In Ukraine, limestone is used in the technological process of sugar production to purify diffusion juice from non-sugars. Initially, limestone is fired at high temperatures to obtain lime and carbon dioxide. Then the lime is quenched with water to obtain lime milk. Lime milk (Ca(OH)₂) and carbon dioxide (CO₂) are used to effectively remove non-sugars from diffusion juice.

The quality of these components (Ca(OH)₂ and CO₂) depends on the degree of the condition of the limestone. According to Ukrainian requirements, the content of calcium carbonate (CaCO₃) in the carbonate rock must be at least 93.00%. The mass fraction of impurities in limestone should not exceed:

Only by using conditioned components for the purification of diffusion juice, it is possible to receive carbonized juices of high purity. After the non-sugars are removed from the diffusion juice, the sucrose content accumulates in the system.

The yield and quality of the final product – sugar – will depend on the concentration of sucrose and the purity of the juice. Sugar is used as a finished product for human consumption and as an intermediate in the production of other foods (Figure 1).

For the needs of the sugar factory, only calcium carbonate is considered a useful component of the carbonate rock, all other components are harmful impurities (Zheplinska et al., 2020). In nature, very rare limestones whose chemical composition would be close to calcite. Magnesium is often a component of limestone, if the content of magnesium oxide (MgO) in it is 4 – 17%, then such limestone is called dolomitized (Zheplinska et al., 2019).

Magnesium carbonate (MgCO₃) contained in the carbonate rock can lead to several subsequent adverse events during its (carbonate rock) firing. Magnesium carbonate, which is part of dolomite (CaMg(CO₂)₂) promotes the formation of a liquid phase during its thermal dissociation. The liquid phase, in turn, envelops the grains of active calcium oxide with a non-reactive film, which leads to the fact that part of the potentially chemically active lime becomes inactive. The formed melt enters the pores of active lime, as a result of which the dissociation of a significant part of the carbonate rock will not be able to pass in full. In addition, the liquid phase in significant quantities can enter the lower zone of the furnace where it meets the colder gas solidifies with the formation of sintered depths. This phenomenon can lead to a "hang" of the furnace, and in less optimistic forecasts, and to the shutdown of the plant.

During the firing of limestone, the proportion of MgCO₃ contained in it decomposes into MgO and carbon dioxide (CO₂). Once on the plant machine during the quenching of magnesium oxide, we obtain magnesium hydroxide (Mg (OH)₂), and later during the saturation of magnesium carbonate (MgCO₃). The latter (MgCO₃) significantly impairs the filtration rates of saturated juice precipitates. In the face of such a situation, when there is a significant content of magnesium in lime milk, the technologist tries to maintain a high pH value (not less than 11) on the first saturation, and before the second saturation not to give lime at all (Sukhenko et al., 2019). Such measures are carried out because Mg (OH)₂ begins to precipitate at pH 11 and above. It is in this state that this component (Mg(OH)₂) has a crystalline structure and is most easily removed from the system in contrast to magnesium carbonate, which has an amorphous structure.

Also, in the case of saturation of juices to a slightly alkaline or neutral reaction, Ca and Mg go into solution in the form of acidic carbon salts and at vigorous boiling of the juice precipitates at the evaporator station.

On the other hand, scientists (Mushtruk et al., 2020a) have studied the use of magnesium oxide for the purification of diffusion juice in the laboratory. As a result of research comparing the classical method of purification and the one proposed by the authors, it was found that the quality of purified juices and syrup was much better, namely the content of colloidal substances and color decreased, but the filtration performance deteriorated.

To date, Ukrainian scientists (Sheiko et al., 2019) have studied the effect of the use of dolomitized limestone on the process of purification of diffusion juice. As a result, recommendations for the use of dolomitized limestone in a sugar, factory have been developed.

Therefore, despite both the positive and negative effects of magnesium on the technological process of the sugar plant, it is very important to monitor the concentration of this component.

The current method of determining the mass fraction of magnesium carbonate in carbonate rocks DSTU 1451-96 (1996), is based on the acidimetric determination of the mass fraction of magnesium carbonate in limestone by calculating the percentage of trilon B solution, which went to titrate the magnesium content to the mass of limestone. The disadvantage of this method is the high error in determining the mass fraction of magnesium carbonate in limestone.

Other authors presented a method (Palamarchuk et al., 2019), which involves analysis by the complexometric method. The disadvantage of this method is its high cost because a large amount of sample is taken for titration, as well as significantly inflated test results.

It is impossible to give a complete objective chemical analysis of limestone without the use of correct methods of analysis of all components of the carbonate rock. Therefore, the inclusion of the proposed method for the determination of magnesium carbonate in the carbonate rock (Mushtruk et al., 2020b) and the method for the determination of calcium carbonate in the carbonate rock to (Somaratne et al., 2019) comprehensive determination of the chemical composition of limestone will provide objective indicators of CaCO₃ and MgCO₃. In turn, this will allow the technologist to take the necessary measures in advance to conduct the technological process, which will give the maximum economic effect and save natural resources (limestone, fuel).

Our research aimed to improve the method of determining the mass fraction of magnesium carbonate in the carbonate rock to increase the objectivity of the results and the introduction of an adjusted method to establish the full chemical composition of limestone.

To increase the accuracy of experimental data, the improved method should be based on the material balance of all components of the carbonate rock. This will make it possible to self-check the accuracy of the results obtained, as having summed up the balance of all components of limestone, approximately we should get a result of 100%. 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. Given this fact and improving the method of determining the content of calcium carbonate and magnesium carbonate in the carbonate rock and entering the results into the material balance, we hope to get the sum of all components approximately 100%, which will indicate the accuracy of the data.

The establishment of the component composition of limestone began with the determination of its moisture content. According to the results of the research (Figure 2), it is seen that all samples contained a low moisture content. 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, the concentration of moisture contained in the experimental samples of limestone will not affect the dissociation process, and consequently the technological process as a whole.

According to the recommendations DSTU 1451-96 (1996), which are put forward to carbonate rocks in Ukraine, to further use them in the sugar industry, the mass fraction of impurities insoluble in hydrochloric acid should not exceed 3%. The obtained data (Figure 3) indicate that samples No. 3, 6, 9, 10 contain an increased concentration of silicon dioxide. The authors of the following scientific papers (Bober et al., 2020; Luo et al., 2019; Dhar et al., 2015) other methods for determining impurities were used for similar scientific research.

During the firing of such limestones, refractory compounds such as calcium and magnesium silicates can be formed. It is these compounds (calcium silicates) that cause irreversible loss of lime. Once on the plant machine, silicates can settle on the heating surface of the evaporator. Given all the above negative consequences, the use of carbonate rocks under No. 3, 6, 9, and 10 in the sugar industry is not possible.

Limestone’s were further analyzed for the content of one and a half oxides of aluminum and iron (AI₂O₃ + Fe₂O₃). The authors of scientific works (Guo et al., 2018; Zheplinska, Mushtruk and Salavor, 2020; Jiang, 2015; Almohammed et al., 2015) recommends not to use such samples in the production process.

Super-important components, as they form low-melting aluminates and ferrites from CaO during firing, which are the main component of the liquid phase in the furnace. It (liquid phase) reduces the mass fraction of free lime in quicklime because a significant part of bound lime is formed (in the form of aluminates and calcium ferrites), and also envelops chemically active lime with non-reactive film, thereby increasing the concentration of inactive lime. Melting leads to soldering of pieces of limestone between themselves and the lining, which in turn increases the amount of overheating and the stability of the lining The authors of the following scientific works (Ivanova et al., 2021; Lee et al., 2012; Lebovka et al., 2007) recommend not to use such samples in the production process. The approximate composition of quicklime is given in (Noguiera Felix et al., 2019).

Impurities AI₂O₃ + Fe₂O₃ passing into lime milk and further on the machine of the plant get into juice promoting formation in the saturated juices of a deposit which is badly filtered.

According to the recommendations of DSTU 1451-96 (1996) AI₂O3 + Fe₂O₃, the content of one and a half oxides in limestones, which are suitable for firing in lime-fired kilns at the sugar factory, should be in the amount of not more than 1.5% by weight of dry matter. 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.

Observing Figure 3, we see that samples No. 1, 3, 5, 6, 7, 9, 10 contain an increased concentration of iron and aluminum oxides.

Therefore, given the above dangers, the use of limestone with a high content of AI₂O3 + Fe₂O₃ can not be recommended for the needs of the sugar factory.

The next test component was calcium sulfate (CaSO₄). The recommended normative value DSTU 1451-96 (1996) of CaSO₄ content in carbonate rock is not more than 0.4% by weight of dry matter. The greatest danger that this impurity can cause is that it easily precipitates on the surface of the evaporator. As well as the process of hydration of lime in the presence of CaCO4 and CaCO₃ is greatly slowed down.

As the results of research (Figure 4) show, samples No. 1, 3, 6, 10 exceed the recommended norms (calcium sulfate content), so their use will lead to the above negative consequences.

Determination of calcium and magnesium carbonate content was carried out as follows:

Pipette 15 cm³ of solution into a 250 cm³ conical flask, add 100 cm³ of distilled water and 25 cm³ of 20% sodium hydroxide solution from a volumetric flask containing the filtrate after precipitation of iron and aluminum oxides. After 1 – 2 min on the tip of a spatula made 0.1 – 0.2 g 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, which was observed on a black background. The authors of scientific works (Dehghannya et al., 2018; Zheplinska et al., 2021; Zhu et al., 2016; Zdziennicka et al., 2017) used other standardized methods to conduct similar studies.

In parallel with the breakdown, distilled water was monitored for calcium carbonate content. To control the distilled water for the content of calcium carbonate, 25 cm³ of 20% sodium hydroxide solution was added to the distilled water. After 1 – 2 min on the tip of a spatula made 0.1 – 0.2 g of murexide in a mixture with potassium chloride. The solution was stirred and titrated with 0.1 M solution of Trilon B until a transition from red to purple, which was observed on a black background.

The difference between the volume of 0.1 M solution of Trilon B, which went to the titration of the sample, which used the filtrate after precipitation of iron and aluminum oxides, and the volume of 0.1 M solution of Trilon B, which went to the titration, which controlled distilled water, makes it possible to set the volume of 0.1 M solution of Trilon B, which went to the titration of calcium – b, cm³. After that, the mass fraction of calcium carbonate in the carbonate rock was calculated (Smetanska et al., 2021).

In Ukraine, only high-calcium carbonate rocks are used for the sugar industry, in which the CaCO₃ content is at least 93% by weight of dry matter. The results of determining the content of the mass fraction of calcium carbonate in the carbonate rock are given in Table 1 and Figure 5.

Observing Figure 5 we see that samples No. 1, 3, 5, 6, 7, 9, 10 are not recommended for use in the technological process of sugar production due to the low content of calcium carbonate.

Control over the content of magnesium carbonate in the carbonate rock was carried out as follows:

The volume of Trilon B spent on titration of magnesium in the carbonate rock was calculated by the difference between the volumes of Trilon B spent on titration of calcium and magnesium – a, and separately calcium – b.

To determine the volume of Trilon B, which went to the titration of calcium and magnesium, from the filtrate after precipitation of iron and aluminum oxides, took a sample of 15 cm³, which corresponds to 0.15 g of carbonate rock and is a sample of carbonate rock – m, in grams, which was taken into account during the calculation of the mass fraction of magnesium carbonate in the carbonate rock according to formula 1. To the sample was added 100 cm³ of distilled water, 10 cm³ of ammonia buffer solution and on the tip of a spatula made 0.1 – 0.2 g of dark chromium acid indicator mixture, stirred vigorously with a glass rod, and titrated with 0.1 M solution of Trilon B until the transition of red-purple to blue. The author of (Sasikumar, Chutia and Deka, 2019) used other standardized methods to conduct similar studies.

In parallel with the breakdown, distilled water was monitored for magnesium and calcium carbonate content. Control of distilled water for the content of magnesium and calcium carbonate involves the introduction into the distilled water of 10 cm³ of ammonia-buffer solution and dark blue acid chromium indicator, followed by titration of water with 0.1 M solution of Trilon B. The difference between the volume of 0.1 M solution of Trilon B, spent on the titration of the sample, which uses the filtrate after precipitation of iron and aluminum oxides, and a volume of 0.1 M solution of Trilon B, spent on titration, where the control of distilled water, made it possible to establish the volume of 0.1 M solution of Trilon B spent on titration of calcium and magnesium – a, cm³.

The mass fraction of magnesium carbonate in the carbonate rock was calculated by the formula:

(1) X = [ ( a − b ) ⋅ K ⋅ 0.004498 ⋅ 100 m ] ⋅ 100 100 − W ,

Where:

The results of determining the mass fraction of magnesium carbonate in the carbonate rock are shown in Table 2 and Figure 7.

From Table 2 and Table 3, and Figure 6 we see that the results of research obtained by the method DSTU 1451-96 (1996) have slightly inflated indicators. Therefore, the above-mentioned improved method for determining MgCO₃ was proposed, which gives more accurate results (Table 3).

Given that the share of magnesium carbonate in carbonate rocks should not exceed 2.5% DSTU 1451-96 (1996), we see that samples No. 3, 5, 6, 9, 10 do not meet the stated standards. Therefore, in those samples where there is an inflated content of MgCO₃ technologist, it is necessary to correct the process following the recommendations given at the beginning of the article.

Control of alkali metal oxides of potassium and sodium (K₂O + Na₂O) is carried out to reduce the concentration of molasses in the system, which they are. K₂O and Na₂O also form fusible compounds during firing. The mass fraction of potassium and sodium oxides in the carbonate rock according to the recommendations DSTU 1451-96 (1996) should not exceed the amount of not more than 0.25% by weight of dry matter. Figure 8 shows that samples No. 1 and 3 cannot be recommended for use in the sugar industry.

To objectively conduct a technological audit of the lime department of the sugar factory, it is necessary to use the correct methods of control of the parameters of technological indicators. One component of the technological audit of the lime department is the chemical composition of the carbonate rock, which is fed for firing. The quality of finished products and their cost will mainly depend on the condition of raw materials. Therefore, to objectively assess the quality of the studied limestones, the concentration of all components included in them was summarized in one table (Table 3) and the balance of all impurities and calcium carbonate was adjusted for each sample. When determining the chemical composition of the carbonate rock, the sum of the mass fractions of CaCO₃ and impurities should be approximately 100%. From the Table 3 we see that all analyzed samples according to the method DSTU 1451-96 (1996) have inflated results. In turn, the lined material balance of all components of the carbonate rock and in all samples according to the proposed method (Cervantes-Elizarrarás et al., 2017) approached 100%. This indicates the high accuracy of the method and the possibility of its use in the production process.

Therefore, the use of the proposed method for determining the content of magnesium and calcium carbonate in limestone will allow the technologist to obtain timely objective information on the component composition of the carbonate rock. Knowing the exact concentration of MgCO₃, you can use its positive effect on the technological process, eliminating the negative factors of technological methods. Thus, as a result, we get finished products of good quality, save natural resources (limestone, fuel) and improve the ecological condition of the planet (by reducing harmful emissions into the atmosphere).