Research on the application of microwave synthesis to obtain aluminum-containing coagulant

The extensive way of industrial development and the rate of population growth determine the increase in the amount of drinking water consumed and the requirements for water treatment. Known and applied technologies for the industrial production of aluminum oxychloride coagulant used in reagent water puriﬁ cation are based on high-temperature processes that require complex hardware design. The paper presents the results of studies on the production of basic aluminum chlorides by the microwave method. The experiments were carried out on a designed laboratory setup based on a microwave oven; data were obtained on the yield of aluminum polyoxychloride depending on the change in process parameters, duration, coeﬃ cient of eﬃ ciency, and 36% concentration of hydrochloric acid solution. Studies of the use of microwave technologies in inorganic synthesis have shown the eﬀ ectiveness of such technologies, a decrease in the reaction time, an increase in the eﬃ ciency of the installation, and the absence of contact between the heating elements and the reaction mixture. The inﬂ uence of sulfuric acid as a process activator substance, which positively aﬀ ects the productivity of aluminum polyoxychlorides, has been established. The results of physicochemical studies of the synthesized samples and their coagulation properties are presented, which indicates the possibility of using microwave technologies in the inorganic synthesis of aluminum polyoxychloride coagulants.


Introduction
Now, a lot of work is done in the Republic of Kazakhstan to provide the population with drinking water.As the population grows, and industrial production develops, the level of water consumption increases, and new water intake points, both surface and underground ones, are commissioned [1,2].Water taken from surface sources usually has a certain turbidity, especially signifi cant during fl ood periods, and the practiced method of purifying natural waters from suspended substances and impurities of varying dispersity is the use of coagulants.Eff ective coagulants are salts of polyvalent metals, the most used are ferric chloride and aluminum-containing coagulants, including aluminum sulfate, aluminum oxychloride, sodium aluminate, and aluminum chloride.It is important how pH, temperature and other characteristics change.For every mg per ml of ferric chloride, the pH dropped by two units.Also, with an increase in each mg per liter of polyaluminum chloride and iron sulfate for coagulants by approximately 0.04 and 0.06 °C, the temperature will decrease, respectively, and for aluminum sulfate and ferric chloride for coagulants by 0.015 and 0.03.The pH value increased with increasing dosages of polyacrylamide coagulants.In addition, TDS, EC and salt values did not change after the addition of a coagulant [3][4][5][6][7][8].The modern coagulant aluminum polyoxychloride − Al(OH) m Cl 3n-m , has several advantages, does not corrode equipment, operates in a larger pH range, requires less coagulant dosage, eff ective for water treatment at low temperatures.The main issues to be solved in the coagulant production technology development are to reduce the product cost and to improve its quality, to simplify and speed up the process.Most methods intended to obtain aluminum polyoxychloride are based on the use as a source raw material − metal aluminum, aluminum oxides or hydroxides.At the same time industry https://doi.org/10.18321/cpc21(3)127-138 has a signifi cant amount of aluminum-containing waste that can serve as a source raw material for aluminum polyoxychloride production.It is possible to simplify the technology intended to obtain aluminum polyoxychloride, to reduce its cost with the use of waste but there are problems of stability of technological and qualitative characteristics of raw materials and the resulting product.Coagulant − Aqua-Aurat 30 which is in demand in the market and has stable characteristics, is produced by the interaction of metallic aluminum with hydrochloric acid.The reaction takes place at high temperatures and high pressure which requires appropriate equipment.A patented aluminum oxychloride production method with the use of the interaction of freshly precipitated aluminum hydroxide and hydrochloric acid of various concentrations is also known.Diffi culties of this method are in the changing aluminum hydroxychloride reactivity, the so-called aging and changing the of Al(OH) 3 aluminum hydroxide structure.Technical aluminum hydroxide is dried in a rotary kiln at 110-130 °C, purifi ed in a pneumatic separator, and then undergoes thermal dehydration.The activated hydroxide is precipitated in a cyclone and then fed to a set of enameled reactors.Multistage technology implies quality compliance issues with the established parameters for each stage complicating the technology.
The same way it is proposed to obtain aluminum oxychlorides by the interaction of aluminum sulfate or chloride with a lack of alkali in the solution.Sodium or potassium aluminate solutions are used as raw materials to increase the stability of the products.Diffi culties in obtaining the product by this technology are in the low interaction rate of products and concentration heterogeneities of hydroxide ions.Solid basic aluminum chloride is obtained by periodic heating and cooling of AlCl 3 water solution, boiling aluminum chloride solutions for 2−4 h in a refl ux condenser and then spray drying.This method produces basic aluminum chloride soluble in alcohol.The same basic aluminum chlorides are obtained by thermohydrolysis of concentrated aluminum chloride solutions (20−50%) in a spray dryer within the temperature range of 200−500 °C [9][10][11][12][13].
One of the developed directions in obtaining coagulants from aluminum polyoxychlorides is the processing of nepheline, bauxite and secondary aluminum smelting waste − salt slags.The technology is complex and power-consuming, the reaction takes place at temperatures up to 110−160 °C, the raw material is used when it is crushed to a fi neness of less than 120 microns.The process is performed at boiling point in heated tube reactors [14].
The proposed technology with the use of microwave inorganic synthesis has the advantage, as it is a one-stage technology for a high effi ciency plant.The process time is signifi cantly reduced, from 4−7 h to 15 min of the reaction, the aggressive medium is separated from the heating elements which implies a simple equipment design.

Methods and materials
The paper presents research on the development of a fundamentally new method for obtaining a reagent for water purifi cation, aluminum polyoxychloride coagulant by microwave method with the use of technical aluminum hydroxide and hydrochloric acid.
Aluminum polyoxychloride was synthesed in a unit designed with a household microwave oven with a power range up to 1000 W and a volume of 20 liters and a reverse stirrer, thermometer and round bottom fl ask on a special plastic stand, refrigerator to condense the vapor and to return the condensate to the reaction mass, installed vertically and connected to cold water (Fig. 1).
Aluminum hydroxide Al(OH) 3 and hydrochloric acid HCl with the concentration of were placed in a round bottom fl ask, the experiments were performed under constant stirring.This article presents data from experiments conducted with a concentration of hydrochloric acid of 36%.
The experiments were carried out: 1 series of experiments − at high heating intensity (1000 W), duration 2.5 minutes and with the addition of one percent of the activator substance, sulfuric acid of 93,6%; 2 series of experiments, with a reduced heating intensity (600 W), an even shorter heating period of 1 min, also with the addition of one percent sulfuric acid as an activator; Series 3 of experiments was carried out with a heating duration of fi fteen minutes, at 800 W, without adding an activator.Besides, when samples of coagulants No. 1 and No. 2 were synthesized, concentrated sulfuric acid was added in an amount of one mass percent, as an activator substance.The synthesis results are presented in Table 1.
The elemental phase composition of the obtained synthesized samples was studied by physical and chemical research methods.
Chemical and physical-chemical analysis methods were used for study and analysis.Thermal analysis of samples was performed with the use of STA 449 F3 Jupiter, a synchronous thermal analysis device.
Heating was performed at a rate of 100 °C/min.in an atmosphere of highly purifi ed argon.Results obtained with the STA 449 F3 Jupiter were processed using the NETZSCH Proteus software.The microstructure, quantitative and qualitative composition was studied with a ISM6610TV Scanning Electron Microscope D8 Advance X-ray Diff ractometer (BRUKER) was used to study the phase composition of the samples, Cu−Kα radiation, total morphological characteristics of the synthesized samples were taken by spectroscopic method, spectra were obtained using Avatar 370 CsI FTIR Spectrometer within spectral range of 4000−250 cm -1 from products in the form of Vaseline oil suspension in KRS-5 windows.The vaseline oil spectrum was taken as a comparison spectrum.Experimental attachment: Transmission E.S.P. elemental analysis was performed by X-ray fl uorescence method on the Axios Panalytical WDXRF.
Coagulation properties of synthesized samples were studied in comparison with coagulant -aluminum polyoxychloride Aquа Aurat 30, experiments were conducted with the use of natural waters taken from the Almaty water intake; the water turbidity was determined by GOST R 57164-2016 [15].
Solutions of samples of synthesized coagulant and Aqua Aurat 30 coagulant were prepared and studied for density and pH value in correlation with the data specifi ed in the «Recommendations for the use of Aqua-Aurat 30».
Coagulant dosage for waters of diff erent composition was set by trial coagulation of treated water under GOST 2919-45 [16,17].

Results and discussion
The following reagents − technical aluminum hydroxide Al(OH) 3 and hydrochloric acid HCl were chosen to study the microwave technology possibilities in the synthesis of aluminum polyoxychlorides.When they are heated the reaction (1) proceeded with obtaining of aluminum polyoxychloride Al 2 (OH) 5 Cl and water.There were certain diffi culties in the reaction, as a long heating process for the reaction mixture, up to boiling, and for several hours was required for the effi ciency of the process according to the literature [18].2Al(OH) 3 + HCl → Al 2 (OH) 5 Cl + H 2 O (1) Microwave radiation can interact with substances in gaseous, liquid, or solid condition.Depending on the composition and morphology of the substance, the eff ects of radiation will vary, as shown in Fig. 1.Noticeable absorption of radiation observed during irradiation of many liquids and liquid solutions.Particularly strong absorption observed in the case of water and aqueous solutions.Microwave radiation technology has been successfully used in analytical chemistry and organic synthesis, direct absorption of electromagnetic energy by materials provides fast and uniform heating, the reaction and analysis time is signifi cantly reduced, and the error of the result is reduced.
There is known experience in the use of microwave technologies in sample preparation, for acid decomposition, to transfer the sample into solution, both in closed microwave preparation systems and in open vessels.The results of studies on the synthesis of a substance that is used as a coagulant are shown in Table 1.Changing the reaction time and radiation power aff ect the result of the synthesis.
In this study of the synthesis of a new coagulant, microwave technology is used by analogy, and considering experience in sample preparation in analytical chemistry.The change in the duration of the processes of sample dissolution in acids, evaporation, and ashing, from 2−11 h without the use of microwave units, to several minutes is indicative.Reactions with the use of conventional energy sources are mostly standardized, while microwave synthesis reactions depend on the reaction conditions and the substances used.
Processes with temperatures of over 1000 °C and above, with a high heating rate of over 100 deg/ min can be used in inorganic synthesis to avoid uncontrolled changes in the composition of the initial reacting substances -charge and to obtain inorganic substances by uniform sintering throughout the initial charge volume.Oxides, sulfi des, carbides, and some oxygen-containing salts capable to absorb microwave (MV) radiation intensively are used in these processes.
Both mixtures consisting of components capable to absorb microwave radiation and to be heated under its action, and mixtures where only one or some of the reagents absorbs microwave radiation are used during these syntheses.In some cases, chemically inert materials with diff erent microwave absorption capacities are introduced into the charge to regulate the heating temperature.In the presented study, intense heating occurs due to the water contained in the mixture, due to an increase in the kinetic energy of dipole water molecules when exposed to microwave radiation.Intensive heating occurs due to the water contained in the charge in the present research.
Absorption of microwave energy by the medium depends on the dielectric properties of the heated substance, that is, how strongly the medium prevents the passage of microwaves through it.Nonpolar substances slightly interact with penetrating microwaves, and therefore are not heated, unlike polar substances, as shown in Fig. 2 [23].
By changing the synthesis conditions, products were obtained − substances that have some diff erences in structure, color, and solubility, but a similar basic composition, including aluminum polyoxychlorides of various basicities, aluminum chloride and aluminum hydroxide.X-ray fl uorescence analysis showed trace amounts of sulfur According to the results of X-ray fl uorescence analysis showed some sulfur in the samples, 2.8 and 2.4%.According to the results of X-ray fl uorescence analysis, as can be seen from Table 2 for samples No. 1 and No. 2, sulfur passes into the composition of the samples during synthesis.The samples had a diff erence in color, sample No. 2 was characterized by a gray tint, and sample No. 3 had a maximum white color.Increasing the duration of synthesis resulted in an increase in product yield by 14%.
The presence of phases Al 9 Cl 6 (OH) 21 •18H 2 O; Al 5 Cl 3 (OH) 12 •7.5H 2 O, which can be called polyoxicchlorides, the total chemical formula of which Al 2 (OH) NCL6-N we see them in the results of the X-ray analysis of synthesized coagulant samples presented in Table 3.
The mineral phase in the Aqua-Aurat 30 sample is 59.6%, the composition AlCl 3 •6H 2 O hydrated chloraluminum by 100% under the diff ractogram analysis in Fig. 4.
The elemental analysis results (X-ray fl uorescence analysis) of synthesized samples No. 1,No. 2,No. 3 show the content of aluminum, oxygen, and chlorine.Sulfur contained in sulfuric acid, an activator of the reaction, practically does not pass into the sample during synthesis.
Based on the analyses results, it can be assumed that more chlorine passed into the composition of sample No. 2 during the synthesis.
Thermal study of sample No. 1 is shown in Fig. 5.The DTA curve had endothermic eff ects of diff erent intensity with maximum development at 157 °С, 264.4 °С, 348.8 °С.The bend at 531.3 °C can also be noted.All the above eff ects developed against the reduction of the sample weight.Each of them on the DTG curve corresponds to its minimum.
Sulfur is present in the X-ray fl uorescence data.The presence of aluminum sulfates is quite possible.But their manifestation takes place at temperatures above 550 °C.Therefore, it is possible to speak about their presence only presumably.In the superposition, the combination of weak minima 139.7 °С, 238.8 °С, 271.3 °С on the DTG curve may refl ect the presence of bischofi te − MgCl 2 •6H 2 О.
The thermogram of sample No. 2 shows the diff erence on the DTA curve, in endothermic eff ects of diff erent intensity with maximum development at 264.80 °C, 344.5 °C.The eff ects developed against the reduction of the sample mass.Each of them on the DTG curve corresponds to its minimum.The minima on the DTG curve appeared at the following temperatures: 242.5 °C, 266.9 °C, 333.8 °C.
There is no eff ect in the temperature range of 400−600 °C in this sample.The minimum of 266.9 °C on the DTG curve has a very low intensity that correlates with dehydration, with an extremum at 344.5 °C.Perhaps, the endothermic eff ect in this sample refl ects complete dehydration and possibly a direct transition to γ-Al 2 O 3 .Therefore, a diff erence in the orderliness of the crystal lattice and other sizes of gibbsite crystals is assumed given the endothermic eff ect with extremum at 264.8 °C.The stripes are recorded in 362, 284 cm -1 in the long length of the spectrum, in the fi eld of manifestation of vibrations − Me -Cl, Me -O.The studied sample No. 2 also contains stripes characterizing Gibbsite γ-Al(OH) 3 are displayed in Table 6 and correspond to − 3620, 3523, 3458, 3372, 1020, 967, 915, 796, 745, 665, 582, 558, 514, 499, 448, 421, 409   Scanning electron microscope study.Comparative characteristics of the obtained spectra of coagulant samples under study obtained with a scanning electron microscope, are presented in the Figs.8-9 and in Table 3.
The aluminum content in the Aqua-Aurat 30 sample is 28.86% which corresponds to the claimed characteristics.The samples of synthesized coagulants have a close quantitative and qualitative composition as in Sample No. 3. Sample No. 2 has a diff erent percentage composition both in terms of oxygen and aluminum content, and the formation of aluminum chloride AlCl 3 can be assumed.When the microstructure of the samples was studied, the diff erence in morphology was also shown.The aluminum content in the Aqua-Aurat 30 sample is 28.86% which corresponds to the a diff erent percentage composition both in terms of oxygen and aluminum content, and the formation of aluminum chloride AlCl 3 can be assumed.When the microstructure of the samples was studied, the diff erence in morphology was also shown.It can be assumed that part of technical aluminum hydroxide did not react during the synthesis or changed its structure [26].The diff erence in the textures of the samples was clearly visible when the pictures were taken with an increase of 10,000 times presented in Fig. 8.The diff erence in the structure of samples is determined by the diff erence in technology for obtaining coagulants and the basicity of the reagent, presented in Fig. 9. Comparative analyzes of aqueous solutions of the synthesized POHA and AA-30 were carried out, as well as the solubility, density and pH of the solutions.When dissolved in water, the synthesized samples showed a signifi cantly lower solubility, even when heated, than the AA-30 reagent, which can be explained by the formation of colloidal solutions and some content of aluminum hydroxide Al(OH) 3 , when heated, then the AA-30 reagent, which can be explained by the formation of colloidal solutions and some content of aluminum hydroxide Al(OH) 3 .Studies of aqueous solutions showed some diff erences in the values of the pH, and density, the values of synthesized samples on the acidity scale are approximately in the same range, pH 2.0; 2.3; 1.8, while the pH AA-30 value is − 1.5.The data is displayed in Table 4.
Samples obtained by evaporation of pre-dissolved coagulant have good solubility in water.
Determination of the optimal dose of coagulant depends on several factors, in particular, indicators of color and turbidity of water, temperature, pH and ionic composition of water, suspended solids content, concentration of colloidal particles and truly dissolved organic substances.

Conclusion
The conducted studies have shown the possibility of using microwave technologies for the synthesis of aluminum polyoxychlorides from aluminum hydroxide and sulfuric acid.The preparation of the reagent was carried out by analogy with the reactions of acid decomposition of samples in analytical chemistry, in a microwave setup for sample preparation.Microwave technologies signifi cantly reduce the duration of the synthesis process, increase the effi ciency of the installation, there is no contact between the heating elements and the aggressive environment.At this stage of the research, household microwave ovens were used, a laboratory pilot plant was designed, research was carried out on synthesis with a change in process parameters.The elemental, phase compositions and morphology of samples of the synthesized reagents were studied in comparison with the widely used coagulant Aqua-Aurat 30.The role of sulfuric acid was revealed as an activator of the process, which increases the effi ciency of synthesis, and practically does not pass into the synthesized sample.The yield of the synthesis product with the use of the activator was 78% at a duration of 1 minute.The synthesis carried out without an activator requires a longer process time for the completeness of the reaction; with an increase in power by 20% and the duration of synthesis by 15 times, the product yield increased only by 13%.It can be assumed that the synthesis of aluminum polyoxychlorides using 600 W microwave radiation and the addition of sulfuric acid is promising, since sample 2 showed more positive results.

Table 1 .
Conditions of microwave synthesis of polyaluminum chloride from aluminum hydroxide and hydrochloric acid 36%.

Table 2 .
X-ray fl uorescence analysis of samples

Table 3 .
Quantitative characteristics of samples of the studied coagulants The samples of synthesized coagulants have a close quantitative and qualitative composition as in Sample No. 3. Sample No. 2 has

Table 4 .
Comparative characteristics of aqueous solutions of coagulants