Management of Solid Waste Marble Powder: Improving Quality of Sodium Chloride Obtained From Sulphate Rich Lake/Subsoil Brines With Simultaneous Recovery of High Purity Gypsum and Light Basic Magnesium Carbonate

Marble industry worldwide produces large amount of non-degradable marble dust powder (MDP) waste during mining and processing stages. MDP mainly comprises of CaCO 3 with small amounts of Mg, Fe or Si in various forms. In India, mainly in Rajasthan state, marble is quarried in huge amounts and MDP thus produced is collected improperly and dumped at any abandoned land or identied disposal sites leading to several environment hazards. On the other hand, the composition of sub soil/lake brines of Rajasthan is typical in nature as it does not have much Ca 2+ and Mg 2+ impurities but contains higher levels of SO 42- . Therefore, the common salt (NaCl) produced from such brines is contaminated with Na 2 SO 4 (8-30 wt%) depending upon SO 42- concentration in the brine. Such a salt produced is neither suitable for edible purpose nor for industrial usage. Herein, we have reacted MDP with HCl, and the resulting solution (CaCl 2 and MgCl 2 slurry) is used in stoichiometric ratio of Ca 2+ to SO 42- in brines to produce high purity NaCl and gypsum (CaSO 4 ·2H 2 O) via fractional crystallization. Remaining magnesium containing solution was reacted with Na 2 CO 3 to prepare high purity light basic magnesium carbonate hydrate. Purity of crystallized NaCl, CaSO 4 ·2H 2 O and MgCO 3 ·6H 2 O has been ascertained through analytical and spectral methods (TGA, FTIR, P-XRD). Field emission scanning electron microscopy (FE-SEM) was used to elucidate morphology of crystals. The method reported for improving purity of NaCl along with CaSO 4 ·2H 2 O and MgCO 3 ·6H 2 O production from sulphate rich brines is simple and economic, and allow management of MDP generated in huge amounts, which poses problems of disposal and creates environment hazards.


Introduction
Marble is a metamorphic rock composed of recrystallized carbonate minerals, most commonly calcite or dolomite (Philip, 2001). Metamorphism causes variable recrystallization of the original carbonate mineral grains. The purest calcite (CaCO 3 ) marble is white in color. Reddish, yellowish or greenish colour in marbles is mainly due to hematite (Fe 2  MDP generated while processing such rocks when reacted with a mineral acid can produce water-soluble inorganic calcium or magnesium based salts solutions. Such salt solutions when mixed with sulphate rich brine are suitable for precipitation of gypsum prior to obtaining high purity solar salt and preparation of magnesium chemicals after the recovery of common salt. Rajasthan is third largest salt producing state in India, after Gujarat and Tamil Nadu, by contributing 10-12 % of total Indian salt production. Rajasthan has no seacoast, and therefore, solar salt production is exclusively dependent on sub-soil/lake brine in the area. Composition of brine of this region is typical and different with sea brine as it does not have much calcium and magnesium impurities but contains high level of sulphate impurities. The major impurities being sodium sulphate (in the form of Glauber salt, i.e. Na 2 SO 4 .10 H 2 O) which seems easy to remove by simple washing with fresh water to achieve good quality of salt. However, Rajasthan having hot climate, this Glauber salt turns to anhydrous Na 2 SO 4 , which has a less solubility and is very di cult remove from salt just by washing. Thus higher percentage of sodium sulphate leads to low purity salt and therefore di cult to cop up with edible or industrial grade salt speci cations. Moreover, during solar salt production, the manufacturers recycle the bittern (left over mother liquor after the separation of salt) back to the weak brine and go on feeding the same bittern every time, which on the contrary increases the concentration of Na 2 SO 4 in the brine. After certain concentration, Na 2 SO 4 not only starts oating on the surface of the brine and reduces the rate of evaporation of the brine, but also results in deteriorated quality of salt.
Many processes have been reported for the production of sodium chloride with lesser impurities. These methods include both chemical and biological interventions in brine systems (Mishra et al., 2004). In order to improve the quality of salt, common ion effects have been utilized to crystallize gypsum from brines forcibly by addition of calcium chloride or sodium sulphate (Hamidani and Sanghavi, 1992;Vohra et al., 2004). Such practices were not found practical in salt elds as large quantities of calcium chloride or sodium sulphate are to be added and thus making process tedious and economically unviable. In order to make salt manufacturing process economically viable, cost effective sources of calcium chloride such as soda ash distiller waste which has signi cant amounts of CaCl 2 and NaCl have been used for desulphatation of subsoil brine (Vohra et al., 2004). Such a practice not only improved the quality and yield of salt but also helped in mitigation of the problems of environmental discharge of e uent in water bodies. However, such a process is more practical when soda ash plants are available near solar salt works. Therefore, we have utilized MDP, which is in abundance in the vicinity of Rajasthan lake brines for improving the quality of salt along with the recovery of high purity gypsum. The remaining solution after recovery of salt and gypsum contain high magnesium content, therefore, magnesium recovery makes the process more signi cant. Magnesium in the form of its carbonate salt is widely needed in industrial process of paints, ceramics, cosmetics, pharmaceuticals, preparation of paper and other magnesium based chemicals. Magnesium carbonate can be precipitated in several forms which include hydromagnesite, nesquehonite or magnesite etc. (Lou et al., 2004). Magnesium carbonate recovery has been reported using Uyuni Salar brine (Tran et al., 2016), using polyacrylamide (Guo et al., 2010), from reaction between magnesium hydroxide and carbon dioxide (Han et al., 2014), from magnesium chloride solution (Hao and Al-Tabbaa, 2014), or by reacting sea bitterns with sodium chloride (Apriania et al., 2018). Herein, we have recovered magnesium carbonate by reacting magnesium rich liquor with sodium carbonate, thus making the process very economic. Common salt, gypsum and magnesium carbonate have been characterized using different techniques in terms of purity and yield.

Materials And Methods
Marble dust powder (MDP) was collected from a marble processing industry located in Rajsamand, Rajasthan. Commercial Grade Hydrochloric acid (11 N) was procured from DCM Shriram Ltd. (Shriram Alkali & Chemicals; CAS number: 7647-01-0), Kota, Rajasthan which is near Didwana Lake. Sulphate rich brine of Didwana lake was used as source of NaCl and Na 2 SO 4 . Lake brine, MDP, slurry generated after reaction of MDP with HCl, and crystallized salts were analysed using standard volumetric, gravimetric or ame photometry techniques (Trivedi et al., 2014), Mg 2+ and Cl − concentrations were determined volumetrically using standard EDTA and AgNO 3 solutions and SO 4 2− was determined gravimetrically using BaCl 2 , with an error of <0.2%. Na + and K + ions were analysed with the help of digital ame analyser (Cole-Parmer, Model 2655-00). Various salts crystallized were characterized using PerkinElmer GX FTIR spectrometer (10 scans with a resolution of 4 cm −1 were made). TGA measurements were performed using a TGA/SDTA851 Mettler Toledo under a nitrogen atmosphere from 0 to 800°C with a heating rate of 10°C min −1 . XRD pattern was recorded using Philips X'pert MPD XRD diffractometer and morphology of precipitated crystals was investigated using FE-SEM (JFM 7100 F; Oxford Inc.) the adopting procedure reported in earlier paper (Shukla et al., 2018).

Results And Discussions
The sub-soil brine was analysed in order to establish its density and ionic composition. The chemical analysis is provided in Table 1 and Table 2.  whereas sea brine analysis shows 13.5 g sulphate at a similar density, with a probable concentration of nearly 5 (w/v) % of Na 2 SO 4 along with 14 (w/v) % of NaCl (Table 2). Lake brine has also very low content of Mg 2+ and Ca 2+ as compared to the seawater. The common salt has been crystalized from this brine via solar evaporation method. The composition of salt crystallized is given in Table 3. As can be seen from Table 3, the salt is highly contaminated with Na 2 SO 4 (23.46 % w/w) which is very di cult to wash, and not suitable either for edible or industrial application. The dolomite rocks of have CaO and MgO in the range of 26-44% and 16-22% respectively. Therefore, MDP generated while processing such rocks was reacted with HCl to generate water-soluble inorganic calcium or magnesium based salts solutions and insolubles. In a typical experiment, 321 g of marble powder was dissolved 535 ml HCl (11 N) and kept overnight for preparation of slurry/solution. Insolubles residue (59 g) was ltered and salt slurry (542 ml) thus obtained was analysed for chemical composition (Table 4, Table 5). Therefore, the ltered slurry (225 ml) was used as economic additives in lake brine (1500 ml brine) to make a stoichiometric proportion of Ca 2+ to SO 4 2− . Composition of the resulting brine is given in Table 6.  As can be seen from Table 7, CaSO 4 ·2H 2 O with a purity of higher than 85 (%w/w) could be directly   The purity of as such collected NaCl was always > 96 %w/w meeting speci cations of edible grade salt, which could be further improved to > 98.5 %w/w by simple washing with water and meeting industrial grade salt speci cations. Purity of washed salt has been ascertained from P-XRD pattern and FTIR spectra of unwashed and washed samples (Fig. 3 left, right).
Morphology of crystallized NaCl indicated growth of crystals from hollow to dense cubic structures ( Figure 4). EDX spectra shows a very high purity of the crystallized salt. After separation of NaCl at 29 o Bé ltrate was recovered and analysed for Mg 2+ content (Table 10). and then allowed to stand. Slurry was put under ltration and washed with fresh water to make it free from chloride and sulphate. Wet cake was then dispersed in fresh water such that the slurry concentration is reduced. This dispersion was heated to 50-60 o C and maintained at this temperature for 30 minutes to transform into dried basic magnesium carbonate. The complete integrated scheme of crystallization of gypsum, sodium chloride and magnesium carbonate is shown in Figure 5.
During investigations it has been found that a rare form of magnesium carbonate hexahydrate (MgCO 3 ·6H 2 O) has been crystallized (Rincke et al., 2020). MgCO 3 ·6H 2 O formed was analysed for chemical composition (Table 11), and characterized for its bulk density, moisture content, purity and morphology. The product was initially characterized using TGA in order to assess water of hydration ( Figure 6). TGA  Morphology of crystals depends upon several conditions viz. composition of reaction mixture, reaction temperature, carbonation time, pH of the solution etc. Figure 9 provides a set of typical SEM images of a dried sample under the investigated conditions. Here, ne nano-sized thick plates like particles are produced which arrange into small rod like structures. Elemental analysis and EDX spectra shows a very high purity of the crystallized magnesium carbonate (Figure 9).

Techno economic analysis
Based on the common salt, magnesium carbonate and gypsum production production rates and chemical input requirements, techno economic feasibility of production of 1 ton/day capacity plant is evaluated (Table 12). The economic viability of salts produced has been assessed based on the following economic indicators: Plant establishment costs: capital expenditure (CAPEX); Operating expenditure (OPEX); Cost recovery from struvite production (revenue). Equipment sizes based on the mass production and cost estimation of the process equipment has been estimated as provided in Table 13. The viability calculations have been done based on pilot scale experiments. Raw materials cost as estimated according to actual consumption from pilot experimental results and prevailing costs, other components of operating costs are presented in Table 14. The rates of raw material and utilities mentioned below are prevailing rates in market. Standard norms have been taken for depreciation, maintenance and repair cost which are applicable for chemical plant. Manpower requirements with their monthly and annual remunerations for running of plant are given below in Table 15. Total 690000 Fringe Bene t @33% 227700 Total 917700 Return on capital investment costing has been done with the assumptions that the economic life of equipment is ten years, two-shift per day basis and no bank borrowings for capital expenditures. The calculations are provided in Table 16. Break even analysis has been done with 100% capacity utilization which is normally achieved during third year of operation. It has been assumed that capacity utilization during rst and second year will be 75% and 80% respectively. The calculations are given in Table 17. Payback period was calculated based on six years of operation using net surplus and cumulative net surplus amounts and is provided in Table 18. Techno-economic analysis indicates that production of common salt, magnesium carbonate and gypsum will generate enough revenue to recover the cost of production and make pro t with a payback period of approximately 4 years. Recovery of salts in current process is thus concluded to be technically feasible and the economically affordable.

Conclusions
We have shown that large amount of non-degradable marble dust powder (MDP) generated from marble industry during mining and processing stages can be used for improving quality and yield of salts produced from sulphate rich lake/sub-soil brines. MDP has been reacted with HCl to generate slurry comprising of Ca 2+ and Mg 2+ ions, which was added directly to lake/sub-soil brines in stoichiometric ratios of Ca 2+ to SO 4 2− in brine. Subsequent brine was subjected to solar evaporation for crystallization of high purity gypsum and common salt. The remaining Mg 2+ rich liquor was processed for preparation    SEM images and EDX spectra of NaCl crystallized from lake brine after addition MDP slurry.

Figure 5
Schematic of recovery of high purity NaCl, CaSO4·2H2O and MgCO3·6H2O from sulphate rich lake brines using marble dust powder.    SEM images, elemental mapping and EDX spectra of MgCO3·6H2O prepared from magnesium rich ltrate after recovery of common salt.