İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü / Cevher Hazırlama Mühendisliği Anabilim Dalı
Akışkan yataklı hava bazlı kömür yıkama sistemlerinde akışkanlaştırma karakteristiklerinin kömür-inorganik madde ayrımına etkisi
Effect of fluidization characteristics on coal-inorganic material separation in air based fluidized bed coal washing systems
Ufuk Aykaç - 2014Teze Git (tez.yok.gov.tr)
Kömürlerin zenginleştirilmesi ya da yıkanması genellikle yaş yöntemlerle yapılmaktadır. Uygulanan yıkama işlemlerin büyük bir çoğunluğunun yaş yöntem olması, yani yıkama (ayırma) ortamının su ve kullanılan suyun büyük oranlarda olması, üretilen temiz kömürlerin ve atılan atıkların susuzlandırılması gerek ekonomik gerekse çevresel sorunlar doğurmaktadır. Susuzlandırma aşamalarında kullanılan yüksek oranda flokülantın kömür yıkama maliyetleri üzerindeki etkisi oldukça önemlidir. Bu sebeple, özellikle su problemlerinin yaşandığı Çin, Avustralya ve donma probleminin olduğu Rusya gibi birçok önemli kömür üreticisi ülkede, son zamanlarda kuru kömür üretimi üzerine çalışmalar başlatılmıştır. Türk kömürlerine uygulanabilirliğinin incelenmesi ve ortaya konulması geniş rezervlerimizin doğru olarak değerlendirilmesi açısından oldukça önemlidir. Bu çalışma kapsamında, -15,-15+4 ve -15+1 mm boyut gruplarındaki kömür numuneleri Allair Jigi ile havalı ayırma işlemine tabi tutulmuştur. Bu ayırma işlemlerinde farklı nemlerdeki malzemeye farklı pülsasyon genlikleri ve farklı hava hızları uygulanmıştır. Uygulanan farklı hava hızlarını yataklanma-tabakalanma (sabit/akışkan yatak) üzerinde etkisi, basınç farklarının oluşturduğu eğriler ile ilişkilendirilip, her bir pülsasyon genliği, boyut ve kömür nemi için minimum akışkanlaştırma hava hızı belirlenmiştir. Soma imbat kömürlerine 120, 180, 240 ve 300 rpm (2, 3, 4 ve 5 s-1) pülsasyon genliğinde uygulanan çeşitli hava hızlarına göre, tabakalandırma sırasında basınç düşüşleri ölçülmüştür. Minimum akışkanlaştırma hava hızı belirlendikten sonra, optimum akışkanlaştırma hava hızının tespiti için, zenginleştirme deneyleri gerçekleştirilmiştir. Akışkanlaştırma hava hızının nem oranına bağlı olarak değiştiği, nem oranı arttıkça akışkanlaştırma hava hızının da arttığı belirlenmiştir. Ayrıca düşük nemlerde ve yüksek pülsasyon frekansı yüksek olduğunda tabakalanmayı olumsuz etkilediği belirlenmiştir. Akışkanlaştırma karakteristiği belirlenmesinde boyutun etkisi araştırıldığında görülmüştür ki malzeme içinde küçük boyut miktarı arttığında hava hızı düşük olduğunda buyuk tanelerin hareketlenmesi ve tabakalanması engellenmekte, hava hızı fazla olduğunda ise ince taneler ayırma işlemine maruz kalmadan pnömatik taşınma ile sistemi terketmektedir.. Dolayısıyla malzeme oldukça dar boyut gruplarına ayrılarak zenginleştirme yapılmasının daha etkili olacağı saptanmıştır. Bununla birlikte ürün boşaltma sisteminin akışkanlaştırma özellikleri ile kontrol edilebilirliği de bu tez çalışmasında incelenmiştir.
Coal enrichment processes or so-called coal washing are usually done in water medium. A vast majority of washing processes are wet using the separation medium of water; so the amount of water used in such processes is considerable. As a result, there are problems in dewatering of both produced clean coal and tailings in economic and environmental terms. Moreover, flocculation agents in high dosage used in dewatering processes increase the production costs. In the light of these facts, there is a tendency to processing of coal in dry basis and thus researchers have started on this issue especially in countries such as China and Australia, and in cold countries where there is icing problems like Russia; in all these countries water resource problems are acute. There is a great deal of work to make the dry beneficiation of coal competitive with conventional wet beneficiation processes. It is important to reveal the applicability of this method in the utilization of Turkish coals. Some technological measures and online data collection systems as well as an optimum process design for specific coal types are required to make this competition realistic as the physical properties of coal and its associated minerals play a major role on the separation efficiency especially in dry coal processing. Dry beneficiation techniques for coal processing such as hand picking, optical or X-Ray sorting, crushing size classification (accelerator) air jigs, air tables, FGX technologies, Akaflow aerodynamic separator, Tribo-electrostatic separator, air-dense medium fluidized bed, electrostatic separation and etc. have many advantages over wet processes in terms of economic and environmental concerns. Pneumatic beneficiations such as air jigs and air-dense medium fluidized bed have been commercialized and being applied in many countries. The efficiency of these processes basically depend on the difference in characteristics of coal and gang minerals such as density, particle size, shape factors, magnetic and electro static susceptibility. In gravity based dry processing, stratification of coal is achieved through fluidizing and pulsating air, vibration and an oscillating deck in gravity based dry separators. There are a number of studies that show successful use of gravity based dry processing for coal cleaning indicating that dry beneficiation techniques can be well adapted to coals with different characteristics. In addition to coal processing, these dry processing methods are also open to beneficiation of different minerals such as sand, ferrous minerals, etc. or coal-like materials such as leonardite. In some applications, gravity based dry processing methods were also developed with modified system using different dense media such as sand, magnetite, magnetite fly ash, magnetic pearl and paigeite in separation of both inorganics, pyrite, and Hg. It was also reported that for the optimal separation of these impurities, using air table or fluidized bed separators, the operational parameters such as vibration amplitude, pulsation frequency, air volume, superficial air velocity, transverse angle, longitudinal angle and also coal properties like size and shape factor should be controlled carefully. Among these operational parameters, pulsation frequencies and superficial air velocity were the most important in governing the density based stratification and separation of the coal particles. In a fluidized bed system, a pack of solids is converted into a ﬂuidized bed by lifting action of air passing through it. Thus, three stages can be identiﬁed in the process of ﬂuidizing a bed of solids based on the velocity of air ﬂow through it; (i) ﬁxed or static bed, where the particles are in direct contact with each other, supporting each other's weight, (ii) expanded/ﬂuidized bed or particulate ﬂuidization where the particles have a mean free distance between particles and the particles are supported by the drag force of the ﬂuid, and (iii) mobilized bed. In fixed bed conditions, the particles remain in stationary contact whereas they are in free motion in a fluidized bed. When the fluid is passed upward through an unrestrained bed of particles, the bed will initially expand slightly to take up a loose packed arrangement. If the flow is increased, the pressure drop across the bed increases. Eventually the pressure drop equals the force of gravity on the particles (weight of the particles) and the particles begin to move. During this period, the porosity increases and the pressure drop rise more slowly than before due to the net effect of increased porosity and velocity. The bed is in extremely loose condition with the particles still in contact. The bed is unstable, the particles begin to lose contact and then adjust their position to present as little resistance to the flow as possible. With further increase on air velocity, the particles are separated and true or complete fluidization begins. By the time this position is reached, all particles are in motion and beyond that stiuation, the bed continues to expand and the particles move in a more rapid and independent motion. The pressure drop across the particles bed reaches a threshold, referred to as the point of incipient fluidization or minimum fluidization velocity. At that sitiution, all of the particles have been entrained in the fluid and pneumatic conveying exists. Dry beneficiation of the tested coals was performed through a Lab scale Allair Jig. Allair jig facility consisits of feed, separation (jigging) and powder filtering units. The fresh run of mine (ROM) coals were fed into a manual feed chute associated with belt conveyor and accumulated in the second feed chute where the feed rate was controlled by the stargates. Following the stratification of the feed in jigging cell by the fluidizing air from the fluidizing and pulsed air production and distribution mechanism towards the bottom part, the lighter coal particles floated and discharged automatically from the channel while the discharge of the dense materials through separated channel is controlled by the stargates by varying the rotational speeds. During stratification, a limited amount of powder is ventilated by the filter unit through the ventilation pipe. During the stratification, the operational conditions are adjusted by the control panel. On separation, pressure drops, Δp, for both part of the jigging cells were measured based on the fluidizing air rate by U-tube manometer to determine the minimum fluidizing air velocities, Umf, Umfa, Umf-cohesive. In this study, coal samples from Soma region of Turkey were used in the experiments. Coal samples of -15, -15+4, and -15+1 mm in size were subjected to Allair jigging process. Samples of various moisture contents were cleaned in different pulsation amplitudes and air speeds. The effect of air speed on bedding and layering (fixed/fluid bed) and the impact of pressure difference was investigated. For each pulsation amplitude, particle size, and coal moisture content a minimum fluidizing air speed is introduced. In Soma-Imbat coal samples, for applied pulsation amplitude of 120, 180, 240, and 300 rpm (2, 3, 4, and 5 s-1) the pressure differences in bedding were measured. After finding minimum fluidizing air speed, coal cleaning experiments were done to determine the optimum air speed. The effect of air speed on bedding and layering (fixed/fluid bed) and the impact of pressure difference was investigated. For each pulsation amplitude, particle size, and coal moisture content a minimum fluidizing air speed was introduced. In Soma-Imbat coal samples, for applied pulsation amplitude of 120, 180, 240, and 300 rpm (2, 3, 4, and 5 s-1) the pressure differences in bedding were measured. After finding minimum fluidizing air speed, coal cleaning experiments were done to determine the optimum air speed. The effect of controlling product discharging system speed on bedding was also investigated in this study. The results obtained in this thesis have revealed that the negative effects arising from coal and device parametres on the separation efficiency would be brought to the minimum levels if the fluidization air velocity is set based on the variable conditions. It is also found that a controlled product discharge/drainage system would be a possible feature for such an application, with a careful implementation of both fluidization characteristics and separation performance.