RHEOLOGY AND PHYSICOCHEMICAL ANALYSIS OF SERPENTINITE DECOMPOSITION IN NITRIC ACID

ABSTRACT

This article presents the results of a study of the rheological properties of a suspension obtained after decomposition of serpentinite from the Arvatin deposit with 100% nitric acid at a concentration of 30% at 90°C and for 3 hours. The raw material decomposition coefficient was 96.50%. In order to clean the solution from impurities, the suspension was neutralized with ammonia to a pH of 5.5. It was found that with an increase in temperature from 20 to 80 °C, all viscosity density indices decrease from 1.3365 to 1.2960 and from 9.81 to 4.89 cP, respectively. This behavior of rheological properties also had a favorable effect on the degree of precipitation of the neutralized nitric acid suspension. It was found that with an increase in time and temperature, the degree of precipitation from 1.77 to 52.29%.

АННОТАЦИЯ

В данной статье представлены результаты исследования реологических свойств суспензии, полученной после разложения серпентинита Арватинского месторождения 100% азотной кислотой при концентрации 30% при температуре 90°С в течение 3 часов. Коэффициент разложения сырья составил 96,50%. Для очистки раствора от примесей суспензию нейтрализовали аммиаком до рН 5,5. Установлено, что при повышении температуры от 20 до 80 °C все показатели плотности и вязкости уменьшаются от 1,3365 до 1,2960 и от 9,81 до 4,89 сП соответственно. Такое поведение реологических свойств также благоприятно влияет на степень осаждения нейтрализованной азотнокислой суспензии. Выявлено, что с увеличением времени и температуры степень осаждения возрастает с 1,77 до 52,29%.

Keywords: serpentinite, nitric acid, decomposition, neutralization, rheological properties

Ключевые слова: серпентинит, азотная кислота, разложение, нейтрализация, реологические свойства.

Introduction

Magnesium is the eighth most abundant element in the Earth's lithosphere (~2 wt.%) and is the third most abundant mineral in seawater, with a concentration of about 1,300 ppm. In nature, magnesium is present in three stable isotopes: ²⁴Mg (78.6%), ²⁵Mg (10.1%), and 26Mg (11.3%), with an average atomic mass of 24.31 amu. There are more than 60 different magnesium-containing minerals in nature, including chlorites, magnesites, pyroxenes, dolomites, amphiboles, saponites, and dolomitized limestones. It should be noted that the content of the main substance in magnesium-containing minerals is different. For example, brucite (Mg(OH)₂) has 41.7% Mg, magnesite (MgCO₃) - 28.8%, dolomite (MgCO₃ˑCaCO₃ - 18.2%, kieserite (MgSO₄ˑH₂O)- 17.6%, bischofite (MgClˑ6H₂O)- 12%, langbeinite (K₂Mg₂[SO₄]₃) - 11.7%, epsomite (MgSO₄ˑ7H₂O) - 9.9%, kainite (KMg(SO₄)Clˑ3H₂O) - 9.8%. In addition, a large reserve of magnesium is concentrated in seawater and marine brines, where the magnesium content in the first case is 0.13%) [1].

In Russia, deposits of serpentinite and related talc and nephrite are known in the Urals (Shabrovskoye, Bazhenovskoye), in the Sayan Mountains, and in Altai. Large deposits of ornamental serpentinite are found in the USA, Canada, Norway, China, Great Britain, and New Zealand [2].

Magnesium has properties several times superior to those of aluminum, iron and other metals, and is also a good conductor of electric current, which plays an important role in the field of metallurgy, mechanical engineering, instrument making and energy storage. In the pharmaceutical, paint and varnish, textile, electronic, chemical, petrochemical and other industries, magnesium and its compounds have proven themselves to be good candidates. Potential use of serpentinite is mainly aimed at the production of building materials (cement, concrete), ceramics, fertilizers (source of Mg), silicate materials (production of glass, silicon, zeolite, etc.), steel production (metal coating, additive r chromium plating of steel), filler for polymers, rubbers and additives for lubricants, geopolymers, adsorbents and catalysts (removal of heavy metals from wastewater, CO₂ sorption and synthesis of biofuels), composite materials (magnetic carbon nanotubes) [3-7].

The Republic of Uzbekistan has its own raw material base of magnesium-containing ores such as dolomite, sopanite, serpentinite, and a very small amount of magnesite. But for the rapid development of refractories, thermal insulation and building materials, raw materials must be processed into high-quality products: brucite, caustic magnesite, hydromagnesite, magnesite, etc. The fact is that the republic's total need for magnesite material (as magnesite refractories) is 18-20 thousand tons per year, and for the chemical industry 3.5 thousand tons. However, due to the lack of this type of raw material, Uzbekistan imports magnesite and other types of magnesite powders from abroad in the amount of 12,233.7 tons worth $ 7.5 million, of which burnt magnesia is about 4 thousand tons worth $ 1.5 million [8, 9]. It should be noted that about 100,000 tons of magnesium chloride are used per year in the production of chlorate-containing defoliants alone. The raw material for its preparation is bischofite supplied from Russia and China [10].

Previously, we investigated the process of decomposition of local serpentinite of the Arvatene deposit with nitric acid. The optimal condition was found and the thermodynamic indices and kinetic regularities were calculated, such as the degree of decomposition, the rate of decomposition and the activation energy, which are in the order of 11.48-82.50; 1.02 ˑ10^-2 τ^-1 - 14.53 ˑ10^-2 τ^-1 and 91.21-31.53 kJ/mol. The average value of the apparent activation energy was 59.68 kJ/mol, which means that the process is accompanied by chemically controlled surface [11].

Of no small scientific and practical interest in this area of ​​research is what exactly the insoluble residue of serpentinite is after its decomposition and neutralization with ammonia, followed by the precipitation of impurities as a whole.

Therefore, the aim of this study is the decomposition of raw materials, the study of their rheological properties and microscopic studies of the sediment of nitric acid decomposition of serpentinite from the Arvatene deposit.

Object and methods

Serpentinite of the following composition (wt.%) was used for laboratory research: SiO₂ - 40.2–41.5; CaO – 0.87-2.12; Al₂O₃ – 0.98-2.34; Fe₂O₃ – 7.27-7.78; MgO – 34.1-38.18; Cr₂O₃ – 0.306; NiO – 0.276; SO₃ – 0.76-1.20; H₂O – 4.35. It should be noted that serpentinite contains soluble SiO₂ – 39.7%. This type of raw material was ground in a laboratory porcelain mortar to a particle size of 0.25 mm. The process of decomposition of the raw material was carried out by 100% nitric acid with a concentration of 30% at a temperature of 90 ° C for 3 hours. The experimental technique is well described in the work [11]. Briefly, the interaction of raw materials with nitric acid in a glass vessel placed in a water thermostat and equipped with a screw stirrer with a speed of 250-300 rpm. The process temperature was regulated using a thermostat. After reaching the specified temperature, the calculated amount of raw material in the amount of 10 g was introduced into the vessel with nitric acid. In this case, the ratio of L:S was equal to 5:1. As soon as the decomposition time expired, the reaction mass was neutralized with gaseous ammonia to a pH value of 5.5. After that, the reactor contents were filtered on a Buchner funnel with a constant vacuum of 150 mm Hg (0.2 atm). The solid precipitate was washed with hot water (85-90°C) at a ratio of Precipitate : H₂O = 1 : 4. The washed precipitate was dried at 100-105°C. It should be noted that the components NiO and Cr₂O₃ are completely soluble in acid, and therefore an analysis was carried out for the content of MgO, CaO, Fe₂O₃ and Al₂O₃ using the methods [12, 13]. To determine the solubility of the components, the raw material decomposition coefficient (K_dec) was adopted as the main indicator.

The value of Kraz is calculated using the formula:

                                                                       (1)

where - content of the general form of MgO, Fe₂O₃ and Al₂O₃ in raw stock, g; - content of the general form of MgO, Fe₂O₃ and Al₂O₃ in in dry sediment.

Microscopic examination of dry sediment was performed using a Transmitted Light Microscope U 300 (made in India). Filming was performed using an ADF PRO20 camera on an ADF U300 reflected and transmitted light microscope, with 5x, 10x, 20x lenses. Specifications. Optical system - infinity. Contrast methods - bright field, transmitted light. Dispersed solution in glycerol. Field of view stitching using a 5x objective allowed us to display a larger field of view to obtain a picture of sample homogeneity. The software used to carry out measurements - MVImage allows you to determine the particle size and perform manual or automatic counting of objects. Linear dimensions are available, as well as the area of ​​the object in square microns.

Since the suspension of nitric acid suspension consists of a complex system of dispersed phases and media, the rheological properties of the nitric acid extract (nitric acid suspension) were determined under kinematic conditions.

The density of the suspension was determined by the pycnometric method, and the viscosity using a VPZh-2 capillary viscometer.

The density value was calculated using the formula:

                                                                                           (2)

where m is the mass of the pulp, g; v is the capacity of the pycnometer, cm³.

Viscosity was calculated according to the following formula:

                                                                                    (3)

where k is the viscometer constant equal to 0.2931.

The degree of precipitation of ammoniated nitric acid suspension was studied at temperatures from 20 to 80 °C and durations from 5 to 480 min.

The calculation of the degree of precipitation ω, % was carried out:

                                                                               (4)

where h₁ is the total height of the liquid, mm; h₂ is the height level of the unclarified part, mm.

Results and Discussion

Calculations of the decomposition coefficient showed that about 96.50% of serpentinite was opened by nitric acid at 90 °C for 3 hours. Figures 1a and b show the dependence of the change in density and viscosity of the nitric acid suspension neutralized with ammonia to pH 5.5 on temperature in the order of 20-80 °C. As follows from the results of the studies, with an increase in temperature, the density and viscosity of the solution decrease monotonously. In this case, one can see that the density and viscosity values ​​decrease with an increase in temperature from 1.3365 to 1.2960 and from 9.81 to 4.89 cP, respectively.

Figure 1. Temperature Dependence of Density (a) and Viscosity (b)

The effect of temperature on the degree of precipitation of ammoniated suspension is shown in Figure 2.

Figure 2. Dependence of the change in the degree of sedimentation on temperature and time

It follows from the figure that with an increase in the duration of time from 5 to 480 min and temperature from 20 to 80 °C, an increase in the degree of precipitation is observed in the order of 1.71 to 41.88 and from 2.75 to 52.29%, respectively. It should be noted here that the difference between the time and temperature of precipitation is in the order of 1.61-1.25 and 24.49-19.01 times.

Micrographs of the precipitate together with sesquimetal hydroxides are shown in Figure 3.

a) 50x magnification of solid phase particle micrographs	b) 100x magnification of particles micrographs of the solid phase

Figure 3. Micrographs of the sediment obtained after neutralization of the nitric acid suspension of serpentinite

As observed, the solid serpentinite sediment consists primarily of two distinct forms: amorphous dark prismatic to round-spherical iron hydroxide particles, ranging in size from 20 to 200 microns. Additionally, fine-grained aluminum hydroxide sludge and other minor phases exhibit a round to round-spherical morphology with no discernible internal structure. Their particle sizes range from 10 to 30 microns in diameter, and their coloration varies from colorless and transparent to yellow and brown translucent. Crystalline transparent prisms with sharp edges are identified as quartz, with particle sizes spanning from 10 to 200 microns. The composition of the sediment includes 20–30% amorphous iron hydroxide, 60–70% quartz, and the remainder consisting of microparticles of heterogeneous minerals.

The results of nitric acid decomposition and subsequent neutralization of serpentinite indicate that the rheological properties of the resulting pulp—such as sedimentation behavior, density, and viscosity—are suitable for storage and transfer between processing units. Microscopic analysis of the acid-insoluble solid phase of serpentinite from the Arvaten deposit suggests that the observed particle size distribution could aid in selecting appropriate decantation equipment.

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