессов, материальные балансы, шесть оригинальных рисунков, технико-экономические расчёты и результаты опытно-промышленных испытаний. Прогнозируемая годовая чистая прибыль составляет 4,16 и 2,38 млрд сумов для направлений переработки ССС и ДЖ соответственно, что подтверждает высокий потенциал импортозамещения для химической промышленности Узбекистана.

Keywords: bischofite; magnesium hydroxide; magnesium oxide; sodium sulfate; saline lake brine; distillery liquid; Karaumbet; Barsakelmes; zero-waste technology.
Ключевые слова: бишофит; гидроксид магния; оксид магния; сульфат натрия; рассолы солёных озёр; дистиллерная жидкость; Караумбет; Барсакельмес; безотходная технология.

1. Introduction

Magnesium and its compounds are among the most strategically critical inorganic materials in the modern global economy. Annual world consumption of primary metallic magnesium exceeds 1.1 million tonnes, driven by aluminium alloy production, automotive die-casting, titanium sponge manufacturing, and a growing range of chemical and pharmaceutical applications [1,2]. China dominates global primary magnesium output at more than 73% of world supply (Figure 1), while demand from developed economies is met largely through imports [3].

Figure 1. Primary magnesium production by country, 2012 (Source: Roskill Information Services Ltd.)

Magnesium consumption is distributed across several industrial sectors, with metallurgy (aluminium alloys) accounting for approximately 44% of total demand, followed by die-casting components for automotive and electronics sectors (~20%), and titanium sponge/steel desulfurization (~17%) (Figure 2).

Figure 2. Global magnesium consumption by industrial sector (Source: Roskill Information Services Ltd.)

Uzbekistan's Karakalpakstan region harbours substantial reserves of sulfate-chloride saline lake brines (Karaumbet and Barsakelmes lakes) enriched in MgCl₂, Na₂SO₄, and NaCl, alongside solid evaporite deposits (dry mixed salts, DMS). These resources remain largely unprocessed industrially despite their clear potential as feedstocks for import-substituting production of bischofite, magnesium hydroxide/oxide, and premium sodium sulfate [4,5,6].

Concurrently, the Kungrad Soda Plant (Karakalpakstan) discharges approximately one million tonnes per year of distillery liquid (DL) — a hazardous calcium chloride-bearing waste containing 8–10% CaCl₂ and 5–6% NaCl — into unlined settling lagoons, causing severe regional environmental contamination and loss of recoverable salt [7,8]. No commercially viable utilization technology has been established for this waste stream.

Earlier work by Bobokulova et al. [4], Mirzakulov and Tojiev [5], and Djuraeva and Mirzakulov  established the physicochemical basis for brine processing and sodium sulfate extraction from local raw materials. The present study extends this foundation to develop, optimize, and pilot-validate an integrated, closed-loop flowsheet covering all stages from brine purification to final product characterization, together with a techno-economic assessment of industrial implementation.

Recent international studies on analogous saline lake systems (Dead Sea, Great Salt Lake, Kara-Bogaz-Gol) and soda plant waste utilization [10,11,12,13] confirm the technical feasibility and economic attractiveness of integrated approaches, providing a global reference framework for the present work.

The aim of the present study is to develop, optimize, and pilot-validate an integrated zero-waste technology for the complex processing of Karaumbet–Barsakelmes saline lake brines and dry mixed salts, as well as Kungrad Soda Plant distillery liquid, enabling co-production of bischofite, magnesium hydroxide, magnesium oxide, anhydrous sodium sulfate, chemical gypsum, and precipitated chalk, with a full techno-economic assessment of industrial implementation.

2. Materials and Methods

2.1. Raw Materials

Brine samples were collected seasonally from Karaumbet and Barsakelmes lakes (North Aral region). DMS were sampled from the Karaumbet evaporite surface. DL was supplied by JSC Kungrad Soda Plant (Karakalpakstan). Chemical compositions of the principal feedstocks are given in Table 1.

Table 1. Chemical composition of principal feedstocks (mass %)

Note: Distillery liquid (DL) composition (mass %): NaCl 5.55; CaCl₂ 8.41; MgCl₂ 0.03; SO₄²⁻ 0.03; density 1.070 g/cm³.

2.2. Analytical Methods

Ion concentrations (Na⁺, Mg²⁺, Ca²⁺, Cl⁻, SO₄²⁻) were determined by standard volumetric and gravimetric methods. Solid phase identification used X-ray powder diffraction (DRON-3M diffractometer, CuKα radiation). Thermal decomposition was studied by TG/DTA (Q-1500 derivatograph). Density and viscosity were measured at 20–60 °C. Vacuum filtration rates were determined at 400 mm Hg using a Buchner funnel–Bunsen flask assembly with filter paper grade 'White Ribbon'. All experiments were performed in triplicate; data represent means ± standard deviation.

2.3. Scale-Up and Pilot-Plant Trials

Laboratory experiments were conducted in stirred glass reactors (250–2000 mL). Scale-up trials (50–200 kg batches) were performed at JSC Ferganaazot (Fergana) in a pilot plant replicating industrial operating conditions. Material balances were calculated per tonne of principal product. Techno-economic assessments used 2020–2021 Uzbekistan market prices and were validated against reference data from JSC O'zkimyosanoat [11].

3. Results and Discussion

3.1. Brine Purification and Two-Stage Evaporative Concentration

The elevated sulfate content of Karaumbet–Barsakelmes brines (MgSO₄ 4.2–6.8%, Table 1) necessitates desulfation before evaporation to prevent contamination of NaCl and bischofite crystals with epsomite (MgSO₄·7H₂O) or astrakanite (Na₂SO₄·MgSO₄·4H₂O). Treatment with DL (Ca²⁺/SO₄²⁻ molar ratio = 1.05, 20–30 °C, 30 min) reduces sulfate to ≤0.30% as CaSO₄·2H₂O; vacuum filtration rate 600–800 kg/m²·h. Residual Ca²⁺ is removed by Na₂CO₃ addition (molar Ca²⁺:CO₃²⁻ = 1:1.05), yielding precipitated CaCO₃ (filtration rate 400–500 kg/m²·h, purity ≥97.9% CaCO₃).

The purified brine (density 1.238–1.245 g/cm³) undergoes two-stage vacuum evaporation. Stage I concentrates the brine from density 1.245 to 1.380 g/cm³, during which NaCl crystallizes (purity ≥98%). In Stage II, after NaCl removal, the residual liquor (density 1.350 g/cm³) yields MgCl₂·6H₂O (bischofite) at ≥99.5% MgCl₂·6H₂O. The complete material balance per tonne of bischofite is summarized in Table 2.

Table 2. Material balance for bischofite production (per tonne of bischofite)

Annual production of 15,000 t bischofite with 23,000 t co-product NaCl, 3,300 t CaSO₄·2H₂O, and 550 t CaCO₃ generates a net economic return of ~5.53 billion UZS/year, with import-substitution value of ~6.75 million USD/year at 450 USD/t import price.

3.2. Sodium Sulfate Production from Dry Mixed Salts

Karaumbet DMS (Na₂SO₄ 55.34%, Table 1) is dissolved at L:S = 3:1, 25 °C, 20–30 min; insoluble residue (2.10%) is removed by settling and filtration. The clarified filtrate (Na₂SO₄ 15.31%, NaCl 4.74%, MgCl₂ 3.97%) is cooled to 0–5 °C, crystallizing mirabilite (Na₂SO₄·10H₂O) at 80–86% yield with filtration rate 4234–5486 kg/m²·h. Thermal dehydration at 200–210 °C yields anhydrous Na₂SO₄ ≥99% meeting GOST 6318-77 Grade A. Per tonne of Na₂SO₄ produced: 10.71 t DMS input; by-products include 1.05 t CaSO₄·2H₂O and 45.95 t NaCl brine. Production cost is 307,000 UZS/t vs. market price 2,000,000 UZS/t — an 85% gross margin.

3.3. Decalcification and Valorization of Distillery Liquid

Distillery liquid from the Kungrad Soda Plant is decalcified by sodium sulfate via the reaction:

CaCl₂  +  Na₂SO₄  +  2H₂O  →  CaSO₄·2H₂O↓  +  2NaCl

The effect of Na₂SO₄ dosage on decalcification degree is shown in Figure 3, and the corresponding liquid-phase compositions are given in Table 4.

Figure 3. Effect of Na₂SO₄ dosage norm and temperature on decalcification degree of distillery liquid (30 min reaction time)

Table 4. Liquid-phase composition after decalcification of distillery liquid (20 °C, 30 min)

At 100% Na₂SO₄ norm, 20 °C, 30 min: decalcification degree = 96.4%; purified liquid contains 10.60% NaCl, 0.31% CaCl₂. Optimal conditions: norm 100–102%, T = 20–30 °C, t = 30 min.

Complete Ca²⁺ removal is achieved by subsequent Na₂CO₃ addition (Ca²⁺:CO₃²⁻ = 1:1.05, 30 min, 25 °C), yielding precipitated CaCO₃ ≥97.9% suitable for construction use. The purified NaCl brine (Ca²⁺ absent; SO₄²⁻ 0.10%; 9.24% Cl⁻) is re-used directly in the soda plant. Pilot-plant trial at JSC Ferganaazot processed 50 kg DL yielding 6.85 kg CaSO₄·2H₂O, 1.25 kg CaCO₃, and 51.1 kg purified NaCl brine — consistent with laboratory predictions.

3.4. Magnesium Hydroxide and Magnesium Oxide Production

The NaCl–MgCl₂ mother liquor (after mirabilite separation) is purified sequentially of residual SO₄²⁻ with DL and of Ca²⁺ with Na₂CO₃, then precipitated with NaOH. Optimized parameters are compiled in Table 3.

Table 3. Optimized process parameters for Mg(OH)₂ and MgO production

Mg(OH)₂ (Grade B, ≥94% Mg(OH)₂, STP TU KOMP 1-276-10) and MgO (≥94.6% MgO, GOST 844-79) fully meet applicable national standards. Bulk density of MgO: 0.43 g/cm³.

The effect of pH on precipitation is shown in Figure 4. Below pH 8, precipitation is negligible (<12%); maximum removal (99%) is achieved at pH 10.5–11 with NaOH. Ca(OH)₂ as precipitant achieves 98.2% yield at the same pH but gives a less uniform particle size distribution.

Figure 4. Magnesium hydroxide precipitation degree vs. pH (NaOH and Ca(OH)₂ precipitants; 40–60 °C, 30–60 min)

Thermogravimetric analysis (Figure 5) shows that Mg(OH)₂ dehydration begins at 315.3 °C and is complete at 410 °C; activation energy of decomposition = −782.8 J/g. Calcination experiments over 400–800 °C confirmed that MgO content ≥94% (GOST minimum) requires 700–800 °C for 120 min.

Figure 5. Effect of calcination temperature on MgO content (calcination time 60 and 120 min; from Mg(OH)₂ obtained via NaOH precipitation at pH 11)

3.5. Integrated Process Flowsheet

The complete integrated flowsheet encompasses: (1) brine/DMS purification from Ca²⁺ and SO₄²⁻ using DL and Na₂CO₃; (2) two-stage evaporative crystallization producing NaCl and bischofite; (3) cooling crystallization of mirabilite from DMS solution and dehydration to premium Na₂SO₄; (4) precipitation of Mg(OH)₂/MgO from purified NaCl–MgCl₂ liquor; (5) decalcification of DL producing chemical gypsum, precipitated chalk, and recyclable NaCl brine. The process is designed as a fully closed loop with no liquid effluent discharge to the environment. All solid by-products are marketable commodities.

4. Techno-Economic Assessment

Table 5 summarises projected annual production capacities and economic performance indicators for each product stream. The economic profile of the integrated technology is visualized in Figure 6.

Table 5. Techno-economic summary of integrated processing technology (2021 Uzbekistan market prices)

Basis: 107,100 t/year DMS and associated brine processed; DL utilization: 100,000 t/year. Prices converted at 1 USD ≈ 10,700 UZS (2021).

Figure 6. Estimated annual revenue and production cost for each product stream (integrated technology, 2021 prices, billion UZS/year)

The DMS processing route generates an annual net profit of ~4.16 billion UZS per 10,000 t Mg(OH)₂ produced, including 47,180 t Na₂SO₄, 10,500 t chemical gypsum, 1,500 t precipitated chalk, and 82,700 t NaCl brine as co-products. DL utilization generates ~2.38 billion UZS annually while simultaneously eliminating the environmental burden of nearly 100,000 t of hazardous soda plant waste per year. The bischofite production route yields ~5.53 billion UZS/year for 15,000 t product, with full import substitution for Uzbekistan's defoliant and fertilizer industries.

Sensitivity analysis indicates that project viability is maintained even if market prices for Mg(OH)₂ and Na₂SO₄ fall by 30–40%, owing to the multi-product nature of the process and the low cost of primary raw materials (DMS: 50,000 UZS/t; DL: essentially zero cost as a waste stream). These economics compare favourably with analogous operations at Dead Sea Magnesium Ltd. (Israel) and Weifang Bell Chemical (China) [10,13].

Conclusions

A comprehensive, scientifically validated integrated technology has been developed for the zero-waste processing of Karaumbet–Barsakelmes saline lake resources and Kungrad Soda Plant distillery liquid. The principal conclusions are:

(1) Two-stage vacuum evaporation–crystallization of purified Karaumbet–Barsakelmes brine produces bischofite MgCl₂·6H₂O (≥99.5% purity) and co-product NaCl (≥98%) simultaneously. The economic return reaches ~5.53 billion UZS/year for 15,000 t bischofite.

(2) Mirabilite crystallization from Karaumbet DMS solution and dehydration at 200–210 °C yields premium anhydrous Na₂SO₄ (≥99%, GOST 6318-77) at a production cost of 307,000 UZS/t vs. market price 2,000,000 UZS/t — an 85% gross margin.

(3) NaOH precipitation at pH 10.5–11 (40–60 °C, 30–60 min) gives Mg(OH)₂ precipitation yield of 99%. Calcination at 700–800 °C for 120 min produces MgO ≥94.6% compliant with GOST 844-79.

(4) Decalcification of distillery liquid with Na₂SO₄ (100–102% norm, 20–30 °C, 30 min) achieves 96.4% Ca²⁺ removal; subsequent Na₂CO₃ polishing gives 100% Ca²⁺ removal. Products: chemical gypsum (≥98%), precipitated chalk (≥97.9% CaCO₃), and purified NaCl brine recyclable to soda production. Annual net profit ~2.38 billion UZS for 100,000 t DL processed.

(5) All technologies were validated at pilot scale (JSC Ferganaazot) and documented with normative-technical specifications (temporary technological regulations and design basis data), confirming industrial readiness.

(6) The integrated closed-loop process generates no liquid effluent, partially neutralizes the environmental impact of soda production waste, and provides full import substitution for Mg(OH)₂, MgO, and premium Na₂SO₄ in Uzbekistan.

Acknowledgements

The author is grateful to Prof. R.R. Tojiev (scientific supervisor), Prof. Kh.Ch. Mirzakulov, and the staff of JSC Ferganaazot for conducting pilot-plant trials. The research was supported by applied project A-6-274 and innovation project ID-2-003 of Tashkent Institute of Chemical Technology, and economic contract No. 06/10 with the State Committee for Nature Protection, Republic of Uzbekistan. Patent IAP 04470 (Uzbekistan, 2012) covers the sodium sulfate production method.

References:

1. Mordor Intelligence. Magnesium Compounds Market — Growth, Trends, and Forecasts (2023–2028). Hyderabad: Mordor Intelligence, 2023. [Online]. Available: www.mordorintelligence.com/industry-reports/magnesium-compounds-market
2. IEA. Critical Minerals Market Review 2023. Paris: International Energy Agency, 2023. Available: https://www.iea.org/reports/critical-minerals-market-review-2023
3. Chernyshev E.A., Shcherbakov V.I., Kupryunin A.A. Global magnesium production and demand outlook to 2030: a review of current trends. Russian Journal of Non-Ferrous Metals. 2021; 62(4): 375–382. DOI: 10.3103/S1067821221040027
4. Bobokulova O.S. Technology for processing of Karaumbet and Barsakelmes lake raw materials to magnesium hydroxide, magnesium oxide and sodium sulfate: PhD Dissertation. Tashkent: Institute of Inorganic Chemistry UAS, 2018.
5. Mirzakulov Kh.Ch., Tojiev R.R., Bobokulova O.S. Complex processing of saline brines of Karaumbet and Barsakelmes lakes for magnesium chloride. Mining Journal of Uzbekistan. 2018; 2(73): 101–105.
6. Tojiev R.R., Mirzakulov Kh.Ch. Processing of saline lake brines of Karakalpakstan into economically valuable products. Theoretical & Applied Science. 2019; 80(12): 235–243. DOI: 10.15863/TAS.2019.12.80.55
7. Reymov K.D. Technology for producing soil-stabilizing agent from distillery liquid of soda production: PhD Dissertation. Tashkent, 2019.
8. Sun Z., Zheng M., Xu J., Shao Y. Zero-discharge processing of high-magnesium brines: integrated evaporation and selective crystallization. Desalination. 2023; 546: 116196. DOI: 10.1016/j.desal.2022.116196
9. Xiao Y., Liu Y., Feng L., Chen R. Magnesium hydroxide precipitation from concentrated chloride brines: effect of temperature, pH and agitation on particle morphology. Chemical Engineering Journal. 2022; 430: 132743. DOI: 10.1016/j.cej.2021.132743
10. Zhao Y., Zhang H., Wang P. High-purity magnesium oxide production via thermal decomposition of magnesium hydroxide: kinetics and process optimization. Powder Technology. 2022; 397: 117068. DOI: 10.1016/j.powtec.2021.117068
11. JSC O'zkimyosanoat. Reference letter No. 14-2353, 22 May 2020 — confirming pilot-plant trial results at JSC Ferganaazot. Tashkent, 2020.
12. Ding T., Li M., Wang Q., Zhou H. Valorization of Solvay process waste liquor: calcium removal and sodium chloride recovery for recycling. Journal of Cleaner Production. 2023; 385: 135698. DOI: 10.1016/j.jclepro.2022.135698
13. Tojiev R.R., Mirzakulov Kh.Ch. Physico-chemical studies of bischofite production from Karaumbet and Barsakelmes lake brines. Eurasian Scientists Union. 2020; 6(75): 102–109.
14. Tojiev R.R., Mirzakulov Kh.Ch. Obtaining magnesium hydroxide, sodium sulfate and sodium chloride from Karaumbet dry mixed salts. Test Engineering & Management. 2020; 83(Mar–Apr): 7101–7108.
15. GOST 6318-77. Technical sodium sulfate. Specifications. Moscow: Standards Publishing House, 1989.