In the sugar industry of Ukraine, calcium hydroxide [Ca (OH)₂] and carbon dioxide (CO₂) are mainly used to purify diffusion juice from non-sugars (Zheplinska et al., 2020). These components can be obtained by firing carbonate rocks and subsequent quenching of lime (CaO), obtained by dissociation of calcium carbonate (CaCO₃). Limestone is a sedimentary rock consisting mainly of calcite and impurities (Zheplinska et al., 2019). The chemical composition of pure limestone is close to calcite, where CaO – 56% and CO₂ – 44%. As is known, limestone is an exhaustible resource, its deposits are limited and make up 19 – 22% of the total mass of sedimentary rocks (Sheiko et al., 2019).

In Ukraine, limestones with a CaCO₃ content of 93.00% and impurities with a mass fraction are allowed for use in sugar production:

Despite the seemingly high condition of limestone, many problems often arise in practice: the formation of melts in the lime kiln, which is the irreversible loss of lime; formation of sediments on the heating surface of the evaporator; saturating juices are poorly filtered; the stability of the furnace lining is reduced; the rate of chemical interaction of CaO with water decreases, so the quenching process is significantly slowed down; the volume of molasses increases; the volume of water on the juicer increases, etc (Alves et al., 2017). All the above technological problems lead to an increase in the cost of lime on the machine juicer and its irreversible losses in the lime department (Palamarchuk et al., 2019). To provide the required amount of CaO, it is necessary to increase the use of limestone, which will increase the amount of fuel used for its firing and increase carbon dioxide emissions into the atmosphere, which worsens the ecological situation of the planet.

One of the primary causes of the above problems is the lack of a correct technological audit of the lime department, which includes the use of correct methods of control of technological parameters (Mushtruk et al., 2020). One of these methods, which is part of the audit of the limestone department, namely the method of determining the mass fraction of calcium carbonate in the carbonate rock, will be presented in this article (Bhatia et al., 2016).

Each technique currently used in practice in the technological flow implies that it should be as accurate as possible, easy to use, and affordable. Often researchers omit some of the points of the above requirements and to date, there is very little research in this area. In particular, there is a method of determining the mass fraction of calcium carbonate in the carbonate rock DSTU 1451-96 (1997), which is based on the acidimetric determination of the mass fraction of calcium carbonate in the carbonate rock by calculating the percentage of Trilon B solution, which went to titrate the calcium content. Other authors presented a method (Zheplinska et al., 2021), which involves analysis by a complexometric method. At the same time, the proposed methods do not provide the high accuracy of the results of determinations.

There are modern techniques that can be used to simultaneously determine the isotopic ratios of calcium and magnesium from a single aliquot of biological and geological samples (Somaratne et al., 2019). However, for the needs of a sugar factory, it is necessary to know the concentration of CaCO₃ in the carbonate rock, and not the isotopes of calcium and magnesium separately. As in the carbonate rock, there are impurities of calcium sulfate, which negatively affect the technological process, the isotopes of calcium from this component will give an error in estimating the content of CaCO₃. Also, this method is highly costly for ongoing control.

Therefore, the audit of the lime department, which includes the use of an adjusted method for the determination of calcium carbonate in limestone, will lead to the fact that the technologist will be able to choose in advance the most effective way to conduct the process. In turn, this will not only ensure energy and resource savings in sugar production but also reduce emissions.

The task of our research was to improve the method of determining the mass fraction of calcium carbonate in the carbonate rock, which will increase the accuracy of the results and reduce its cost.

The results (Figure 1) obtained by repeating the experiment twice show that the moisture content in all samples was not significant. Only in sample no. 6 a jump in moisture, concentration was observed, which is explained by the fact that chalk-like limestone was analyzed. Cretaceous limestones are characterized by increased hygroscopicity. The authors of scientific works (Buniowska et al., 2017; Verma et al., 2018; Shmyrin, Kanyugina and Kuznetsov, 2017) engaged in similar research but obtained other scientific results. The presence of moisture in limestone has almost no effect on the firing process. However, if the moisture concentration significantly exceeds the norm, it will increase fuel consumption and reduce the concentration of CO₂ in the furnace gas. The last component (CO₂) is very important in the technological process of sugar production. The authors of scientific works (Nadeem et al., 2018; Avalos- Llano, Molina and Sgroppo, 2020; Rahimi et al., 2020) do not believe that the concentration (CO₂) is not an important factor in the technological process of sugar production.

Therefore, all samples of the studied limestones contained the normative concentration of moisture, which indicates that this indicator will not adversely affect the firing process of carbonate rocks. The next test component was impurities that are insoluble in hydrochloric acid. The standard concentration of these impurities in the carbonate rock for Ukrainian plants is not more than 3%. The authors of the following scientific papers (Dalfré Filho, Assis and Genovez, 2015; Luo et al., 2019; Dhar et al., 2015) other methods for determining impurities were used for similar scientific research. According to the results of experiments (Figure 2) we observe that samples no. 2, 3, and 7 have an inflated content of the investigated component of the carbonate rock.

Given that during the firing of limestone SiO₂ can form silicates of calcium and magnesium, which is an irreversible loss of limestone, the use of these samples in the process is not recommended. Besides, silicates contribute to the formation of sediment on the heating surface of the evaporator. The authors of scientific works (Guo et al., 2018; Ozerov and Sapronov, 1985; Jiang, 2015; Almohammed et al., 2015) recommend not to use such samples in the production process.

Carbonate rocks were analyzed for their content of iron and aluminum oxides. Since this component (both AI₂O₃ and Fe₂O₃) harms the filtration rates of saturating juices, and iron compounds reduce the stability of the lining and slow down the process of slaking lime, control over its content is necessary. The content of the sum of oxides of aluminum and iron in the carbonate rock according to DSTU 1451-96 (1997) should not exceed 1.5%. As the results of research (Figure 3) show, samples no. 3, 5, and 8 exceed the recommended norms, so their use in the technological process will lead to a number of the above negative consequences. The authors of the following scientific works (Johnson, Zhou, and Wangersky, 1986; Lee et al., 2012; Lebovka et al., 2007) recommend not to use such samples in the production process.

Control of the content of calcium sulfate in the carbonate rock should be carried out to prevent the formation of scale on the surface of the evaporator and slow down the hydration of lime. The mass fraction of calcium sulfate according to the recommendations DSTU 1451-96 (1997) should not exceed 0.4%. Figure 4 shows that samples no. 2, 7 and 8 cannot be recommended for use in the sugar industry. In subsequent scientific works (Katariya, Arya and Pandit, 2020; Kim et al., 2016; Kozelová et al., 2011; Krasulya et al., 2016) such studies were not conducted.

Determination of calcium carbonate content in the carbonate rock was carried out as follows. Pipette 0.15 m³ of solution into a 2.50 m³ porcelain cup or conical flask, add 1 m³ of distilled water and 0.25 m³ of sodium hydroxide solution with a mass fraction of 20% from a volumetric flask containing the filtrate obtained after precipitation of iron and aluminum oxides. After 1 – 2 minutes on the tip of a spatula made from 0.1 to 0.2 kg of murexide in a mixture with potassium chloride. The solution was stirred and titrated with 0.1 M solution of Trilon B until the transition from red to purple color, observed on a black background. Water was monitored by titration of the blank. The authors of scientific works (Dehghannya et al., 2018; Moser et al., 2017; Zhu et al., 2016; Zdziennicka et al., 2017) used other standardized methods to conduct similar studies.

Our proposed method (Noguiera Felix et al., 2019) differs from the method presented in DSTU 1451-96 (1997) in that the filtrate after precipitation of iron and aluminum oxides was additionally introduced murexide in a mixture with potassium chloride and then titrated with 0.1 M solution of Trilon B to the transition from red to violet color, and the mass fraction of calcium carbonate in the carbonate rock was calculated with a factor of 0.005008.

Thus, the mass fraction of calcium carbonate (CaCO₃) in the carbonate rock (based on the dry matter) (X) in percent was calculated by the formula (1):

(1) X = ( B ⋅ K ⋅ 0.005008 ⋅ 100 m − 0.735 ⋅ a ) ⋅ 100 100 − W

Where:

B – is the volume of 0.1 M solution of Trilon B, which went to the titration of calcium, m³;

K – is the correction factor for the solution of Trilon B; 0.005008 – the mass of calcium carbonate corresponding to 0.01 m³ of a solution of Trilon B with a concentration of 0.1 mol.m^-3, kg;

a – is the mass fraction of calcium sulfate in limestone,%; 0.735 – conversion factor of calcium sulfate to calcium carbonate;

m – is a sample of limestone in the amount of 0.15 kg;

W – it is the mass fraction of moisture in percent.

The results of determining the mass fraction of calcium carbonate in the carbonate rock are shown in Table 1 and Figure 5.

Based on Table 1 and Table 2, all analyzed samples, which were carried out according to the method DSTU 1451-96 (1997), have slightly inflated results. Therefore, an adjusted technique was proposed that gives more accurate results.

According to Figure 5, we see that the concentration of calcium carbonate in samples no. 2, 3, 5, 6, and 7 does not reach the recommended DSTU 1451-96 (1997). Therefore, the use of such limestones is economically and technologically unprofitable.

The share of magnesium carbonate in carbonate rocks should not exceed 2.5%. Since the excess of this impurity contributes to the deterioration of the filtration rates of juices. From Figure 6 we observe that samples no. 2, 3, 5, 6, and 8 do not meet the recommended standards. It is also observed that the indicators obtained by the method DSTU 1451-96 (1997) are slightly inflated (Table 2). Therefore, this method of determining magnesium carbonate in carbonate rocks requires clarification, which will be discussed in the next article. The author of (Sasikumar, Chutia and Deka, 2019) used other standardized methods to conduct similar studies.

Alkali metal oxides of potassium and sodium also attract a lot of attention and should not exceed 0.25% DSTU 1451- 96 (1997). K + and Na + ions are active molasses-forming agents. Therefore, control over their concentration must be carried out. From Figure 7 it is seen that all samples corresponded to the normative concentration.

To provide an objective assessment of limestones, in terms of their technological condition, it is not enough to consider their individual components. It is necessary to consider a comprehensive complete chemical composition of carbonate rocks and only then provide certain technological characteristics of limestone. For this purpose, all the obtained data were summarized (Table 2) and the balance of all components of carbonate rocks was summed up. When determining the chemical composition of the carbonate rock, the sum of the mass fractions of impurities and calcium carbonate in the rock should be approximately 100%. According to the results of the final calculations, the total sum of the components of carbonate rocks in all samples according to the method DSTU 1451-96 (1997) gives slightly inflated results, the data obtained by the proposed method are almost close to 100%. Therefore, our proposed method for determining the mass fraction of calcium carbonate in the carbonate rock (Cervantes- Elizarrarás et al., 2017) which is based on the material balance makes it possible to most accurately assess the chemical composition of the carbonate rock.

The obtained data indicate that the use of the proposed method for determining the mass fraction of calcium carbonate in the carbonate rock makes it possible to obtain more accurate indicators of its concentration and, accordingly, the chemical composition of limestone.

Thus, the inclusion of the proposed improved method for determining the content of calcium carbonate in the carbonate rock, as one of the elements of the technological audit of the lime department will allow to give an objective assessment of the processes of the sugar plant and their early correction, which will lead to economic and environmental efficiency.