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Original article / Оригинальная статья УДК 669.782
DOI: http://dx.d0i.0rg/l0.21285/1814-3520-2020-2-444-459
Effect of silicon oxide reduction operational aspects on material and heat flow ratio in ore-thermal furnace
© Nikolay N. Zobnin****, Sailaubai O. Baisanov**, Alibek S. Baisanov**, Azat M. Musin**
*Tau-Ken Temir LLP, Karaganda, Kazakhstan,
**Zh. Abishev Chemical-Metallurgical Institute, Karaganda, Kazakhstan
***Karaganda State Industrial University, Temirtau, Kazakhstan
Abstract: A study was carried out into the effect of the loading method and surface distribution of charges at the furnace top on the parameters of carbon reduction of silica in submerged arc furnaces. In the study, the direct physical modelling method was applied. The experiments were carried out under large-scale laboratory conditions using a 200 kVA single-electrode furnace of the Zh. Abishev Chemical and Metallurgical Institute (Kazakhstan). An additional dynamic industrial experiment was conducted in the 30 MVA furnace of Tau-Ken Temir LLP. In a large laboratory furnace, two smelting campaigns were performed, each using different approaches to the treatment of the furnace top. In the first campaign, no balancing in the energy and material flows of the system took place, resulting in forced slipping and an uncontrolled feed rate of the charge. The second campaign, conversely, included the specified balancing measures. A similar study took place over 3 days under industrial conditions. The following technological parameters were empirically determined: specific electricity consumption, furnace average active power and performance, as well as power per unit area of the furnace hearth. The Fe, Al, Ca, Si material balance of the smelting was compiled. In order to assess the load balance, the concept of charge excess in relation to charge demand at the current furnace power was used for the first time. This concept value was determined as the ratio of number of batches actually loaded relative to the theoretically-calculated number ensuring the harmonisation of material and energy flows in the furnace. As a result of the research, an increase in the interval between the furnace top treatments up to 30 min and maintenance of harmonisation between material and thermal flows in the ore smelting furnace was established for increasing silicon extraction efficiency by 10-15%. In addition, oscillations in the phase current were stabilised. The proposed balancing concept was established to support the rapid elimination of crisis conditions in an industrial furnace.
Keywords: silicon metallurgy, technical silicon, thermal and material balances, electric current distribution, ore-thermal furnace, charge loading
Acknowledgments: The authors are deeply grateful to the management of Tau-Ken Samruk NGK JSC, the parent company of Tau-Ken Temir LLP, namely the chief business development director of Tau-Ken Samruk NGK JSC A. N. Ar-shabekov, who supported the financing of present scientific research as part of the execution of the contract No. 04-02437 of November 10, 2017 on the topic: "Development of the technology for smelting of commercial silicon using a br i-quetted mono-charge obtained from various types of low-ash reducing agents and gas cleaning dust (microsilica)". We are grateful to all the operators and employees of Tau-Ken Temir LLP and the Zh. Abishev Chemical and Metallurgical Institute, who showed patience and understanding in the implementation of this work, and hope to continue in mutually beneficial cooperation.
Information about the article: Received January 09, 2020; accepted for publication March 11, 2020; available online April 30, 2020.
For citation: Zobnin NN, Baisanov SO, Baisanov AS, Musin AM. Effect of silicon oxide reduction operational aspects on material and heat flow ratio in ore-thermal furnace. Vestnik Irkutskogo gosudarstvennogo tehnicheskogo universiteta = Proceedings of Irkutsk State Technical University. 2020;24(2):444-459. https://doi.org/10.21285/1814-3520-2020-2-444-459
Влияние операционных аспектов процесса восстановления оксида кремния на соотношение материального и теплового потоков в рудно-термической печи
Н.Н. Зобнин****, С.О. Байсанов**, А.С. Байсанов**, А.М. Мусин**
ТОО «Tau-Ken Temir», г. Караганда, Казахстан
**Химико-металлургический институт им. Ж. Абишева, г. Караганда, Казахстан ***Карагандинский государственный индустриальный университет, г. Темиртау, Казахстан
ВЕСТНИК ИРКУТСКОГО ГОСУДАРСТВЕННОГО ТЕХНИЧЕСКОГО УНИВЕРСИТЕТА 2020;24(2):444-4Б9
Резюме: Цель - влияние способа осуществления технологических операций загрузки сырья на колошник печи и его распределение по поверхности на показатели процесса восстановления кремнезема углеродом в печах с погруженной электрической дугой. Применяли прямое физическое моделирование. Эксперименты проводились в крупнолабораторных условиях на одноэлектродной печи мощностью 200 кВА Химико -металлургического института им. Ж. Абишева (Казахстан). Также производили активный промышленный эксперимент в печи мощностью 30 МВА ТОО «Tau-Ken Temir». В крупнолабораторной печи проведено две кампании плавок с различным подходом к обработке колошника: в первый период - без соблюдения баланса энергетического и материального потоков в системе с принудительной осадкой шихты неконтролируемой интенсивностью подачи шихты, во втором - с обеспечением указанного баланса. Аналогичная работа проводилась в течение 3 суток в промышленных условиях. Были рассчитаны фактически достигнутые технологические показатели: удельный расход электроэнергии, средняя активная мощность и производительность печи, мощность на единицу площади пода печи. Составлен материальный баланс плавки по Fe, Al, Ca, Si. Впервые для оценки сбалансированности загрузки использовали понятие избытка шихты по отношению к потребности в шихте при текущей мощности печи. Принимали эту величину как отношение фактически загруженных в печь навесок к теоретически рассчитанному количеству навесок, обеспечивающему гармонизацию материального и энергетического потоков в печи. В процессе исследований было доказано, что увеличение интервала между обработками колошника до 30 мин и достижение гармонизации между материальным и тепловым потоками в рудно-термической печи позволяют увеличить извлечение кремния минимум на 10-15%. Стабилизируются колебания фазного тока. Установлено, что на основе концепции балансирования возможно в краткие сроки вывести промышленную печь из сложных кризисных состояний.
Ключевые слова: металлургия кремния, технический кремний, тепловой и материальный балансы, распределение электрического тока, рудно-термическая печь, загрузка шихты
Благодарности: Авторы высказывают глубокую благодарность руководству АО «НГК «Tau-Ken Samruk», которая является материнской компанией ТОО «Tau-Ken Temir»: главному директору по развитию бизнеса АО «НГК «Tau-Ken Samruk» А.Н. Аршабекову, поддержавшему финансирование данных научных исследований в рамках исполнения договора № 04-02-437 от 10 ноября 2017 г. по теме: «Разработка технологии выплавки технического кремния с использованием брикетированной моношихты, полученной из различных видов низкозольных восстановителей и пыли газоочистки (микросилики)». Благодарны всем операторам и сотрудникам ТОО «Tau-Ken Temir» и Химико-металлургического института им. Ж. Абишева, которые проявили терпение и понимание при осуществлении этой работы, и надеемся на продолжение взаимовыгодного сотрудничества.
Информация о статье: Дата поступления 09 января 2020 г.; дата принятия к печати 11 марта 2020 г.; дата он-лайн-размещения 30 апреля 2020 г.
Для цитирования: Зобнин Н.Н., Байсанов С.О., Байсанов А.С., Мусин А.М. Влияние операционных аспектов процесса восстановления кремния на соотношение материального и теплового потоков в рудно -термической печи. Вестник Иркутского государственного технического университета. 2020. Т. 24. № 2. С. 444-459. http://dx.doi.org/10.21285/1814-3520-2020-2-444-459
1. INTRODUCTION
smelting furnace (OSF). In industrial settings, both single- and three-phase ore smelting furnaces are utilised. Here, a key role in the smelting process is played by the magnitude of the current flowing through each electrode. At the same time, temporal changes in current magnitude also appear to be of great importance. An example of a phase current diagram under conditions of silicon production at the FESIL RANA Metall AS company (Norway) is shown in Fig. 1 [16]. As can be seen from the figure, the arc repeatedly jumps onto the cavity wall, about 30 cm up the lateral surface of the electrode, and then gradually descends again after a period of about half an hour ending the full charge "accumulation-collapse" cycle having 60 min duration.
The production of silicon is based on carbon thermal reduction of silica-containing raw materials in an ore smelting furnace. This development of this branch of metallurgy proceeds in the directions of expanding the silica-containing raw material base and carbon reducing agents, as well as solving various environmental problems associated with silicon production1 [1-14]. Thus, despite positive results achieved in terms of technological parameters, the problem of improving parameters for commercial silicon production remains highly relevant [15-17]. However, smelting parameters are often affected by instability associated with the electrical mode of the ore
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Fig. 1. Example of an electric arc phase current diagram Рис. 1. Пример электрической дуги
Although under ideal silicon smelting conditions, a constant phase current should be ensured, in practice, this is unachievable, since changes in current strength are determined by the various technological operations. Among the most important of these are charge loading and OSF top treatment.
The charging of furnaces is carried out in a cyclic manner. At the first stage, fresh raw material is loaded in the top without movement. Next, heating proceeds with the removing of moisture and volatiles. In this case, a fresh charge is supported by a layer of raw material sintered into the monolith. This monolith forms the cover of the gas cavity (Fig. 2). At the bottom of the cavity, thermally-prepared materials are localised, having fallen there during the previous smelting period, referred to in terms of "accumulation". At the bottom of the cavity, reduction processes occur with the formation and accumulation of liquid silicon. During this period, the volume of the cavity increases. The electric arc moves from the side of the electrode down to the end part. The majority of the current flows not through the solid wall of the cavity, but rather through the liquid melt. The electrical resistance of the melting bath decreases, leading to a gradual increase in current from the minimum to the maximum value (see Fig. 1). At the end of the accumulation, the partially-melted sintered layer flows down under the fresh charge. During this time, the fresh charge has already been heated up and is losing moisture and
volatiles due to evaporation. The thickness of the sintered layer in the upper part of the cavity then decreases, followed by a new portion of the charge falling into the cavity under its own weight. This period is referred to as "charge collapse".
The new portion of raw materials accumulating at the bottom of the cavity cools the melt, thereby reducing its electrical conductivity. The strength of the arc current in the end part of the electrode ("end current") is reduced and a current redistribution appears in the increase direction of the side arc current ("side current"). A gradual decrease in the current is observed due to an increase in the electrical resistance of the smelting bath. As can be seen from the diagram presented in Fig. 1, the periods of accumulation and collapse, involving the possible appearance of anomalies, last about 30 minutes. Thus, following the end of the collapse period, at about 10:15, a period of sharp increase in current strength is observed for 5 min (see Fig. 1). Such sharp fluctuations are undesirable, resulting in a violation of the OSF cyclic operation, increased energy consumption and reduced extraction of silicon (XSi). Several possible explanations for such fluctuations can be provided. Some malfunctions in furnace operations can be attributed to operator error when loading raw materials into the furnace using a special machine for their surface distribution (Fig. 3).
1
Katkov OM. Smelting of commercial silicon: textbook. Irkutsk: ISTU Publishing House, 1999. 243 p. / Катков О.М. Выплавка технического кремния: учеб. пособ. Иркутск: Изд-во ИрГТУ, 1999. 243 с.
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Fig. 2. Schematic representation of the area around one electrode (based on the drawings of Schei, Tveit and Tuset [15]) Рис. 2. Схематическое изображение области вокруг одного электрода (данный рисунок основан на рисунках Шея, Твита и Тусета [15])
Fig. 3. Surface distribution of raw materials in the furnace: a - correct operation; b - incorrect operation Рис. 3. Распределение сырья на поверхности печи: a - правильное ведение операций;
b - неправильное ведение операций
If the operator inserts the working mechanism too deeply into the OSF, wall destruction occurs in the silicon carbide-containing cavity where low electrical resistance is present. When SiC falls to the bottom of the cavity, the current increases rapidly due to the low electrical resistance of silicon carbide. A similar carbide effect on the distribution of current (lateral and end) is described in detail in the literature [18, 19]. This represents a negative factor: the mechanism of the process changes, carbide in the normal state acts as a collector of gaseous silicon monoxide and reaction (1) proceeds. In the case of silicon carbide entering the bottom of the cavity, reaction (2) occurs [2]. When the silicon
carbide in the wall of the cavity is consumed, silicon monoxide is carried out by the exhaust gases from the furnace, resulting in decreased silicon extraction:
SiCs + SiOg = 2Sil + COg, (1)
2SiCs + SiO2l = 3Sil + 2COg. (2)
In addition, if insufficient time is provided for heating the charge in the upper part of the furnace, it enters the cavity too early. As a result of this, the temperature at the bottom of the cavity decreases. The cold charge then mixes with liquid products, giving rise to a high viscosity magmatic "swamp" formed at the bot-
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tom of the cavity. During the tapping of liquid silicon, this magma closes the tap hole and prevents liquid silicon from flowing freely. In such cases, high-viscosity magma has to be extracted through the tap hole using manual devices (Fig. 4 contains a sample of such material extracted through the tap hole of the OSF).
The magmatic mixture has the form of cylinders with a diameter of 100-150 mm and a length of 3-4 m with characteristic transverse notches from the hand tool.
In cases where OSF operators lack adequate training, the number of above-described smelting violations increases. In trying to speed up the process, such a poorly-trained operator typically loosens the raw materials in the upper part of the furnace when loading the charge by immersing the working device of the processing machine deep in the charge. As a result, the current diagram acquires the form shown in Fig. 5 (this diagram was obtained under the production conditions of Tau-Ken Temir LLP). At this enterprise, the furnaces are installed according to the description provided in a previous work [20]. As can be seen from Fig. 5, the phase current is very unstable. These effects are partially explained in terms of phase transformations of quartz into its modification, cristobalite, and the physical processes of softening and melting of quartz [21, 22]. From the standpoint of physical modelling, the cyclical nature of the side and end current distribution is described by the authors of [19]. However, is also necessary to establish a connection between the process model and the practical implementa-
tion of technological operations.
Intensive loosening of the charge leads to an imbalance between material and thermal flows in the furnace. Observing the reduction in the volume of the cavity, a poorly-trained operator may wrongly conclude that the next portion of the charge should be fed into the furnace. Thus, the actions of the operator are based on a subjective assessment of the OSF loading requirement, but the quantity of charge introduced into the furnace is not provided with sufficient energy. The importance of accurate exergetic balancing is discussed by the authors in [23]. However, this technique must also be coordinated closely with the order of the operator actions, with raw materials loaded per unit of time in accordance with the consumption of electricity for the same previous period. With insufficient energy, the reaction remains incomplete, stopping at the stage of SiO formation. This partially explains the increased formation of silica fume under the conditions of Tau-Ken Temir LLP equal to 1 t/t Si.
It is important that the tapping of liquid silicon be accomplished following 1-2 cycles of "accumulation - collapse", i.e. at the end of accumulation under maximum phase current strength and not at an arbitrary moment. If the tapping of silicon is carried out during the collapse period, the mixture falling to the bottom of the cavity will impede the tapping.
However, since there are differences of opinion among experts in the field concerning the above-described approach to OFS charging, the present study is devoted to demonstrating the correctness of the concept.
Fig. 4. Magmatic mixture of fused silica and slag with various content of residual silicon oxide and reduced silicon Рис. 4. Магматическая смесь плавленого кварца, шлака с различным содержанием неразложившегося
оксида кремния и восстановленного кремния
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Fig. 5. An example of the phase current diagram under conditions of incorrect technological operations
at Tau-Ken Temir LLP with an "accumulation - collapse" cycle time of 15-20 min Рис. 5. Пример диаграммы фазного тока в условиях неправильного ведения технологических операций ТОО «Tau-Ken Temir» с продолжительностью цикла «накопление - обвал шихты» 15-20 мин
2. MATERIALS AND METHODS
At the initial stage, the study of the charge loading operation was carried out in a 200-kVA electric arc furnace located in the laboratory of the Zh. Abishev Chemical and Metallurgical Institute (Karaganda, Kazakhstan). The main silica-containing raw material used for the study was quartz obtained from the Sarykul deposit. The chemical and granu-lometric composition of raw materials and car-
bonaceous reducing agents are presented in [20].
The study was carried out in a two-electrode electric furnace with one electrode coked in the hearth with a bottom mass (Fig. 6), that is, the electric furnace has a structure similar to an Mige type electric furnace (Japan). The transformer was powered by a voltage of 380 V. The electric furnace was powered from two OSU-100/0.5 single phase dry type transformers connected in parallel.
Fig. 6. The structure of the melting bath of the furnace with a 200 kVA transformer: 1 - electrodes; 2 - initial charge; 3 - softened charge zone; 4 - transition zone; 5 - wall lining; 6 - melt and silicon, carbide crust Рис. 6. Строение ванны рудно-термической печи с трансформатором мощностью 200 кВ-А: 1 - электроды; 2 - исходная шихта; 3 - зона размягченной шихты; 4 - переходная зона; 5 - пристенный гарнисаж; 6 - расплав и кремний, карбидная настыль
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The arc discharge temperature of 2500-4500°C was provided by a graphite electrode having a diameter of 150 mm. The furnace is lined with fireclay bricks and the round furnace bath is 40 cm in diameter. The distance from the electrode to the tap hole block and the rear wall of the furnace comprises 21-22 and 29-30 cm, respectively. The bath depth is equal to 40 cm. Up to the furnace tap hole level, the hearth is sintered of the electrode mass coked for 12 hours under current with periodic shutdown of the furnace. The electrode is moved manually. The furnace is equipped with an electric meter connected via a 400/5 current transformer (with a ratio of 80). Additional devices are used for measuring secondary and primary currents, as well as secondary voltage. On the low-voltage side, the variation limits of current and voltage comprise the value of 0-4000 A and 0-50 V, respectively. Silicon is drained through a tap hole closed and opened by a wooden pole. The possibility of a stepwise change in the secondary voltage was also considered, since the furnace transformer has four voltage levels of 18.4, 24.5, 36.8 and 49.0 V. The voltage reduction in the furnace operation was about 4-8 V, depending on the voltage stage (a larger voltage drop was recorded at high stages).
The main electrical parameters of the ore smelting electric arc furnace with a 200-kVA transformer are given in Table 1.
3. RESULTS OF THE LARGE-SCALE LABORATORY STUDY
Prior to the start of study, work was carried out to prepare the OSF for electric smelting. The electric furnace was heated for 12 hours on a coke bed in order to conduct electric current and maintain hearth integrity. Following heating, the electric furnace was completely cleaned of the coke bed remnants. The electric heating mode period involved a secondary voltage value of 24.6 V and current strength
from the high side equal to 150-200 A.
The start of the furnace for heating took place on 2 December, 2017 at 20:00 followed by charging on 3 December, 2017 at 08:30. The following electrical parameters of the smelting were maintained: nominal secondary voltage of 36 V (32 V given the voltage drop) and current strength of 2500 A. For the formation of the lining, the first two heavy charges composed of coal, charcoal and quartz in quantities of 4.2, 2.3 and 20 kg, respectively, were loaded in the absence of a reducing agent. The lack of reductant from stoichiometry comprised 56%. These charges were loaded until 12:00 on 3 December. Next, the loading of the standard charge was commenced. The standard charge composition included coal, charcoal, special coke2, wood chips and quartz in quantities of 8.3, 4.7, 1.2, 3 and 20 kg, respectively. The excess reducing agent was 3.3%. For more precise control over the process, the charge volume was halved to 10 kg in terms of quartz content starting from the second charge. Loading of the standard charge was carried out until 19:00 on 3 December. At the same time, there was an increase in the characteristics of the furnace operation in the absence of a reducing agent, i.e. intense gas emission from the tap hole, deep electrode landing, sintering of the top and unstable current load. In order to solve this problem, the composition of the charge with an excess of carbon was established containing coal, charcoal, special coke, wood chips and quartz in amount of 3.8, 0.6, 1.5 and 10 kg, respectively. The excess reducing agent comprised 42%. The top was raised to the upper level by 01:00 on 4 December. The charge mass of 246.2 kg was spent to fill the melting bath. The process was conducted with such a composition of the charge until 09:00, December 4. By 03:00, 4 December, the first tapping of silicon took place, with subsequent tapping carried out every 2 hours. From this moment, the excess of reducing agent was
2
Ulieva GA. The study of the physical-chemical properties for special types of coke and its application for smelting highsilicon alloys: Abstract. Cand. Thesis: 05.16.02. Yekaterinburg, 2013. 151 p. / Ульева Г.А. Исследование физико-химических свойств специальных видов кокса и его применение для выплавки высококремнистых сплавов: дис. ... канд. техн. наук: 05.16.02. Екатеринбург, 2013. 151 с.
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Table 1
OSU-100/0.5x2 transformer rating for a furnace with a total power of 200 kVA
Таблица 1
Номинальные электрические характеристики трансформатора ОСУ-100/0,5х2 на печи общей мощностью 200 кВ А
Power, kVA High voltage side Low voltage side
Outputs U, V I, A Connections Outputs U, V I, A Connections
200 AX 380 526 x-a1, x-a x3-a 49.0 4070 -
150 AX 380 395 x-a, x2-a x3-a 36.8 4070 a3-x4
100 AX 380 263 arxi x3-a 24.5 4070 x1-a2
75 AX 380 197.6 a2-x2 x3-a 18.4 4070 a4-x2
optimised at the level of 15-25% creating the conditions for the commencement of the study. The countdown for the amount of accumulated silicon and the charge used for its production was started from the moment of the second tapping.
The effect of charge loading operations on smelting process parameters was studied over the course of large-scale laboratory tests. The time between the technological operations of top processing and the intensity of their performance was alternated with silicon extraction monitoring. The study was carried out in two stages. At the first stage, smelting was carried out for 14 hours in the mode of continuous top processing, with new portions of the charge being supplied with minor interruptions in this process. The cavities were intentionally pierced and a fresh mixture loaded into the thus-formed voids (this represents the process under the conditions of Tau-Ken Temir LLP). After analysing the intermediate results, the second mode was implemented with pauses between top treatments equal to 30 minutes. In this case, only natural changes in the structure of the cavity occurred. The duration of the second stage comprised 65.5 hours. Following completion of the large-scale laboratory tests, experiments were carried out under semi-industrial conditions of Tau-Ken Temir LLP enterprise using the previously-used equipment as described in [20].
4. RESULTS OF INDUSTRIAL TESTS
The methodology for achieving the balance of material and thermal flows in indus-
trial OSF of the Tau-Ken Temir LLP was adopted as follows. For example, the amount of electricity consumed by the furnace over the past period, e.g. consumed power for 1 hour, was determined from the control devices. Consumption for 1 h was equal to 19.5 MW of active power. Hence, the consumption of raw materials per elemental silicon is 19.5/12.5 = 1.56 t/h. Here, 12.5 MWh/t is the generally accepted energy consumption per 1 tonne of commercial silicon according to published data [15-17]. Equivalent to silicon dioxide, this value comprises 1.56-60/28 = 3.3428 t/h. At TauKen Temir LLP, it is customary for the charge consumption to be recorded in batches. A batch consists of a charge portion of a certain composition having a typical batch weight of 1100-1200 kg. The amount of quartz in the batch is always fixed and equal to 600 kg (0.6 t). Therefore, the number of batches is 3.3428/0.6 = 5.57 batches/h. In weight terms, this comprises 5.57 • 1.15 = 6.4 tonnes of charge/h.
The existing control system provides no direct accounting the amount of charge loaded for a certain period of time. Therefore, to account for and maintain a fixed charge feed rate, the following method was applied. The charge was loaded approximately 2 times per hour for implementing the cycle according to Fig. 1. Hence, 6.4/2 = 3.2 tonnes of charge must be placed in the furnace in one load. The charge is fed evenly from six furnace bunkers with 3.2/6 = 0.53 tonnes of charge each. The furnace bunker charge control has the discrete property of a fixed amount of charge being provided with a single press of the control but-
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ton. For our purposes, this is « 0.25 t. In each particular case, this value must be determined empirically. Therefore, when feeding the charge from each bunker, the button should be pressed by operator no more than 0.53/0.25 « 2 times. The amount of charge can be increased by «20%, corresponding with achievable silicon extraction of 80%. Here, the human factor appears to be key: the operator should not feed the OSF with more charge than calculated even if this amount subjectively seems insufficient. The operator's skill consists in distributing the calculated amount of the charge so as to evenly cover the entire top and prevent the development of local gas emissions, i.e. so-called "blowholes". The technologist must monitor the consumed power hourly and adjust the amount of charge fed when the active power consumption changes.
In practice, it was not always possible to maintain a 30-minute pause between the top treatments. In addition, the treatment is carried out at different speeds by different operators. In this regard, under industrial conditions, the parameter of Ik intensity of furnace top processing in time was used. By this definition, the ratio refers to the time spent on top treatment to the total time of treatment and pauses between treatments (this parameter was evaluated every hour during operation).
Unfortunately, it was not always possi-
ble to convey an understanding of the balanced charge loading to each operator. Here, the lack of operator experience was the main adverse factor. In this regard, over a certain period of time, a charge was loaded into the furnace in various quantities, both upward and downward. In order to assess the load balance, the concept of charge excess (CE) in relation to the required charge at the current furnace power was used. This value is taken as the ratio of the actually charged batches to their theoretically calculated number. The ratio was also calculated every hour with XSi estimated by the weight of solid commercial silicon after refining.
5. RESULTS AND DISCUSSION
The consumption of raw materials and the yield of silicon, obtained as a result of the study for a 200-kVA furnace, are presented in Table 2.
As can be seen from the results presented in Table 2, the change in top-treatment and charge-feeding modes significantly reduced the consumption rate of raw materials. Thus, the consumption of quartz comprised 2,7 t/t Si approached the values characteristic for the production of silicon in Norway, i.e. 2,5 t/t. The magnitude of the phase current was significantly stabilised, with a large and constant cavity volume and freely tapped silicon.
Table 2
The amount of consumed charge and obtained silicon during experiments
in a 200-kVA furnace
Таблица 2
Количество израсходованной шихты полученного кремния в ходе опытов _на печи мощностью 200 кВА_
Smelting campaign Charg e composition
Hard coal Charcoal Quartz Special coke Wood chips Silicon, kg
First stage Consumed per stage, kg 45 35 90 13 5 28
Consumption rate, t/t 1.61 1.25 3.21 0.18 0.46
Second stage Consumed per stage, kg 134 128 340 8 46 126
Consumption rate, t/t 1.06 1.02 2.70 0.06 0.37
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In addition, the most important technological parameters were calculated as follows: XSi - amount of electricity consumed per stage (consumed power); Wt - silicon specific consumption of electricity; average active power of the furnace; G - furnace performance; and Ws - power per unit area of the furnace hearth. The above parameters are presented in Table 3. As can be seen from the Table 3, due to a change in the method of charge loading in the reduction process, XSi is significantly increased, yet no rise in power and performance was established. This is possibly due to the small number of experiments; in the case of a larger number, other positive changes can be expected.
The calculation of silicon-, iron-, aluminium- and calcium- extraction from charge materials into commercial silicon with other elements is presented in Tables 5 and 6 as
the balance of reduction smelting process in the first and second stages of study. In Table 4, the calculated amount of the above elements transferred into a commercial silicon product at the first stage of the study is provided. These data were used in calculating the material balance. As can be seen from the results, the XSi value is justified and takes the amount of technical silicon obtained in each tapping into account along with its chemical composition. The reliability of the results is also confirmed by the coincidence of the process balance in relation to the amount of iron completely transferred to silicon melt, according to the published data [24]. This statement is confirmed by conducted experiments. Although an uncertainty is observed regarding the extraction of aluminium and calcium, aluminium extraction was 78 and 46% in the first and second stages, respectively, yet the XSi is
Table 3
The main technological parameters of the commercial silicon smelting during the study
Таблица 3
Основные технологические показатели при выплавке технического кремния _в ходе проведения исследований_
Stage Consumed power, kWh Active power, kW Wt, kWh/kg Xsi,% G, kg/h Ws, kW/m2
First 1096 78.2 39.14 63.62 2.00 622.6
Second 5112 78.0 40.50 78.07 1.92 621.0
Tapping No Mass of commercial silicon, kg Chemical composition
Fe Al Ca Si
% k g % k g % k g % k g
1 2.2 3.03 0.066 3.99 0.087 0.12 0.002 92.36 2.031
2 4.3 2.4 0.103 3.66 0.157 0.21 0.009 93.28 4.011
3 2.2 2.57 0.056 3.8 0.083 0.23 0.005 92.96 2.045
4 0.8 2.67 0.021 2.97 0.023 0.2 0.001 93.68 0.749
5 3.0 2.77 0.083 2.48 0.074 0.16 0.004 94.27 2.828
6 4.7 2.7 0.126 3.21 0.150 0.26 0.012 93.49 4.394
7 6.0 2.17 0.130 3.6 0.216 0.3 0.018 93.61 5.616
8 4.9 1.1 0.121 3.93 0.192 0.31 0.015 92.9 4.552
Total 28.0 - 0.709 - 0.986 - 0.068 - 26.228
Table 4
Calculation of iron, aluminium and calcium amounts transferred to commercial silicon during smelting
Таблица 4
Расчет количества железа, алюминия, кальция, перешедших в технический кремний при плавке
ВЕСТНИК ИРКУТСКОГО ГОСУДАРСТВЕННОГО ТЕХНИЧЕСКОГО УНИВЕРСИТЕТА 2020;24(2):444-4Б9
Table 5
Iron, aluminium, calcium and silicon balance of the reduction smelting
(first stage of study)
Таблица 5
Баланс процесса восстановительной плавки по железу, алюминию, кальцию _и кремнию (первый этап исследований)_
Consumption Charge mass, kg Fe Al Ca Si
% k CO % k O % k O % k O
Quartz 90 0.65 0.59 1.14 1.03 0.50 0.45 44.88 40,39
Hard coal 45 0.19 0.09 0.44 0.20 0.16 0.07 1.71 0.77
Charcoal 35.1 0.22 0.08 0.04 0.01 0.73 0.25 0.10 0.04
Special coke 5.4 0.17 0.01 0.38 0.02 0.17 0.01 0.52 0.03
Total 0.76 1.26 0.79 41.22
Yield Fe Al Ca Si
k со X*Fe,% k O Xai,% k O XCa,% k CO XSi,%
Silicon 0.71 93.86 0.99 78.09 0.07 8.71 26.23 63.62
* extraction of the corresponding elements
shown to increase. The extraction of aluminium at the first stage is close to the literature data of 85%. Possibly, the deviation at the second stage is explained by fluctuations in the chemical composition of quartz raw material. This usually occurs due to a change in the fraction of Al2O3 in the form of inclusions and clay impurities entering the OSF. In order to identify the causes of this contradiction, longer studies are required. Calcium extraction is at a relatively low but stable level of 8-13%. Alt-
hough this is slightly lower than the generally accepted value (40-70%), this discrepancy can be explained in terms of the minimisation of excess carbon in the mixture with respect to stoichiometry.
In general, the adequacy of the results is confirmed by the extraction of silicon at the first stage of the study coinciding with the results obtained at Tau-Ken Temir LLP under the same conditions of technological operation.
Table 6
Iron, aluminium, calcium and silicon balance of the reduction smelting
(second stage of study)
Таблица 6
Баланс процесса восстановительной плавки по железу, алюминию, кальцию и кремнию (второй этап исследований)
Consumption Charge mass, kg Fe Al Ca Si
% k CO % k CO % k CO % k CO
Quartz 340.3 0.70 2.38 1.14 3.89 0.50 1.70 44.85 152.61
Hard coal 134.3 0.19 0.26 0.44 0.60 0.16 0.22 1.71 2.29
Charcoal 8.1 0.22 0.02 0.04 0.00 0.73 0.06 0.10 0.01
Special coke 18.7 0.17 0.03 0.38 0.07 0.17 0.03 0.52 0.10
Total 2.69 4.56 2.01 155.01
Yield Fe Al Ca Si
k CO XFe,% k CO Xai,% k CO XCa,% k CO XSi,%
Silicon 2.67 99.28 2.14 46.86 0.28 13.94 121.01 78.07
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Based on the results of the large-scale laboratory studies, new approaches for the implementation of the reduction smelting process were evaluated. The results of studies conducted under industrial conditions of TauKen Temir LLP are as presented in Tables 7 and 8. As can be seen from the results of 26 December, 2017, the applied charging mode was characterised by significant instability.
With a charge excess was 70-90%, the cavities were small and the furnace could not gain full power due to the regular collapses of the thermally-unprepared charge to the bottom of the cavity. As a result, a severe violation of technology and poor process performance was observed. The tapping of silicon was greatly hampered by abundant slag clumps (see Fig. 4). Over the next two days (8 hours
Time, h Number of charge batches per hour Consumed power, MWh Ik,% CE,%
26 December, 2017
8:48 5.8376 17.7 37 15.4
9:48 3.976 18.1 38 -23.1
10:48 8.964 18.3 38 71.4
11:48 4.074 18.6 43 -23.3
12:48 3.2812 16.1 45 -28.7
13:48 5.951 17.2 49 21.1
14:48 10.104 18 50 96.5
15:48 6.8052 19.2 57 24.1
16:48 6.869 19.7 54 22.0
27 December, 2017
8:20 7.629 19.8 50 34.9
9:20 6.169 18.1 37 19.3
10:20 5.822 18.7 37 9.0
11:20 6.337 19.7 36.2 12.6
12:20 5.9563 19.7 42 5.8
13:20 6.9817 19.8 42 23.4
14:20 6.4561 19.6 30.7 15.3
15:20 6.342 19.6 28.6 13.3
16:20 6.62 19.7 56.8 17.6
28 December, 2017
8:50 6.479 19.55 51.6 16.0
9:50 7.734 19.45 60.5 39.2
10:50 5.2443 20 43.4 -8.2
11:50 6.726 18.9 53.5 24.6
12:50 6.9222 16.8 43.7 44.2
13:50 4.6155 19.8 60 -18.4
14:50 7.8447 19.9 60 38.0
15:50 5.6397 19.8 41 -0.3
Table 7
Parameters of the reduction smelting process in conditions of Tau-Ken Temir LLP
Таблица 7
Параметры процесса восстановительной плавки в условиях
ТОО «Tau-Ken Temir»
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Table 8
Comparative analysis of various options for conducting the reduction smelting
in conditions of Tau-Ken Temir LLP
Таблица 8
Сравнительный анализ различных вариантов ведения процесса восстановительной
плавки в условиях ТОО «Tau-Ken Temir»
Date Consumption The amount of commercial silicon, kg Consumed power, MWh Wt, kWh/kg Xsi,%
The number of charge batches, pcs Quartz, t
26 December, 2017 55,862 55.862 * 0.6 = 33.5172 6240 162.9 162.9/6.24 = 26.1 39.89
27 December, 2017 58,313 58.3131 * 0.6 = 34.987 11140 174.7 174.7/6.24 = 15.68 68.22
28 December, 2017 51,2054 51.2054 * 0.6 = 30.7232 10620 154.2 154.2/10.62 = 14.52 74.07
per day), an excess of charge was maintained in the range of 10-40%. Initially, the charge level in the OSF decreased significantly, but later the cavity acquired a stable skeleton, the top became more rigid and less loose, the charge level returned to normal and the charge went into the reaction zone independently without forced treatment. Within 2 days, the furnace was removed from the state of crisis.
6. CONCLUSION
On the basis of our studies, the effect of technological operation mode on the Si reduction parameters in an ore smelting furnace can be considered as having been demonstrated. Increases in silicon extraction of at least 10-15% were achieved by means of ex-
tending the interval between furnace top treatments to 30 min and achieving a balance between material and thermal flows in the furnace. By this means, oscillations in the phase current were stabilised. By maintaining the correct balance, it is possible to quickly rescue an industrial furnace from complex crisis conditions. However, further studies are required for solving the issue of coordinating the top treatment and the tapping of silicon. The further implementation of smelting method with liquid silicon tapped at the end moment of its accumulation at maximum phase current strength appears to be promising. The authors considering all options for collaboration aimed at a further practical implementation of new operational aspects in the production of commercial silicon.
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ВЕСТНИК ИРКУТСКОГО ГОСУДАРСТВЕННОГО ТЕХНИЧЕСКОГО УНИВЕРСИТЕТА 2020;24(2):444-4Б9
Authorship criteria
Zobnin N.N., Baisanov S.O., Baisanov A.S., Musin A.M. declare equal participation in obtaining and processing of scientific results and are equally responsible for plagiarism.
Conflict of interests
The authors declare that there is no conflict of interests regarding the publication of this article.
The final manuscript has been read and approved by all the co-authors.
INFORMATION ABOUT THE AUTHORS
Nikolay N. Zobnin,
Cand. Sci. (Eng.),
Associate Professor of the Department of Metallurgy
and Materials Science,
Karaganda State Industrial University,
30 Respubliki Ave., Temirtau 101400, Kazakhstan;
Production Engineer,
Tau-Ken Temir LLP,
accounting quarter 018, bldg 133, Oktyabrsky district,
Karaganda 100018,
Kazakhstan;
H e-mail: zobninnn@mail.ru
Sailaubai O. Baisanov,
Dr. Sci. (Eng.), Professor, Director,
Zh. Abishev Chemical-Metallurgical Institute, 63 Ermekov St., Karaganda 100009, Kazakhstan; e-mail: hmi2009@mail.ru
Alibek S. Baisanov,
Cand. Sci. (Eng.), Associate Professor,
Head of the Laboratory of Pyrometallurgical Processes,
Zh. Abishev Chemical-Metallurgical Institute,
63 Ermekov St., Karaganda 100009, Kazakhstan;
e-mail: alibekbaisanov@mail.ru
Azat M. Musin,
Doctoral Student, PhD, Zh. Abishev Chemical-Metallurgical Institute, 63 Ermekov St., Karaganda 100009, Kazakhstan; e-mail: mam.9@mail.ru
Критерии авторства
Зобнин Н.Н., Байсанов С.О., Байсанов А.С., Мусин А.М. заявляют о равном участии в получении и оформлении научных результатов и в равной мере несут ответственность за плагиат.
Конфликт интересов
Авторы заявляют об отсутствии конфликта интересов.
Все авторы прочитали и одобрили окончательный вариант рукописи.
СВЕДЕНИЯ ОБ АВТОРАХ
Зобнин Николай Николаевич,
кандидат технических наук,
доцент кафедры металлургии и материаловедения,
Карагандинский государственный индустриальный
университет,
101400, г. Темиртау, пр. Республики, 30, Казахстан; инженер производственно-технической службы, ТОО «Tau-Ken Temir», 100018, г. Караганда, Октябрьский район, учетный квартал 018, корп. 133, Казахстан; Н e-mail: zobninnn@mail.ru
Байсанов Сайлаубай Омарович,
доктор технических наук, профессор, директор,
Химико-металлургический институт им. Ж. Абишева, 100009, Караганда, ул. Ермекова, 63, Казахстан; e-mail: hmi2009@mail.ru
Байсанов Алибек Сайлаубаевич,
кандидат технических наук, ассоциированный профессор,
заведующий лабораторией пирометаллургических процессов,
Химико-металлургический институт им. Ж. Абишева, 100009, Караганда, ул. Ермекова, 63, Казахстан; e-mail: alibekbaisanov@mail.ru
Мусин Азат Мергенович,
докторант PhD,
Химико-металлургический институт им. Ж. Абишева, 100009, Караганда, ул. Ермекова, 63, Казахстан; e-mail: mam.9@mail.ru
ВЕСТНИК ИРКУТСКОГО ГОСУДАРСТВЕННОГО ТЕХНИЧЕСКОГО УНИВЕРСИТЕТА 2020;24(2):444-4Б9
