Brine sludge waste from a Chlor-alkali industry: characterization and its application for non-structural and structural construction materials

Introduction

The Chlor-alkali industry is essential to the chemical industry because it is responsible for the production of chlorine, which has many applications such as chemicals, pharmaceuticals, ultra-pure metals, polymers, and others [1,2], besides the caustic soda production, which can be applied in the manufacture of almost all chemical products.

Regardless of the type of technology adopted for chlorine and caustic soda production (membrane cells, asbestos diaphragm, or mercury cell), salt is used as raw material and needs treatment before electrolysis. During the salt treatment, the Chlor-alkali industry generates large amounts of brine sludge (BS). That industrial waste is generated in large amounts in Brazil and usually disposed into industrial landfills. Although brine sludge generated from the different Chlor-alkali industries from different countries presents potential recycling and reuse [3-6], there are no many studies that address the characterization of this type of waste generated in Brazil.

Therefore, the present work aims to characterize, evaluate, and correlate the physical and chemical characteristics of two brine sludge samples from the Unipar Carbocloro S. A. chemical industry (Brazil) to propose possibilities of application.

Materials and Methods

Materials

The brine sludge wastes were collected at Unipar Carbocloro S.A. units located in Cubatão (São Paulo/Brazil) and Santo André (São Paulo/Brazil). The samples were first dried at 105 ± 5℃ for 24 hours. After cooling to room temperature, the brine sludge samples were grounded.

Characterization of brine sludge samples

The brine sludge samples collected at Cubatão City (BSCB) and Santo André City (BSSA) were characterized by different techniques. The chemical compositions in the form of oxides were analyzed by energy dispersive XRFX-ray fluorescence spectrometry using Malvern Analytical, Zetim model. X-ray diffraction (XRD) analyzes were performed on an X'Pert, Philips diffractometer, operating with Cu-Kα radiation (λ = 1,542 Å), scanning from 5 to 50° at a rate of 1°/min with a voltage of 40 kV and current of 50 mA. Particle size distribution of samples was determined by DPSA using Malvern Instruments, Mastersizer 2000, Ver. 5.54, with the sample dispersed in isopropyl alcohol and analyzed by He-Ne laser. Functional groups within the material structure were identified by FTIR (Perkin Elmer Frontier) using KBr as a medium. IR spectra were scanned in the range of 4,000 cm^-1 to 400 cm^-1 with a resolution of 4 cm^-1. Thermogravimetric analysis was conducted using a Q500 thermogravimetric analyzer (TA Instruments). Thus, samples were heated from 50 to 1000 °C at a rate of 10 °C/min under air at a flow rate of 60 mL/min. Samples had an average mass of 9 mg. The pH was measured as follows: 0.25 g of each sample was placed in 25 mL of deionized water and the mixture was stirred during 24 h in a shaker at 120 rpm (Ética — Mod 430). After filtration, the pH of the solutions was measured with a pH meter (MSTecnopon — Mod MPA 210). The bulk density was determined from the weight of the sample divided by the volume of 35 mL of the sample in a beaker. All BS samples characterization was performed at the Laboratório de Caracterização Tecnológica - Escola Politécnica da Universidade de São Paulo (LCT-POLI/USP).

Results and Discussion

Mineralogical composition

Identification of the phases present in the brine sludge samples was carried out by comparing the experimental interplanar spacing (d values) with those of the respective likely substances listed in the PDF2 of ICDD (International Centre for Diffraction Data) and ICSD (Inorganic Crystal Structure Database). The X-ray diffraction spectra (XRD) of BS samples from Santo André and Cubatão are shown in Figure 1 and Figure 2, respectively. There is a great similarity between the diffractograms of the samples, despite being of different origins. The XRD patterns are composed of sharp multiple peaks indicating that the samples have a predominance of crystalline structure.

Figure 1.Figure 1. XRD pattern of brine sludge from Santo André (BSSA)

Figure 2.Figure 2. XRD pattern of brine sludge from Cubatão (BSCB).

The observed d values confirm the presence of multi-elemental, diverse mineralogical phases as shown in Table 1 and Table 2 for BSSA and BSCB, respectively.

*Possible presence

*Possible presence

Chemical composition and physical-chemical properties

The chemical composition of the brine sludge wastes is given in Table 3. The majority composition is calcium. The other most abundant elements are Si, Mg, Na, Cl, and Fe. These elements are the main constituents of minerals present in the wastes, such as calcite, halite, quartz, and brucite (Figure 1 and Figure 2). In addition, the BSCB also presents a relatively large presence of Al.

(*) Loss on Ignition

The high loss on ignition presented in the chemical composition of 41% and 36% for BSSA and BSCB, respectively can be associated with volatile constituents and the presence of carbonate materials in the residues.

The results of physical and chemical analysis of sludge samples are given in Table 4. The bulk density of a powder is dependent on its particle size distribution. A coarse homogenous powder has a lower bulk density than a fine homogenous powder, therefore, the value of the sample BSCB is less than that of the sample BSSA. The pH value indicates that samples were moderately alkaline in nature due to the presence of oxides, which hydrolyze in the presence of water.

Particle size distribution

The differential and cumulative particle size distributions are shown in Figure 3 and Figure 4 for BSSA and BSCB, respectively. The differential size distribution of BSSA is relatively uniform with a bell-shaped asymmetric distribution curve while that of the BSCB sample tends to show a bimodal particle size distribution.

Figure 3.Figure 3. Particle size distribution of BSSA

Figure 4.Figure 4. Particle size distribution of BSCB.

The particle size distributions of materials are given in Table 4. These distributions specify that the majority of particles (90%) lie below ~18 µm for BSSA and ~130 µm for BSCB. Therefore, most of the particles have a size that corresponds to the silt fraction in BSSA and the sand fraction in BSCB. A previous study showed that the particle size distributions of a brine sludge sample presented a range from 0.075 mm to 4.75 mm for up 60 % of particles [6].

FTIR studies

The relevant vibration bands of the brine sludge samples in FTIR spectra were determined in the range of 400–4000 cm^-1 and are shown in Figure 5 (a) and (b) for BSSA and BSCB, respectively.

Table 6 summarizes the wavenumbers at which the absorption maxima appear in the spectra of the materials and their vibrational assignments.

Figure 5.Figure 5. FTIR spectrum of (a) BSSA and (b) BSCB

Calcium carbonate is a major component of brine sludge samples and the FTIR spectrum is very useful for distinguishing among different phases of this compound. The crystal phases of calcium carbonate can be discriminated by detecting the absorption bands.

The vibration frequencies observed were characteristic of two crystalline calcium carbonate phases (calcite and aragonite). Typically, the absorption bands were divided into four parts: the symmetric stretch of the carbonate ion at about 1080 cm^-1 (ν1); the out-of-plane bending absorption at about 870 cm^-1 (ν2); the asymmetric stretch at about 1400 cm^-1 (ν3) and the in-plane bending at about 700 cm^-1 (ν4) [7]. Each phase of calcium carbonate has some characteristic absorption bands. The bands at ~1480 cm^-1, 1083 cm^-1, 858 cm^-1 are characteristics of aragonite [7,8].

The presence of H₂O is detected by two characteristic absorption bands in the 3670-3440 cm^-1 region corresponding to the stretching and deformation modes of OH^- and in the 1640 cm^-1 region associated with bending vibration mode of H₂O [9].

A broad absorption band at ~3670 cm^-1 indicate also the presence of the characteristic frequency of the hydroxyl group (OH^-) stretching vibration of magnesium hydroxide [10,11].

The minor peak at 2900 cm^-1 is associated with a saturated C-H vibration from some impurity in the KBr crystals and those minor peaks observed at ~ 2500–2300 cm^-1 were due to the adsorption of the atmospheric CO₂ [12-14].

The presence of quartz was identified by the characteristic absorption bands at 448, 780, 1010-1040, and 1640 cm^-1. Among these bands, the absorption at ~ 800 cm^-1, which is due to Si-O-Si symmetrical stretching vibration is the most suitable band for silica determination [15,16]. The absorption bands at 585-575 cm^-1 indicate the formation of the Si-O-Al bond [17].

Thermal analysis

The thermogravimetric analysis is important to check the thermal stability of the material and it is useful for the strategic design of sludge management when thermal treatment is involved. Figure 6 and Figure 7 show TGA and derivative TGA (DTG) curves of BSSA and BSCB samples, respectively. The peak of the DTG curve represents the temperature corresponding to the highest mass loss rate for the samples. Analyzing the thermogravimetric curves, it is possible to observe a thermal behavior similar between the two samples. The first peaks observed at ~ 306-338 ºC corresponds to the decomposition of magnesium compounds [18]. For sample BSCB (Fig. 8), the DTG curve in this temperature range shows two peaks, which are related to two reactions that are partially overlapping. Around 650-860 ºC is observed the signal due to the CO₂ loss corresponding to the decarbonization of the calcite [19]. BSSA sample presented higher content of CaCO₃ in agreement with the data reported by the chemical analysis. The peak near 430ºC for the BSCB sample may be attributed to the quartz dehydroxylation due to the condensation of silanol groups resulting in the loss of water [20]. High residual mass was found for both samples at the end of the analysis (~60%) probably due to high inorganic contents, mainly salts.

Figure 6.Figure 6. TG and DTG curves of BSSA

Figure 7.Figure 7. TG and DTG curves of BSCB

Evaluation of brine sludge waste applications

Many advantages recycling the wastes can bring, such as avoiding pollution caused by incorrect disposal, reducing the number of raw materials used to produce products, and reducing costs. Therefore, applications for brine sludges will be discussed.

Utilization of Brine Sludge samples in Nonstructural Building Components

Portland cement is the most used binder in the manufacture of concrete. Greenhouse gas emissions owing to Portland cement production correspond to 5–8% of anthropogenic emissions. To limit the impact of climate change, it is mandatory to reduce carbon emissions and to lower the CO₂ concentration in the atmosphere. The use of supplementary cementitious materials in concrete reduces CO₂ emissions and saves non-renewable resources [21-23].

The results indicated that brine sludge (both samples) has the potential to be used as a limestone filler alternative due to its fine granulometry and its composition. The limestone filler is a product of the crushing of the limestone rock, which can be calcitic or dolomitic. It is an extremely fine powder, which is widely used as a filling material in cement, concrete, and mortar, increasing compactness through the best particle size distribution [24].

The two samples can be used in partial replacement of Portland cement and the nonstructural construction materials, such as paver blocks, mortars, and bricks [25]. The effect of binders with different particle size distributions on cement-based materials was reported by Mehdipour and Khayat [26]. In addition, the sludge samples have also the potential to be used as a chemically reactive binder. The Ca²⁺ ions can react with silica and alumina present in cement forming calcium silicates and aluminates which bind the entire mass together producing a solidified matrix [23,4]. Further, magnesium oxide content present in brine sludges can cause a decrease in the shrinkage strain of cementitious materials improving their structural and durability [27]. Another advantage in preparing construction materials with the incorporation of these residues is that the toxic metal ions present in them will be immobilized in the prepared composite.

Utilization of Brine Sludge samples in Geopolymerized Based Composites

A geopolymeric composite can be developed by incorporating industrial waste. The utilization of industrial waste to develop geopolymer concrete has a significant positive impact on the environment based on sustainability indicators [28-31].

The incorporation of brine sludge samples containing calcium carbonate, magnesium hydroxide, sodium chloride, and silica can cause improvements in the interfacial bonding of the geopolymeric matrix due to the additional formation of C-S-H gel or A-S-H gel together with N-S-H or N-A-S-H / C-A-S-H gel [3, 5]. The simultaneous presence of this variety of gels modifies the physical and mechanical properties of geopolymer materials. There should be improvements in resistance to compression and flexion due to the formation of dense microstructure [32-34].

The high calcium content can contribute to the greater compaction of geopolymer structures. Chloride, on the other hand, works as a catalyst improving the geopolymerization reaction and leading to the formation of geopolymeric gel [3,5]. In addition, geopolymer materials also have the potential to immobilize toxic ions present in wastes.

Conclusions

The two brine sludge samples presented Ca, Si, Na, Mg, Al, Cl, and Fe in their chemical compositions. The majority composition was calcium for both samples. In addition, the BSCB also presents a relatively large presence of Al. The high loss on ignition presented in the chemical composition of 41% and 36% for BSSA and BSCB, respectively can be associated with volatile constituents and the presence of carbonate materials in the residues.

The XRD results confirmed the crystalline nature of compounds and indicated that the main compounds in were calcium carbonate, sodium chloride, magnesium hydroxide, and quartz. The pH value indicates that samples were moderately alkaline in nature due to the presence of oxides, which hydrolyze in the presence of water.

The differential size distribution of BSSA is relatively uniform with while that of the BSCB sample tends to show a bimodal particle size distribution. The particle size distributions of materials specify that the majority of particles (90%) lie below ~18 µm for BSSA and ~130 µm for BSCB. Therefore, most of the particles have a size that corresponds to the silt fraction in BSSA and the sand fraction in BSCB. The two brine sludge samples can be considered as a fine powder with the mean diameter (d₅₀) of 4.984 µm and 24.574 µm, for the BS from Santo André and Cubatão, respectively.

FTIR showed the presence of varying functional groups like carbonate, siloxane, and hydroxide for both samples. BSSA and BSCB also presented a very similar thermal behavior, with the decomposition of magnesium compounds and CO₂ loss due to decarbonization of calcite.

The results indicate that the brine sludge samples present favorable characteristics for to be used as limestone filler and binder alternative to Portland cement in the nonstructural construction materials. The incorporation of brine sludge in geopolymeric materials is another possible use in sustainable construction material products. The production of value-added products from brine sludge will be an important contribution towards sustainable development adopted by the Chlor-alkali industry.

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