The influence of initial concentration of sulfuric acid on the degree of ...

J Therm Anal Calorim (2016) 124:355–361 DOI 10.1007/s10973-015-5114-y

The influence of initial concentration of sulfuric acid on the degree of leaching of the main elements of ilmenite raw materials Maciej Jabłon´ski1 • Sandra Tylutka1

Received: 31 March 2015 / Accepted: 19 October 2015 / Published online: 6 November 2015 Akade´miai Kiado´, Budapest, Hungary 2015

Abstract There are a variety of incidents in the chemical industry, which occurred in bursts of heat at different stages of technological process and whose main cause was the reactive compounds. Consequences of unfortunate events were large loss of property and in some cases deaths. These events show how important are the conditions under which the process takes place and how important is knowledge about them. On the other hand, very important is the level of leaching of the main elements of the titanium raw materials, which influences the efficiency of the process. One of the important parameters affecting the safety and the degree of leaching of elements is the concentration of the sulfuric acid. In this research article, we propose parameter for determination of risk of thermal explosion in the reaction of sulfuric acid with titanium raw materials and results of investigation of sulfuric acid influence on the degree of leaching of the main elements. The investigations were realized for the Norwegian and Australian ilmenites analyzing the degree of leaching of elements such as TiO2, Fe2O3, MgO and MnO. Keywords Ilmenite Products of reaction Degree of leaching Titanium dioxide Runaway reaction

& Sandra Tylutka [email protected] 1

Institute of Chemistry and Environmental Protection, West Pomeranian University of Technology, Al. Piasto´w 42, 71-065 Szczecin, Poland

Introduction Chemical reactions, such as the resins polymerization, the reaction of the pharmaceutical industry and many others, are often exothermic and called hazard reactions [1, 2]. The reaction with a high coefficient of thermal explosion may also include reactions for preparing titanium dioxide. Titanium dioxide plays an enormous role in the global economy, due to its variety of applications. Titanium occurs in many natural mineral ores: ilmenite (FeOTiO2 or TiFeO3), rutile (TiO2) and leucoxene (Fe2O3nTiO2) are the major ones. Over 91 % of the demand for raw materials is an ilmenite, which contains 40–65 % TiO2. In 2010, the world’s ilmenite production reached about 5.8 million tons [3]. The biggest used of titanium ores is applicable in the production of titanium dioxide. TiO2 is one of the most common inorganic pigments in the world. The titanium ore where ilmenite is exploited industrially is located in countries such as Norway, India, China and Australia. Due to the significant differences in climate (temperature and humidity) and geological conditions (nature and age of the source rock deposits) in the places of origin, it can be expected the differences in chemical composition [4]. In order to obtain the highest quality of TiO2 from titanium ore, accompanying chemical compound should be removed by the use of proper technological processes. There are two commercial processes for titanium dioxide (TiO2) production: the sulfate process and the chloride process [5]. Reaction of the sulfuric acid with ilmenite is the first step in sulfuric technology of the titanium pigments production [6]. This reaction is highly exothermic and is characterized by high reaction temperature and the possibility of toxic gas emissions into the atmosphere [6, 7]. Owing to the risk of thermal explosion, reaction belongs to the hazard-type reaction.

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The next process is the dissolution of the reaction products in water. As a result of this process, we obtain two phases: a solution phase and the insoluble residue as a solid phase. Distribution of elements between these the two phases is important for the quality of the product. An important element might have an impact on safety, and the degree of leaching of elements is the reaction rate. The visible effect of the reaction is an increase in the temperature of reaction mixture caused by the accumulation of heat evolved during the process. The rate of temperature change of the reaction mixture is related to the rate of the reaction. Thus, the rate of the temperature change of the reaction mixture might has an impact on the degree of conversion of the raw material and further the degree of leaching of major elements present in the raw material titanium. The rate of temperature increase has an impact on the safety of the process, the higher the rate of temperature rise, the higher the probability of thermal explosion. To determine the optimum reaction, kinetics is necessary to find the relation between the rate of temperature rise, the degree of leaching of major elements, and the safety of the reaction. On thermokinetics reaction, sulfuric acid with ilmenite raw material influences initial concentration of the sulfuric acid, the initial temperature of the reaction, phase and elemental composition the material used and particle size distribution of the ilmenite raw material [8, 9]. One of the important factors that influence on the reaction rate is the concentration of the sulfuric acid, so the aim of this study was to determine the effect of sulfuric acid concentration on the thermokinetics reaction and the degree of leaching of major elements present in the raw material.

200–400 g. To calorimeter vessel sulfuric acid was introduced at a given concentration, and after reaching thermal equilibrium at a given temperature, the reaction was initiated by the introduction of ilmenite. Throughout the reaction, the amount of heat generated is analyzed, which informs about thermokinetics process. After completing the reaction, reaction products were dissolved in water for approximately 2 h at a temperature of about 50 C. After completing the leaching process, obtained suspension was filtered. The filter cake after washing was calcined at 900 C for 1 h. After cooling, obtained sample was analyzed using a Philips PW1480 XRF spectrometer. The composition of ilmenite samples was also analyzed using XRF spectrometer.

Results and discussion The evaluation of the thermokinetics reaction is necessary to analyze the changes of thermal power during the process. The nature of these changes is the basic information regarding the safety of running reactions. On the one hand, the higher the maximum thermal power reached during the reaction, the higher the probability of thermal explosion is, while on the other hand, the low heat output might indicate a unreacted mass. A factor that significantly affects the value of the thermal power arising in the course of the reaction is the initial concentration of sulfuric acid. Figures 1 and 2 show the measurement results of thermal power for the different initial concentrations of sulfuric acid with Australian and Norwegian ilmenites. These two raw materials are significantly different in both elemental and phase composition. In the case of Norwegian ilmenite, content of main and selected associated elements determined using XRF spectrometer was as follows (as oxides):

Investigations of ilmenite with sulfuric acid reaction were realized in the calorimeter described in [10] because the particular conditions in which the reaction occurs (highly corrosive environment, the possibility of thermal explosion, emission of gases, etc.) were used calorimeter own design. Ilmenite reaction with sulfuric acid was investigated in non-isothermic and non-adiabatic calorimeter with isothermal cover (isoperibol). The main unit of the apparatus was a calorimetric vessel volume about 0.6 dm3, equipped with a heater, a stirrer, a temperature Pt 100 sensor, feeder and safety valve. The installed electric heater served to calibrate the calorimeter determination of time constant (257.5 min) and heat transfer coefficient (0.098 J K-1 s-1). The used sample of ilmenite was about 100 g, and mass of sulfuric acid was in the range of

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Thermal power/W g–1

Experimental

2.5 2.0 1.5 1.0 91.8

0.5

88.2

0 20

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80

Time/min

100

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83.5 Concentration H2SO4 /%

Fig. 1 Thermal power changes in calorimeter during the reaction of Norwegian ilmenite with different initial concentrations of sulfuric acid

91.8 88.2 20

40

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Time/min

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84.4 Concentration H2SO4 /%

Fig. 2 Thermal power changes in calorimeter during the reaction of Australian ilmenite with different initial concentrations of sulfuric acid

TiO2—44 %, Fe2O3—51.5 %, MgO—4.4 % and MnO— 0.3 %; however, in the case of Australian ilmenite, the content was: TiO2—52.3 %, Fe2O3—47.5 %, MgO— 0.25 %, MnO—1.6 %. The main phases in Norwegian ilmenite are: ilmenite (FeTiO3), hematite (Fe2O3), geikielite (MgTiO3), enstatite (MgSiO3) and MnTiO3, whereas, in Australian ilmenite, the main phases are: ilmenite (FeTiO3), pseudorutile (Fe2Ti3O9), hematite (Fe2O3), geikielite (MgTiO3) and MnTiO3 [8]. The reactions of the individual components of titanium raw materials with sulfuric acid are presented as follows: TiO2 þ H2 SO4 ! TiOSO4 þ H2 O FeO þ H2 SO4 ! FeSO4 þ H2 O Fe2 O3 þ 3H2 SO4 ! Fe2 ðSO4 Þ þ 3H2 O MgO þ H2 SO4 ! MgSO4 þ H2 O MnO þ H2 SO4 ! MnSO4 þ H2 O The thermokinetics curves in Figs. 1 and 2 show two characteristic peaks. The first is the moment of initiation of the reaction (narrow peak), associated with the process of wetting the interfacial surface of the titanium raw material with sulfuric acid. The second much larger peak corresponds to a maximum output and is related to the maximum rate of the reaction. Both graphs show the value of the maximum heating power and the time it became strongly dependent on the initial concentration of sulfuric acid. This dependence is nonlinear as shown in Fig. 3, which illustrates the influence of the initial concentration of sulfuric acid on the maximum thermal power in reaction with the Norwegian ilmenite. The value of the thermal power varies in the range of 0.5–2.6 Wg-1, reaching a maximum value in the range of sulfuric acid concentrations of 87–88 %.

Maximum of thermal power/W g–1

0.7 0.6 0.5 0.4 0.3 0.2 0.1 0

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3 2.5 2 1.5 1 0.5 0 82

84

86

88

90

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Concentration H2SO4 /%

Fig. 3 Influence of the initial concentration of sulfuric acid on the maximum thermal power in the reaction with the Norwegian ilmenite

In conversion per one ton of raw material, maximum of thermal power varies in the range of 0.5–2.6 MW. This creates a real danger of thermal explosion. Significant difference between the titanium raw materials (Australian and Norwegian ilmenites) is associated with achieved maximum heat output, and with the width of the main peak which is associated with the rate of the reaction. In the case of Australian ilmenite, thermal power values are lower, while the peak width is larger. Lower concentrations of sulfuric acid may be observed to be unstable nature for the reaction; at a concentration of sulfuric acid 88.2 %, there is an extra small peak associated with the solidification of the reaction mass. Figure 4 shows the influence of the initial concentration of sulfuric acid on the maximum thermal power in reaction with the Australian ilmenite. The obtained values of the maximum thermal power are much lower than in the case of Norwegian ilmenite 0.9

Maximum of thermal power/W g–1


The influence of initial concentration of sulfuric acid on the degree of leaching of the main…

0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 83

85

87

89

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Fig. 4 Influence of the initial concentration of sulfuric acid on the maximum thermal power in the reaction with the Australian ilmenite

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3.5

ΔTmax /Δτ /K min–1

3 2.5 2 1.5 1 0.5 0 82

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86

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Fig. 5 Influence of the initial concentration of sulfuric acid on the parameter DTmax/Ds in the reaction with the Norwegian ilmenite

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3

ΔTmax /Δτ /K min–1

2.5 2 1.5 1 0.5 0 83

85

87

89

91

93


Fig. 6 Influence of the initial concentration of sulfuric acid on the parameter DTmax/Ds in the reaction with the Australian ilmenite


(Fig. 3), and the maximum of these values is in the concentration range 89–91 %. The maximum thermal power values are in the range of 0.4–0.7 Wg-1, and the lowest values are obtained for the reaction of the Norwegian ilmenite with sulfuric acid. Figure 4 shows the reaction of sulfuric acid with Australian ilmenite, which is the process with little risk of explosion heat (thermal explosion) input. In industrial conditions, values of the thermal power were determined relatively difficult, and a much easier way is parameter DTmax/Ds. This parameter can be defined as the ratio of the maximum temperature increase in response time to the time in which this growth has been achieved. Figures 5 and 6 show variations of this parameter depending on the initial concentration of sulfuric acid. Similar nature of changes are observed in the case of thermal power depending on the initial concentration of sulfuric acid (Figs. 3 and 4). Maximum values DTmax/Ds occur in the same concentration range 87–88 % in case of Norwegian ilmenite. A similar relationship is observed in the case of ilmenite Australian (Figs. 4 and 6) with maximum values in the range of concentrations 89–91 %. As mentioned previously, one of the important parameters affecting the shape of the thermokinetics curve is phase and elemental composition the ilmenite raw material [9]. These parameters also influence the degree of separation between the solutions and the solid phase of reaction products in the leaching process. In order to compare the results of the investigations of individual raw materials should be chosen suitable reaction conditions for each of the raw materials. Based on extensive investigations, following conditions for the Norwegian ilmenite reaction with sulfuric acid are as follows: initial temperature of 80 C, the initial concentration of sulfuric acid—84 %, and the particle size distribution of the raw

0.7 0.6 0.5 0.4 0.3 0.2 0.1 0

C B

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A

Time/min

Fig. 7 Thermal power changes in calorimeter during the reaction of ilmenite raw materials with sulfuric acid concentration of 84 % for different initial temperatures, A—Australian ilmenite, T0 = 80 C, B—Australian ilmenite, T0 = 90 C, C—Norwegian ilmenite, T0 = 80 C

material in a range of values from 8 to 12 % of the residue on the sieve 40 lm. Figure 7 shows the changes of thermal power during the reaction of Norwegian and Australian ilmenites in the above conditions and additionally for Australian ilmenite with increased initial temperature of reaction to 90 C. Reaction of Australian ilmenite with sulfuric acid at initial temperature of 80 C takes a longer time compared to Norwegian ilmenite, reaching considerably lower value of thermal power. Additionally, in the final stage the reaction is unstable. Such course of reaction may indicate incomplete conversion of the raw material in these conditions. As a result of temperature increase to initiate the reaction to 90 C in the case of Australian ilmenite, we obtain comparable changes of thermal power as in the case of Norwegian ilmenite reaction with sulfuric acid.

The influence of initial concentration of sulfuric acid on the degree of leaching of the main… 100

Degree of leaching/%

95 90 85 80 75 70 82

84

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Concentration H2SO4 /% TiO2

Fe2O3

Fig. 9 Degree of leaching of TiO2 and Fe2O3 from reaction mixture, depending on the acid concentration in the reaction with Norwegian ilmenite

100 95


The same investigation of thermal power changes was realized for a higher concentration of sulfuric acid 88 %, and the measurement results are shown in Fig. 8. Similarly to the previous case (Fig. 7), Australian ilmenite with sulfuric acid reaction at initial temperature of 80 C takes a longer time compared to the Norwegian ilmenite. Increasing the initial temperature of the reaction causes a significant acceleration. Despite the initial temperature of the reaction increase, maximum of thermal power in the reaction is lower compared to Norwegian ilmenite. This indicates a milder reaction. On the basis of measurements for further study in the case of Australian ilmenite, 90 C was accepted as a more optimal temperature to initiate the reaction. Figure 9 shows the obtained measurements results of the degree of leaching of TiO2 and Fe2O3 contained in Norwegian ilmenite, depending on the concentration of sulfuric acid. The presented results of investigation show that the maximum values of the degree of leaching in the case of TiO2 are in the range 94–95.5 % at a concentration of sulfuric acid 83–84 %. A similar degree of leaching is observed in the case of Fe2O3. Above the concentration of sulfuric acid, 84 % is observed the decrease the degree of leaching. At concentration of sulfuric acid about 87–88 %, this value falls below 90 %. On further increasing the acid concentration, the degree of leaching drops to a level of about 85 %. The conversion of Fe2O3 shows the same nature as the leaching of TiO2, and the differences between them are not large and do not exceed 2.5 %. Titanium in Norwegian ilmenite occurs mainly in two phases: FeTiO3 (ilmenite) and MgTiO3 (geikielite), while the iron is present in phases: ilmenite and hematite (Fe2O3) [8].

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90 85 80 75 70 65 60 82

84

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Concentration H2SO4 /% MgO

MnO


Fig. 10 Degree of leaching of MgO and MnO from reaction mixture, depending on the acid concentration in the reaction with Norwegian ilmenite

2.5 2.0 1.5 1.0 C

0.5

B

0 20

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A

Time/min

Fig. 8 Thermal power changes in calorimeter during the reaction of ilmenite raw materials with sulfuric acid concentration of 88 % for different initial temperatures, A—Australian ilmenite, T0 = 80 C, B—Australian ilmenite, T0 = 90 C, C—Norwegian ilmenite, T0 = 80 C

Figure 10 shows the degree of distribution contained in Norwegian ilmenite magnesium and manganese elements between the solution and the solid phase. Magnesium oxide (MgO) in the Norwegian ilmenite occurs as an accompanying element, and the contents do not exceed 4.5 %. The degree of leaching of this element is in the range 72–83 %, and the nature of the changes depending on the acid concentration does not show a particular regularity (lack of a characteristic maximum as in the case of TiO2 and Fe2O3). This dependence may be due to the type of phases in which the magnesium occurs in the Norwegian ilmenite: TiMgO3 (geikielite) and MgSiO3 [8]. According to the Jablonski [11], MgSiO3 weakly reacts with sulfuric acid and a large part remains in its solid phase. In the case of manganese, which occurs mainly in the MnTiO3 [4], the nature of the changes of leaching differs

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360 100


95 90 85 80 75 70 82

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Concentration H2SO4 /% TiO2

Fe2O3

Fig. 11 Degree of leaching of TiO2 and Fe2O3 from reaction mixture, depending on the acid concentration in the reaction with Australian ilmenite

100


95 90 85 80 75 70 82

84

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Concentration H2SO4 /% MnO

Conclusions

MgO

Fig. 12 Degree of leaching of MgO and MnO from reaction, depending on the acid concentration in the reaction with Australian ilmenite

significantly from the degree of distribution of MgO, however, and is similar to the nature of changes of leaching of TiO2 and Fe2O3. Figure 11 shows the degree of distribution contained in Australian ilmenite titanium and iron between the solution and the solid phase. The highest concentration of TiO2 and Fe2O3 in solution phase in this case is obtained for the concentration of sulfuric acid in the range 85–89 %. It can be concluded that this range of concentration moves to the solution phase about 90–95 % titanium and iron.

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A small difference in the degree of leaching between Fe2O3 and TiO2 is observed, similarly as in the case of the Norwegian ilmenite. Titanium as described by Jablonski et al. [8] in Australian ilmenite occurs in two main phases: FeTiO3 (ilmenite) and Fe2Ti3O9 (pseudorutile), similarly as in the case of iron. This implies that the degree of leaching of these elements from the reaction of sulfuric acid will be similar. The relatively high degree of leaching (85–97 %) shows the magnesium from Australian ilmenite (Fig. 12). The nature of variation of this parameter is similar to that as in the case of the iron and titanium. The nature of this parameter depending on the concentration of sulfuric acid is similar to that of TiO2 and Fe2O3. Magnesium in Australian ilmenite occurs in the form of MgTiO3 [8]. Also, the degree of leaching of manganese in Australian ilmenite in the reaction with sulfuric acid obtains the level of 86–90 %. The nature of variability of this parameter is similar to the previously mentioned elements. Manganese just like magnesium in Australian ilmenite occurs as MnTiO3. The results show some scatter, which may be due to classical measurement error, and high heterogeneity of the titanium raw material [4]. The solubility of the reaction products influences the presence of other accompanying elements in the ilmenites. Even small changes in the concentration of various elements may influence the final distribution of the elements between solution and the solid phase, which results from the solubility products.

Maximum of thermal power generated during the reaction of the sulfuric acid with Norwegian ilmenite occurs in the concentration range of sulfuric acid 87–88 %. In the same range of concentrations, parameter DTmax/Ds also obtains the maximum value and the maximum degree of leaching of TiO2 and Fe2O3 occurs in the concentration range of sulfuric acid 83–84 %. It can be concluded that the maximum value of the leaching of these elements appear at lower value of thermal power and DTmax/Ds parameter, which indicates a lower rate of reaction that creates less risk of thermal explosion. In the case of the reaction of Australian ilmenite with sulfuric acid, maximum of thermal power and DTmax/Ds parameter occurs in the concentration range 89–91 %. The maximum values of leaching of elements TiO2 and Fe2O3 occur in a concentration range of 86–89 % and

The influence of initial concentration of sulfuric acid on the degree of leaching of the main…

indicate lower rates of reaction. In the case of Australian ilmenite, obtained values of thermal power and the parameter DTmax/Ds are significantly lower compared to the values obtained for the Norwegian ilmenite; therefore, these values indicate a lower probability of thermal explosion. On the basis of the results, it can be concluded that the parameter DTmax/Ds can be used to estimate the safe range of thermokinetics of titanium raw materials reaction with sulfuric acid. The degree of leaching of TiO2 and Fe2O3 is on the similar level, and this difference is not more than 2.5 % for both ilmenite ores. The degree of leaching of more than 90 % for TiO2 and Fe2O3 was obtained in reaction with a sulfuric acid concentration range of 83–88 % for Norwegian ilmenite and 85–90 % for the Australian ilmenite. On the other hand, the degree of leaching of magnesium (MgO) in the Norwegian ilmenite reached the level of 82–90 %. The occurrence of MgSiO3 (weakly reacts with sulfuric acid) in Norwegian ilmenite is probable the reason a lower degree of leaching of MgO. Manganese in Norwegian ilmenite is present in a small amount (0.3 %), but in investigated range of concentrations, the degree of leaching was in the range 85–95 %, similarly as in the case of TiO2 and Fe2O3. In Australian ilmenite, manganese and magnesium obtain the similar range of the degree of leaching as titanium and iron.

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