Synthesis of humic acid with the obtaining of potassium humate based on coal waste from the Lenger deposit, Kazakhstan

: This article a synthesis of humic acid with the obtaining of potassium humate based on coal waste from the Lenger deposit. Accumulated industrial waste heavily pollutes the environment and has a direct impact on all living things. The accumulation of waste in land ﬁ lls increases the pollution level of the atmosphere, soil, groundwater, and surface water, destroys the functioning of ecosystems, and damages agriculture and construction. A sieve analysis was carried out to study the ﬁ ne fractions of coal waste, and a scanning electron microscope analysis was performed to study the mineralogical, structural state and X-ray chemical phase composition. The chemical composition of coal waste was studied using di ﬀ erential thermal analysis during heat treatment. The optimal parameters of the process for obtaining humic acids are established, and the results of experimental work are presented. The results are con-ﬁ rmed by mathematical planning of the experiment using the method of orthogonal plan of the second order. The mathematical planning results were tested according to the Student and Fischer criteria. Based on the conducted studies, it was identi ﬁ ed that the degree of extraction of humic acid reaches up to 95.90% in terms of the organic phase, and the concentration of humic acid is 49.13%. From the humic acids obtained using potassium hydroxide, potassium humate was synthesized. The potassium humate obtained in its composition has fertilizer properties. Therefore, the obtained potassium humate will be used for the production of humic fertilizers to improve soil fertility and crop yields. The synthesis of humic acids with the production of potassium humate is aimed at reducing the accumulated industrial waste, which in turn allows you to regulate and improve the ecological situation and green ecology in the region.


Introduction
At the present development stage of science and technology, one of the most important problems is the processing of natural and industrial waste.Coal is mined all over the world, as coal is an energy carrier.Coal production leaves behind many tons of waste, which in turn pollute the environment.As a result of coal mining in Kazakhstan, more than 6 million tons of waste were generated, and the Lenger deposit of brown coal is located in south Kazakhstan.According to the data, the reserve balance part is 33,956 thousand tons, and the off-balance part is 3,244 thousand tons.The use of the balance part is currently economically feasible, whereas the processing of the off-balance part at the given level of technology and technology development is not economically feasible but can be later transferred to the balance [1,2].
The deposit [3] is named after a German miner of the Lenger developer.The Lenger coal deposit has been known since the nineteenth century.The Lenger coal deposit is located 35 km east of the city of Shymkent, 1 km north of the village of Lenger, and contains mostly brown coal (grade B3) with a heat release of up to 7.3 thousand kcal•kg −1 .
Coal mining waste from the Lenger deposit is a promising and valuable source of organic and mineral raw materials that contain a wide variety of trace elements and organic substances with fertilizing properties, and in this regard, it is a valuable secondary raw material suitable for processing into humic acid [4].
The coal waste of the Lenger deposit consists of organic matter and mineral impurities.The main elements included in the organic mass of coal are carbon, hydrogen, oxygen, and organic sulfur.They have various forms of connection with the organic and mineral parts of coal and differ from each other in the content and nature of their active groups.Coals also contain chemical compounds of some metalscalcium, iron, potassium, magnesium, etc. [5,6].The location and view of the coal waste in the Lenger deposit are shown in Figure 1.
The relevance of scientific work lies in the fact that the accumulation of coal waste creates serious environmental problems in the regions.The accumulation of coal waste increases the pollution level of the atmosphere, soil, ground, and surface waters, destroys the functioning of ecosystems, damages agriculture, and has a negative impact on climate change.Environmental pollution from industrial waste is a global environmental problem of our time.This research has implications beyond chemical technology and environmental impact.It is also aimed at reducing and recycling accumulated coal waste [7].
In the work of the authors, the possibility of synthesis of humic acid from coal waste and the study of their composition and radioactivity are investigated.Humic acid is considered for practical use in the production of organic and mineral fertilizers and feed additives.In this work, the authors show the yield of humic acid with a description of the obtained elemental analysis [8,9].
Humates are natural organic mineral fertilizers that stimulate growth; humates are potassium salts obtained from humic acid.Potassium humate is most widely used in pre-sowing seed treatment and foliar feeding of plants during the growing season.Potassium humatean organic and mineral fertilizer with a stimulating effect and fungicidal activity, is a product of high-tech processing of lowlying peat, coal, and coal waste.
The authors in their work indicate the scientific novelty of the work on the use of technogenic resources in the form of coal mining waste for the extraction of organic and mineral raw materials.The novelty of the scientific work lies in the decomposition of coal waste using 50% hydrochloric acid (HCl), followed by the release of humic acid by 95.9% in terms of organic mass and the determination of optimal technological parameters of the process.The purpose of the scientific work is to synthesize humic acid based on coal waste from the Lenger deposit, followed by the production of humate-containing fertilizers with a high content of useful components and trace elements for crop yields [10,11].

Materials and methods
For the processing of coal mining waste under experimental conditions, test methods were selected using a Jeol JSM-6490l V scanning electron microscope (SEM), a multiparametric portable cyber scanner (PCS 650 Eutech), an IR Fourier spectrometer (Zhimadzu IR Prestige-21), a MicroXRF Analysis Report, an incident beam monochromator D878-PC75-17.0,and a Q-1500 derivatograph.
Experimental scientific work has been carried out on the obtaining of humic acids as a raw material, plant litter is used, which is pre-crushed to a fraction with a diameter of not more than 1 mm and are treated in heat at t = 145-150°C.Next, extraction is carried out, followed by filtering of the extract and precipitation of humic acids from the solution by adding HCl, then the deposition is separated from the solution [12].

Sieve analysis to determine the particle size distribution of raw materials
In the study of coal waste from the Lenger deposit, it was identified that its density is −0.67 kg•dm −3 , and its specific gravity is −0.57kg•dm −3 .To determine the particle size distribution, a sieve analysis was carried out, and the results are shown in Figure 2.
On the basis of sieve analysis (Figure 2), it was identified that in the total mass of the waste, about 15% are large fractions with sizes of more than 7.10-5.0mm.About 35% of coal waste falls on the share of the middle fraction with a particle size of 3.50-2.00mm.Small particles account for about 50%.The coal waste from the Lenger deposit is made up of fractions with the following sizes: 2.00-1.50,1.50-1.00,and 1.00-0.75[13].

Differential thermal analysis of raw materials
To study the chemical composition of the coal under study during heat treatment, a differential thermal analysis was carried out in the derivatograph Q-1500D (DEMO).The derivatogram of coal waste is shown in Figure 3.
The curves of differential thermal analysis in Figure 3 are characterized by three endoeffects at 150°C, 330°C, 530°C, and 830°C.The first one at 150°C characterizes the removal of crystallohydrate moisture.The other three endoeffects characterize the decomposition of carbonate compounds of iron 330°C and magnesium 530°C, as well as 830°C calcium carbonates.Exoeffects at 130°C, 200°C, 470°C, and 750°C.They are characteristic of burnout reactions of sulfur compounds of iron and sulfate impurity metals.

Spectral analysis of raw materials
The infrared spectral analysis of the coal waste (Figure 4) was carried out using an IR Fourier spectrometer, Shimadzu IR Prestige-21, with a frustrated total internal reflection device, Miracle (Pike Technologies Kyoto, Japan).The IR Prestige-21 uses a bright ceramic light source, a high-sensitivity DLATGS detector, and high-throughput optical elements.Optimization of optical/electronics/signal systems minimizes noise and maximizes the S/N ratio (40,000:1 and better).
The infrared spectrum of the coal waste, represented in Figure 4, shows that less intensive absorption spectra at 1,585.5 cm −1 are characteristic for sodium-containing carboxyl groups -C-O-K, absorption spectra with wavelengths of 1,090-1,020 (1,033.8)cm −1 show the presence of silicates with valency bonds Si-O-Si and Si-O-C; in addition, they are characteristic for oxygen-containing ether groups, intensive fluctuations in the range of 910.4 cm −1 inorganic metal (Mg, Al, Fe, and Ca) compounds, absorption spectra in the interval of 794.6-752.2cm −1 characterize organic functional groups.

Elemental analysis of raw materials
The elemental composition and micrograph of coal waste (Table 1, Figure 5) were determined using scanning microscopy (JSM-6490lV, Jeol, Tokyo, Japan).The micrograph of the coal waste (Figure 5) is characterized by the indistinct dense congestion of irregularshaped crystals.The irregular hexahedral single minerals are evidence of the presence of calcium aluminates.The dark aggregates around the fine-crystalline aluminate minerals are characteristic of calcium and silicon ferrites.The sample contains quartz minerals (as confirmed by fine chain aggregates), silicon aluminates, and iron-containing minerals [14].
From the analysis of Table 1, it follows that in the elemental composition of coal waste, the content of the main useful component is (C) carbon -51.23%.This carbon content is sufficient for humic acids [15].

X-ray phase analysis of raw materials
To determine the structural state and chemical phase composition of coal waste, X-ray phase analysis using an incident beam monochromator D878-PC75-17.0(London, England) was carried out, and the results are shown in Figure 6.
From Figure 6, it follows that the analysis of the X-ray pattern shows that the structure of the test sample is amorphous.Diffraction peaks with values of interplanar distances A 0 = 4.24-3.84-2.45-2.28-1.81-1.53indicate the presence of quartzite -SiO 2in the crystal structure of the sample, which is the main component.The composition of the test sample contains in significant quantities: iron oxide -Fe 2 O 3 , with diffraction maxima A 0 = 2.77-2.56-2.21-2.08-1.66-1.48,and gypsum -CaSO 4 •2H 2 O, for which the diffraction maxima A 0 = 7.14-3.34-3.02-2.70-2.12-1.66,are characteristic.The presence of coal impurities is evidenced by diffraction peaks with low intensity A 0 = 3.12-2.08-1.63-1.48[16].
In order to increase the yield of humic acids in this coal, the process of coal oxidation with the use of HCl has been studied.The oxidation process was carried out at a concentration of HCl of 40-50% at a temperature of 30-60°C for a duration of 10-60 min, with a weight ratio of coal:HCl from 1:0.5 to 1:2.Moreover, the ratio of coal:acid meant the ratio of the organic part of coal to HCl [17].
Experimental work was carried out in a glass cylindrical reactor equipped with a thermostatic jacket and a screw-type stirrer.Acid was poured into the reactor, a set temperature was set, a stirrer was turned on, and coal was loaded.At the end of the process, the reaction mass was divided into liquid and solid phases.The solid phase was washed with distilled water from HCl to a neutral reaction, then dried to an air-dry state, and ash  content, humidity, organic substances, and humic acid yield were determined in it by a known method.Humic acid was synthesized from coal waste from the Lenger deposit using sodium hydroxide (NaOH -10%), with a concentration yield of 27.8% in terms of organic mass.The synthesis of humic acid was also carried out using NaOH and ammonium hydroxide (NH 4 OH) with a concentration yield of 22.5% and 18.3%.Synthesis of humic acid based on coal waste from the Lenger deposit using nitric acid (HNO 3 -50%), with a concentration yield of 45.2% in terms of organic mass [18][19][20][21][22].The experimental work carried out on the synthesis of humic acids based on coal waste using HCl (50%) is a new method.

Results and discussion
During the research, it was determined that the coal waste of the Lenger deposit had the following composition (wt%) after drying to an air-dry state: moisture 17.26, determined by GOST 9516-92; ash 31.51,determined by GOST 11022-95, ISO 1171-97; organic matter 51.23.Microscopic examination revealed that the structure of coal has a microcellular composition, and the cells are rounded or slightly lenticular and are not large (0.0001-0.0002 mm).This structure is due to the high porosity and moisture capacity of coal.Under experimental conditions, it was identified that the degree of decomposition of coal waste is 95.9% with a solution of HCl concentration of 50% at a temperature of 30-60°C.The scientific novelty of this work is the yield of humic acid by 95.9% in terms of organic mass and the yield of acid by 49.13% (GOST 9517-94, ISO 5073-85).
It follows from Table 2 that when coal is decomposed with 50% HCl in the ratio T:W, equal to 1:2,0 and the degree of extraction of humic acid reaches up to 95.90% in terms of the organic phase, and the concentration of humic acid is 49.13%.Chemism of this process can be described as follows: where CW denotes coal waste and (R-OH), (R-COH), and (R-COOH)-are functional groups in humic acid.According to the chemical characteristics of humic acids isolated according to GOST 9517-94, they contain approximately 8.40 mg-eq•g −1 (COOH -4.97, OH -2.49, and CO -0.94) of the sum of functional groups.The synthesis of humic acid was analyzed using an IR spectrometer (Shimadzu IRPrestige-21), and the results of the studies are shown in Figure 7.
For the reliability of the experimental data obtained, mathematical methods of processing the process were carried out.For processing, the methods of mathematical planning of the experiment according to the orthogonal plan of the second order for k = 3 were used.The results of studies on the oxidation of brown coals were used as initial data.At the same time, the concentration of HCl varied from 10% to 50%, the temperature regime of the process varied from 30°C to 60°C, and the duration of the oxidation process varied from 10 to 60 min.As a result of the studies carried out to optimize the HCl oxidation process of brown coals, the reproducibility variances for the criteria y coefficients of regression equations were determined as an evaluation criterion.The plan and results of the experiment are shown in Table 3.
The dependences of the following criteria were investigated: Y is the yield of humic substances, %; Z 1 is the temperature (25-60°C); Z 2 is the HCl concentration (10-50%); and Z 3 is the oxidation time (10-60 min).
( ) Using formulas 2-4, the results of four experiments in the center of the plan were calculated using reproducibility variances for the criterion yyield of humic substances.

∑ ∑
Based on the results of the experiments, the coefficients of the regression equations were calculated according to formulas 5 and 6.The calculation results are shown in Table 4.
The significance of the regression equation coefficients was verified (7) by the Student's criterion, and the adequacy of the regression equation to the experiment was verified (8) by the Fisher criterion.After excluding insignificant coefficients of regression equations, the regression equations adequate to the experiment have the following form:  According to the results of mathematical planning of the process, the optimal conditions ensuring the maximum yield of humic acids (up to 95.90%) during the HCl oxidation process of coal fines are as follows: HCl concentration -50%, process temperature -60°C, and oxidation duration -60 min.The results of humic acid yield are shown (Figure 8) by 3D modeling [23].
Based on the data in Figure 8, an increase in the yield of humic acid under the influence of HCl concentration and temperature during the decomposition of coal waste is characterized by a change in the square appearance of the plane from green to saturated red.Many scientific research works have been carried out for the synthesis of humic acids.These studies were carried out to establish the regime conditions for the synthesis and output parameters for the production of humic acids.For the synthesis of humic acids, various inorganic alkalis and acids were used, among them are commonly used alkalis such as sodium, potassium, and NH 4 OH, as well as HNO 3 s [23].
Due to an increase in the concentration of alkali used in the production of potassium humate, the yield of the resulting potassium humate increases.Chemism of this process can be described in the following way: The elemental composition of the resulting potassium humate was determined by an instrumental method using scanning microscopy (JSM-6490lV, Jeol, Tokyo, Japan).The results of the experimental work are shown in Figure 9.
From Figure 9, it can be seen that the results of microscopic studies of potassium humate carried out using a SEM JSM-6490l using the energy-dispersive method are shown in the form of yellow spectra.These yellow spectra, respectively, indicate the elements that are present in a given sample and their mass fraction in ratios, %.Based on these data, the elemental composition of the test sample is determined and converted to the oxide form (Table 5).
From Table 5, it follows that the carbon content is 58.2% and potassium is 19.4%.Such a content of useful components of potassium humate is enough to use it as a humate-containing fertilizer to increase crop yields.

Conclusions
A synthesis of humic acid with the obtaining of potassium humate based on coal waste from the Lenger deposit using inorganic solutions was studied.The synthesis of humic acid with the obtaining of potassium humate is aimed at reducing accumulated industrial waste, which in turn allows regulating and improving the environmental situation in the region [5,24,25].
The elemental and mineralogical composition of coal waste has been studied, as well as the yield and structuralfunctional composition of humic acid.The chemical composition of coal waste was studied using differential thermal analysis during heat treatment.A sieve analysis was carried out to study fine fractions of coal waste, and an X-ray phase analysis was carried out to study the mineralogical, structural state, and chemical phase composition of coal waste from the Lenger deposit [26].
Based on the results of experimental work, the optimal parameters of the process of obtaining humic acids were determined and established.Also, the results were confirmed by the mathematical planning of the experiment using the method of the orthogonal plan of the second order.The mathematical planning results were tested according to the Student and Fisher criteria.Based on the conducted studies, it was identified that the degree of extraction of humic acid reaches up to 95.90% in terms of the organic phase, and the concentration of humic acid is 49.13%.Potassium humate is involved in the formation of soil structure and the accumulation of nutrients and microelements in a form available to plants and helps regulate the content of metals in water and soil ecosystems.
Based on the synthesized humic acids, potassium humate was obtained using potassium hydroxide.Based on the results of experimental work, the optimal technological parameters for the process of obtaining potassium humate and the elemental mineralogical composition were determined.The resulting potassium humate, in its composition, has fertilizer properties.Therefore, the resulting potassium humate will be used for the production of humic fertilizers to improve soil fertility and crop yields.Author contributions: Bakyt Smailov: conceptualization, methodology, formal analysis, investigation, data writing original draft preparation, writingreview and editing, visualization, project administration, and funding acquisition.Usha Aravind: software, validation, resources, and supervision.

Conflict of interest:
The authors state that there is no conflict of interest.
Data availability statement: The datasets generated and/ or analyzed during the current study are available from the corresponding author on reasonable request.

Figure 1 :
Figure 1: Location and view of the Lenger deposit, the Republic of Kazakhstan.

Figure 2 : 3 Figure 3 :
Figure 2: The sieve analysis of coal waste from the Lenger deposit.

Figure 4 :
Figure 4: The IR spectrum of the coal waste from the Lenger deposit.

Figure 5 :
Figure 5: The micrograph of coal waste from the Lenger deposit.

Figure 6 :
Figure 6: The X-ray image of coal waste from the Lenger deposit.

( 9 )
where x 1 = z 1 − 42.5/12.38,x 2 = z 2 − 25/14.14, and x 3 = z 3 − 40/17.68.The regression Eq. 9 allows us to calculate the values of the selected criterion for any study factors' combination in the studied range, analyze the intensity of the influence of individual factors on the process indicators, and determine the optimal conditions for the oxidation of coal fines.

Figure 8 :
Figure 8: Dependence of humic acid yield on HCl concentration and temperature.

Funding information :
This research was funded by the Science Committee of the Ministry of Science and Higher Education of the Republic of Kazakhstan, Grant No. AP14972664.

Table 1 :
Elemental composition of coal waste from the Lenger deposit

Table 2 :
The dependence of the yield of humic acid on temperature and acid concentration )

Table 3 :
Experimental plan and research results on optimizing the HCl process of coal oxidation

Table 4 :
Coefficients of regression equations

Table 5 :
Elemental composition of potassium humatePotassium humate synthesis from Lenger coal waste  9