2014EPD Congress

New proceedings volumes from the TMS2014 Annual Meeting, available from publisher John Wiley & Sons:

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www.tms.org

2014EPD Congress

Proceedings of symposia sponsored by

the Extraction & Processing Division (EPD) of

The Minerals, Metals & Materials Society (TMS)

held during

February 16-20, 2014San Diego Convention Center

San Diego, California, USA

Edited by:

Copyright © 2014 by The Minerals, Metals & Materials Society. All rights reserved.

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TABLE OF CONTENTSEPD Congress 2014

EPD Council 2013-2014................................................................................... xiiiEPD Honors and Awards 2014...........................................................................xv

Fluidization Technologies for the Mineral, Materials,and Energy Industries

Fluidised Bed Technology in Practical Examples ................................................5 A. Krzysik

Fluidized Bed Applications for the Minerals Industry and Renewable Energy..13 M. Runkel, A. Wirtz, J. Hammerschmidt, and K. Pope

Evaluating a Fluidized-bed Process through Applied Research and Development: A Practical Approach to a Successful Project ......................21 L. May and H. Mudgett

Energy Efficient Fluidized Bed Systems ............................................................29 K. Adham

The Use of Pilot Scale Fluidized Beds for the Development of a CommercialPlant Design........................................................................................................37 J. White and A. Olson

Advanced Green Petroleum Coke Calcination in an Electrothermal FluidizedBed Reactor ........................................................................................................45 A. Kozlov, Y. Chudnovsky, M. Khinkis, H. Yuan, and M. Zak

Study on Phosphorus Removal of High-phosphorus Iron Ore by MicrowaveCarbothermic Reduction and Separation ............................................................55 S. Zhou

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General Recycling

General Recycling Poster Session

Characterization of Printed Circuit Boards from Scrap Printers.........................67 F. Silvas, V. de Moraes, G. Bortolini, O. Gomes, S. Gaydardzhiev, D. Espinosa, and J. Tenório

Convert Melting Slag Directly into High Basicity Glass-ceramic ......................75 Y. Li, X. Liu, D. Cang, and Y. Zong

Determination of Apparent Dry Density for Ternary Mixture of Crushed Marble Waste......................................................................................................83 C. Ribeiro, R. Rodriguez, and C. Vieira

Experimental Study on Reduction-magnetic Separation Process of Pickling Sludge...............................................................................................91 X. Liu, J. Zhang, Q. Xiao, and Q. Li

Indium Recovery from Discarded Light Emitting Diode (LED) Liquid Crystal Display (LCD) TV: Influence of Leaching Reagents.............................99 H. Hashimoto, P. Hanashiro, V. de Moraes, and D. Espinosa

Kinetic Study of Acid Copper Leaching from Waste Printed Circuit Board ....105 F. Ramunno, V. de Moraes, D. Espinosa, and J. Tenório

Life Cycle Based Greenhouse Gas Footprints of Metal Production with Recycling Scenarios..................................................................................113 N. Haque, T. Norgate, and S. Northey

Optimal Leaching on Hydrometallurgical Process of Recycling Batteries Using Less Energy and Reactants.....................................................................121 F. Bertin, R. Pereira, D. Espinosa, and J. Tenório

Production of Ornamental Compound Marble with Marble Waste and Unsaturated Polyester ................................................................................129 C. Ribeiro, R. Rodriguez, and C. Vieira

Removal of Heavy Metals and Upgrading Crude Bio-oil from Pteris VittataStems and Leaves Harvest Using Hydrothermal Upgrading Process ...............137 J. Yang, Z. Deng, J. Li, and X. Zhang

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Research on the Fundamental Characteristic of Dust and Sludge Containing Iron from Steel Enterprise.................................................................................151 R. Mao, J. Zhang, X. Xu, M. Wei, and K. Jiao

Silver Recovery from Industrial Wastes Using an Electrochemical ReactorREOV-01 ..........................................................................................................159 P. Ortega, J. Islas, L. Lechuga, and L. Garcia

Study of Degradation of Ceramic Bodies Incorporated with Ornamental Rock Waste Obtained from Test of Wetting and Drying Cycles ......................167 G. Xavier, J. Alexandre, F. Saboya, P. Maia, and A. Azevedo

The Effect of Ethanol Concentration for the Separation of ABS and HIPS From Waste Electrical and Electronic Equipment (WEEE) by Flotation Technique .........................................................................................................173 S. Utimura, J. Tenório, and D. Espinosa

Materials Processing Fundamentals

Thermodynamic

Thermodynamic Properties of Equilibrium Phases in the Ag-Cu-S SystemBelow 500 K: Experimental Study ...................................................................185 F. Tesfaye and P. Taskinen

Iron-carbon Phase Diagram: A Century at Variance with ChemicalThermodynamics ..............................................................................................195 H. Näfe

Effect of Water on S and P Distribution between Liquid Fe and MgO-saturated Slag Relevant to a Flash Ironmaking Technology ............203 M. Mohassab-Ahmed and H. Sohn

Numerical Analysis of Thermo-mechanical Behavior of Laser Cladding Process ..............................................................................................211 T. Tang and S. Felicelli

Process & Properties Control

Effect of Different Parameters on Breakouts in Billet Caster ...........................221 R. Singh, Devilal, S. Jha, S. Shekhar, E. Chacko, and R. Sahu

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Metal Extraction

A Sintering Ore Blending Optimization Model Based on “Iron Increase andSilicon Reduction” Ore Dressing Processes .....................................................233 C. Liu, J. Li, H. Tang, and W. Liu

Electrodeposition of Cobalt from Air and Water-stable Ionic Liquid 1-Butyl-3-Methylimidazolium Tetrafluoroborate.............................................241 M. Li, Z. Wang, and R. Reddy

Effects of Ultrasound on Al2O3 Extraction Rate during Acid Leaching Process of Coal Fly Ash....................................................................................251 K. Liu, J. Xue, and W. Luo

Separation of Nickel and Cobalt in Acidic Aqueous Solution by SelectiveReduction of Metals..........................................................................................259 S. Shiryama and T. Uda

TWIP / Steelmaking

Formation of Non-metallic Inclusions in the Molten Steel in MgO Crucibles .............................................................................................269 W. Yang, L. Zhang, H. Duan, Y. Ren, J. Wang, and X. Liu

Experimental Research of Continuous Temperature Measurement for MoltenMetal Bath through Bottom-blowing Component ............................................277 Y. Ren, S. Niu, W. Li, and X. Hong

AlN Formation in High-Al and High-Mn Alloyed Advanced High StrengthSteels.................................................................................................................285 J. Jang, D. Kim, M. Paek, and J. Pak

Interfacial Reactions between Slag and Melt in the New World of HighManganese Steels..............................................................................................291 M. Peymandar, S. Schmuck, P. von Schweinichen, and D. Senk

Poster Session

Computational Study of Texture Development during Templated Grain Growth ....................................................................................................301 J. Zhou and Y. Wang

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Dissolution Behavior of Magnesia in Hydrochloric Acid with Strong Brine ...309 Z. Hu, W. Ding, D. Tan, and S. Guo

Distribution of P2O5 between Solid Solution and Liquid Phases in CaO-SiO2-Fe2O3 System Containing Na2O or B2O3.........................................317 S. Xie, L. Zhou, and W. Wang

Effect of M-EMS on the Macroscopic Quality of TP347 Heat-resistantStainless Steel Billet .........................................................................................325 S. Zhou

Effect of Mn, Ni Contents on Microstructure and Rust Layer of BridgeWeathering Steels under Atmosphere Containing Cl-1 ....................................331 G. Fu, D. Jin, X. Gao, Q. Li, and M. Zhu

Effects of Niobium Alloying on the Microstructure and Mechanical Properties of Bainite Ductile Iron .....................................................................339 L. Chang, Y. Yan, X. Chen, Q. Hua, and Q. Zhai

Effects of Solidification Conditions on As-cast Structure of Ferritic StainlessSteel in Continuous Casting..............................................................................345 J. Sun, J. Ye, H. Zhong, W. Du, and Q. Zhai

Electrowinning of Silicon with Liquid Electrodes............................................353 M. Jia, Y. Cheng, Z. Tian, Y. Lai, and Y. Liu

Experimental Study on the Influence of Vacuum Carbonitriding Process for 20Cr2Ni4A Steel.........................................................................................361 Y. Zhang, S. Du, W. Zhao, G. Wang, and Y. Rong

Hybrid Porous Metal of Nano-micro Double Size and Regular-random Bimodal ............................................................................................................367 X. Zhang, H. Zhang, and Y. Li

Hydraulic Simulation of Fluid Flow in Beam Blank Continuous Casting Mold with Double Nozzles ...............................................................................375 L. Zhang, D. Chen, M. Long, X. Xie, X. Zhang, and Y. Ma

Isothermal Bainite Transformation of Cr5 Steel under Pulsed Current and Pulsed Magnetic Field Treatment ..............................................................385 X. Xia, L. Li, Z. Lu, Q. Zhai, and Q. Li

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Microstructural Characterization of Aluminum Metal Matrix CompositePrepared by In-situ Method ..............................................................................393 D. Mitrica, M. Burada, R. Florea, M. Ghita, E. Alexandrescu, V. Soare, and P. Moldovan

Microstructure Modification for Semisolid Slurry of Al-4.5wt.%Cu Alloy byPulse Magneto-oscillation Treatment ...............................................................401 Z. Xu, Z. Zhang, Q. Li, Q. Zhai, and Y. Gong

Modeling of Magnetohydrodynamic, Thermal and Solidified Behavior inElectroslag Remelting Process..........................................................................409 Q. Wang, Z. He, and B. Li

Phase Composition of Scale Layer Formed during Continuous Casting ..........417 N. Wang, J. Dong, B. Li, M. Chen, and C. Huang

Properties of Cu-based Oxygen Carrier Used in Chemical Looping AirSeparation (CLAS) ...........................................................................................423 K. Wang, Q. Yu, Q. Qin, and W. Duan

Removing Fluorite and Calcite from Scheelite during Flotation SeparationProcess with Calcium- and Sodium-containing Reagents ................................431 L. Liu, J. Xue, and J. Zhu

Simulation of Solidification Microstructure in Austenitic Stainless Steel Twin-roll Strip Casting Based on CAFE Model ...............................................441 J. Ma, J. Zhang, B. Wang, J. Zhao, S. Zhao, and G. Wu

Simulation of Solidification Process of Steel Ingot under Different ThermalBoundary Conditions ........................................................................................449 J. Zhao, J. Zhang, B. Wang, Z. Chen, and J. Ma

Statistical Estimation of Dislocation Pinning at Precipitates, Voids and Bubbles ......................................................................................................457 A. Dutta, M. Bhattacharya, and P. Barat

The Effect of Cooling Intensity on the Solidification Structure and Ferrite Phase Fraction of a Duplex Stainless Steel.......................................................463 C. Zhang, J. Ye, C. Wu, J. Hu, H. Zhong, and Q. Zhai

Study on the Purification of Nickel by Vacuum Directional Solidification......471 G. Wang, K. Wei, W. Ma, W. Yu, and C. Zhang

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Synthesis of Titanium Dioxide by Microwave Solid State Method and ItsPhotocatalytic Property.....................................................................................481 K. Yang, J. Peng, L. Zhang, H. Zhu, Y. Xia, and J. Jia

The Solidification Structure and Ferrite to Austenite Transformation of a Novel Lean Duplex Stainless Steel............................................................487 J. Ye, C. Zhang, C. Wu, H. Zhong, H. Song, X. Cao, and Q. Zhai

The Study of Refinement Mechanism of Pure Aluminum under Surface Pulsed Magneto Oscillation ..............................................................................495 Z. Zhang, Z. Xu, Q. Li, D. Liang, and Q. Zhai

Thermal and Metallographic Parameters Evolution during Solidification of Zn-Sn Alloys ................................................................................................501 W. Desrosin, C. Schvezov, and A. Ares

Thermodynamic Relation between Chromium and Sulfur in Fe-Cr Melts .......509 H. Do, Y. Kim, D. Kim, and J. Pak

Recycling and Sustainability Update

Recycling

Challenges to the Biotechnological Recycling of Precious and Rare MetalsSourced from Post-consumer Products .............................................................521 N. Saitoh and Y. Konishi

Sustainable Recycling of Solid Wastes via In-process Separation ...................529 N. Ma

Recovery of Valuable Metals from Lead Flue Dust by a Integrated Process ...537 X. Yang, H. Li, C. Li, and Y. Wang

Recycling of Valuable Metals from Poly Cracker Ash of Printed Circuit Boards (PCBS) by Physical Beneficiation and Hydrometallurgical Treatment..........................................................................................................545 V. Kumar, A. Kumari, M. Jha, A. Vidyadhar, and B. Soni

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Waste

Characterization of Components of Liquid Crystal Displays: The End-of-LifeManagement .....................................................................................................555 T. Moreno, P. de Hanashiro, H. Hashimoto, V. de Moraes, and D. Espinosa

Kinetics and Equilibrium Studies for the Removal of Tannin Acid fromAqueous Solutions by Regenerated Activated Carbon .....................................563 A. Ma, L. Zhang, J. Peng, H. Xia, C. Sun, Y. Luo, T. Hu, and Y. Zuo

Recovery of Copper and 1, Hydroxyethane-1, 1-Diphosphonic Acid (HEDP)from Cyanide-free Electroplating Wasterwater by Electrodialysis...................571 T. Scarazzato, D. Buzzi, A. Bernardes, J. Tenorio, and D. Espinosa

The Life Cycle Assessment of Copper Metallurgical Process..........................579 X. Yang, X. Hao, H. Li, and S. Sun

Zinc Oxide Preparation Using Rotary Hearth Furnace Secondary Dust...........587 H. Tang, H. Zhang, L. Fan, and Z. Guo

The Estimation of Waste Packaging Containers Generated by Householdsin Taiwan ..........................................................................................................595 E. Hsu and C. Kuo

Author Index.....................................................................................................603

Subject Index ....................................................................................................607

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EPD COUNCIL 2013-2014Adrian C. Deneys

ChairPraxair Inc.

San Ramon, CA USA

Mark E. SchlesingerVice Chair

Missouri University of Science & TechnologyRolla, MO USA

Thomas P. BattlePast Chair

Midrex TechnologiesPineville, NC USA

Corby G. AndersonMembership & Student Development Committee Representative

Colorado School of MinesGolden, CO USA

Neale R. NeelamegghamHydrometallurgy & Electrometallurgy Committee Chair

IND LLCSouth Jordan, UT USA

Kevin M. JaansaluContent Development & Dissemination Committee Representative

Royal Military CollegeKingston, ON Canada

John S. CarpenterMaterials Characterization Committee Chair

Los Alamos National LaboratoryLos Alamos, NM USA

Tao JiangPyrometallurgy Committee Chair

Central South UniversityChangsha, China

Anne KvithyldRecycling & Environmental Technologies Committee Chair

SINTEFTrondheim, Norway

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EPD COUNCIL 2013-2014 (CONT.)

James A. YurkoProcess Technology & Modeling Committee Chair

MaterionElmore, OH USA

Jaroslaw W. DrelichEnergy Committee Chair

Michigan Technological UniversityHoughton, MI USA

Rachel A. DeLucasEducation Committee RepresentativeMassachusetts Institute of Technology

Cambridge, MA USA

Sergio N. MonteiroInternational Liaison

Instituto Militar de Engenharia - IMERio de Janeiro, Brazil

Soobhankar PatiMaterials & Society Committee Representative

Indian Institute of TechnologyBhubaneswar, India

Christina Elizabeth MeskersPublic & Government Affairs Committee Representative

�������������� ������������Antwerp, Belgium

Michael L. FreeProgramming Representative

University of UtahSalt Lake City, UT USA

Shijie WangProgramming Representative

Rio Tinto Kennecott Utah Copper CorporatrionSouth Jordan, UT USA

Neale R. NeelamegghamSymposium Sponsorship Committee Representative

IND LLCSouth Jordan, UT USA

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EPD HONORS AND AWARDS 2014

Distinguished Lecturer AwardBrajendra Mishra

Colorado School of Mines

Distinguished Service AwardThomas Battle

Midrex Technologies

Science AwardNazmul Huda, Jamal Naser, and Geoffrey Brooks

Swinburne University of Technology

Markus A. Reuter and Robert W. MatusewiczOutotec Limited

Technology AwardGwang Seop Lee

Korea Resources Corporation

Masahito Uchikoshi, Kouji Mimura, and Minoru IsshikiTohoku University

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2014EPD Congress

Fluidization Technologies for the Mineral, Materials, and

Energy Industries

Lead OrganizerJerome P. Downey

Co-OrganizerLawrence D. May

2014EPD Congress

Fluidization Technologies for the Mineral, Materials, and

Energy Industries

Session ChairsJerome P. DowneyLawrence D. May

FLUIDISED BED TECHNOLOGY IN PRACTICAL EXAMPLES

Andre Krzysik

Metso Minerals; 1110 Hay St, West Perth, WA 6005, Australia

Keywords: fluidization, dryer, calciner

Abstract

Fluidized bed technology has been used in the industry for almost 100 years. To fully understand the process hundreds if not thousands of researchers were and are involved in finding the way to predict the behavior of the solids when subjected to the flow of media through them and the conditions of the solids in the fluidized state. The difficulties in predicting the behavior of solids in the fluidized state stem from the complexity and abnormality of the parameters related to the solid particles. Theoretical calculations can in some degree account for some of them. Particle size distribution, their shape, density, moisture content and physical and chemical reaction that may exist in the fluidized bed reactor play an extremely important role and influence thebehavior of the fluidized bed. This paper focuses on the practical examples of the fluid bed equipment and the use of different conditions to process various products.

Introduction

Each process application has its own unique requirements and the designers of the equipment must take into consideration specific process requirements, specific properties of the material to be processed (both physical and chemical), and physical and chemical reactions/changes that are taking place in the reactor (heat and mass exchange, chemical reactions, isomorphic change etc.).Each individual piece of equipment must also satisfy specific requirements/ preferences of the client that will operate and maintain the equipment. Those often represent greater challenges thanthe process on its own, and are equally important to the satisfactory operation of the plant. That includes aspects like location of the equipment, environmental effects, and expected life of theequipment, process control, access, maintenance, and many others.

Over the period of time a great deal of research, both theoretical and practical have been carried out and the results of that work are available for equipment designers. For the simple applications that information is usually sufficient and the selection of a suitable solution is relatively simple. In many instances, the available information is not sufficient, and often specialized test work needs to be conducted to obtain additional data. In many cases a pilot installation is built in which the required process is tested. Only then the commercial equipment is designed. The following deal with specific installations that were designed to satisfy specific requirements. In some instances, additional problems were discovered during the commissioning of the equipment that required on site modification. The continuous monitoring of the equipment that was supplied to the various clients allowed us to further develop and improve our designs.

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EPD Congress 2014Edited by: James Yurko, Lifeng Zhang, Antoine Allanore, Cong Wang, Jeffrey S. Spangenberger,

Randolph E. Kirchain, Jerome P. Downey, and Lawrence D. MayTMS (The Minerals, Metals & Materials Society), 2014

Practical Application of Fluid Bed Systems

Bubbling Bed With Fine-Particle Removal

The application involves a simple drying process of chromite based foundry sand with relatively narrow particle size distribution, 200 – 350 [μm] ��� ������� �s= 4.3 [g/cm3]. The moisture content in the feed is ~10%. In addition the dryer must not produce any fine particles (<20 μm) as this will decrease the quality of the final product. Before the fluidized bed dryer was installed,the product was dried using a rotary dryer. The discharge from the rotary dryer contained unacceptable level of fines. The main challenge was to reduce the amount of generated fines and if possible remove the unwanted fines that were already present in the feed. Test work conducted in the fluidizing velocity test tube has shown that the fluidized bed process not only stopped producing fine particles but it also removed the very fine particles that were included in the feed. The minimum fluidizing velocity for the drying process only was relatively easy to calculate as the feed could be classified as group B particles1. However, satisfying the additional requirement, which is the removal of fine particles, required a series of tests. The sample of feed material was fluidized for a period after which it was removed from the test tube and tested for turbidity. The same sample was tested at various fluidizing velocities. Analysis of the conducted test indicated that the use of higher fluidizing velocity resulted in greater removal of the fine particles. This is attributed not only to higher freeboard velocity that carries the fine particles away but mostly to the fact that greater turbulence of the bed is rubbing off the fines attached to the surface of the larger particles.

Bubbling Bed With Re-circulating Inert Material

The following example describes a very specific drying process. One of our potential clients had a product that was precipitated from a chemical reaction in a form of small, needle-type crystals. The product included about 15% free moisture. The goal of the drying process was to produce dry (<0.5%) and free flowing powder ready to be packaged. The challenges facing the design included:

1. Material would melt if the temperature exceeded 120oC. 2. Material would dissolve in its own moisture in a temperature exceeding 80oC.3. Material when squeezed would form lumps that would solidify when dry, and a

subsequent crushing process would be required.

The final product of our design is a dryer filled with the inert bed consisting of soft and light polypropylene beads. The beads are placed in the dryer and fluidized. The fluidizing velocity was selected at such a value that the bed is very vigorously agitated but the beads do not have a chance to be lifted with the air and carried out to the product collection system that consists of areverse pulse bag filter. The beads are circulated in the bed using a screw conveyor located at the bottom of the dryer and the pneumatic conveying system. The fresh feed is added to the stream of beads just before they enter the dryer where they impact onto the impact plate. The lumps consisting of beads mixed with the feed break on impact into smaller lumps. As the large lumps are breaking the smaller lumps that are completely dry break too, liberating small particles of dry material that is fine and light enough to be entrained in the stream of air. This process is repeated

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many times and, eventually, once the feed was stopped all the particles would be elutriated leaving behind clean beads.

Figure 1. FB dryer with impact plate

Entrainment Dryer With Circulating Inert Bed

Another process involves drying of 120 tph of very fine and wet copper concentrate. The fundamentals of the fluidization process divide the solid particles to four groups that exhibit different properties when fluidized with gas (Geltart, Powder Technol., 7, 285-292, (1973))1.Previous examples were dealing mostly with particles belonging to group A & B. The application described below deals with particles that belonged to group A and C. The major characteristic of particles belonging to group C is that they exhibit cohesive tendencies, and when the gas flows through them, it would “rat-hole” opening channels between the air distributor and the surface of the bed and do not form an evenly fluidized bed. Such particles require mechanical agitation or vibration.

In the process of drying, the problem is amplified even further by the moisture contained with the particles that would form solid clusters. One of the methods commonly used is the introduction of an inert bed that can be easily fluidized. Such a solution has been used in the process described below.

The task was to dry a large quantity of copper concentrate (120 tph) with the particle size between 10–70 μm and the moisture content up to 15% (normally 12%). The inert bed consistedof gravel with particle sizes evenly distributed between 2 and 6 mm. The fluidizing velocity needed to fluidize the gravel particles was approximately 2.5 m/s. At that velocity, the bed isvery vigorous and allows for very good mixing of the wet concentrate fed into the dryer. As the

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drying process progresses, the larger conglomerates of the concentrate break into small distinctive particles that are entrained in the stream of the gas flowing through the bed and arecarried away to the baffle chamber and further to the bag filter that separates the product from the exhaust gases.

Very vigorous fluidization of the gravel bed creates considerable attrition of the gravel particles, which with time reduce their size to the point that they could be entrained with the product. To avoid the contamination of the product with gravel particles, the bed is circulated through a screen that removes the particles smaller than 2 mm. The screen also removes agglomerates larger than 8 mm and prevents build-up of the bed.

Iron Ore Dryer – Turbulent Bed And Entrainment Reactor

As discussed at the beginning of the paper, one of the major problems/challenges when designing the fluidized bed reactor is the establishment of the fluidizing velocity. When the feed to the fluidized bed reactor is relatively uniform, the fluidizing velocity can be predicted using theoretical calculations or can be established experimentally by conducting a series of tests. In this particular application the particle size distribution is so wide and can change depending of the source of the product that it is impossible to select a fluidizing velocity that is suitable for all conditions. The other challenge is a relatively high feed rate that would require a very large reactor if particles of all sizes were to be retained in the bed. Yet another challenge is substantial changes of the moisture content in the feed and their influence on the selected design.As usual, the size and the cost of the design equipment plays a very important role and cannot be overlooked. Forced to reduce the cost of the equipment, we were required to reduce the size of the reactor and use high fluidizing velocity. This provided us with a very vigorous and well-mixed bed that would fluidize even the largest particles, but created a problem because a large portion of the bed was elutriated. During the test work that we conducted, we noticed that the entrained material included many fine particles or conglomerates of small particles that did not fully dry in the bed and were drying in the freeboard (disengagement chamber). This led to substantial drop in the temperature of the off gas and condensation in the exhaust system. The elutriation of fine and still wet particles is greater when the moisture content of the feed is lower. This is caused predominantly by the feed distribution system that spreads the feed across the bed and exposes the fine particles to the high velocity air. Due to large feed rate to the dryer a proper distribution is absolutely crucial in order to avoid localized cold spots in the dryer. The normal reaction to low exhaust temperature would be to increase the bed temperature. This however would lead to increased fuel consumption and therefore increased operating cost as the portion of the dried material leaving the bed would be hotter than required.

In order to find a compromise we decided to supply additional hot gas into the freeboard of the dryer by installing a second burner. The air is supplied to the expansion chamber through a ring main and injected via a number of nozzles distributed around the dryer. Having another source of heat in the freeboard, the bed temperature can be set at any value as long the product issufficiently dry. Installation of the second heat source in the freeboard provided full control of temperature and moisture content of the fine particles that are elutriated from the bed. In fact, with this configuration, there are two drying systems in one reactor.

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The discharge over a weir or overflow pipe from the fluidized bed reactor is one of the most commonly used solutions for controlling bed height. It provides an easy way of maintaining the correct bed level without the need for a complicated control system. The use of the replaceable weir in the design allows easy adjustment of the bed level in case such an adjustment is required. This system however can only be used if there are no large particles in the feed that have a tendency to remain at the bottom of the bed and accumulate over time. Accumulation of large particles in the bed practically reduces the bed depth and at the same time reduces the bed volume and the time for which the smaller particles remain in the bed as they virtually float on top of the accumulated large particles. In such an instance, other methods need to be employed and they will be discussed later on.

Lime Calciner With Elutriated And Partially Re-circulating Bed

The process and equipment described in this section was one of the most challenging designs I ever performed. It was an innovative design and loaded with various challenges to overcome. It involved very extensive test work, complicated and detailed calculations and many assumptions that carried substantial risk. In certain areas, provisions for alternative solution and modifications were made just in case the first option failed.

Calcination of the limestone is known since ancient times. When the limestone – calcium carbonate (CaCO3) is exposed to high temperature (above 550oC) a process of decomposition to carbon dioxide, (CO2) and calcium oxide, (CaO) begins. This process however depends on the concentration of carbon dioxide in the atmosphere surrounding the decomposing limestone and temperature. When too much carbon dioxide is present and temperature drops the process reverses and calcium oxide reacts back with the carbon dioxide. For practical purposes, at which the reaction is fast enough the temperature must be above 898oC when the equilibrium pressure is greater than the atmospheric pressure.

In the industry two major methods of lime calcining dominate – vertical shaft kilns and rotary kilns. Those methods have been perfected for many years. There is however a limitation that restricts the use of those two methods – the size of the limestone. After the limestone is quarried, it needs to be crushed and screened for a very specific particle size range. As a result relatively large quantities of small limestone, <10 mm are discarded. Some of it finds use in road building, back fill, and agriculture, but much of it is simply stockpiled.

The fluidized bed technology has been used in the lime calcining process but unfortunately not widely. One of the reasons is the higher energy cost in comparison to the other methods and the other is so-called scaling problem. In some instances, the vertical shaft kiln or rotary dryer cannot be used due to the properties of the limestone that need to be calcined. One of such examples was a deposit in South Africa where initially a vertical shaft kiln was built to calcine the limestone but never worked properly. The major problem was the extremely dense limestone that when exposed to high temperature would virtually explode to very small particles. It would simply turn to dust. That gave us an opportunity to introduce the fluidized bed technology and design a plant that is one of a kind.

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The process described below is shown on Figure 2. The feed is introduced into the preheating section of the plant where the exhaust gases from the calciner, carrying huge amounts of energy,dry and pre-heat the cold limestone. The temperature of the gas leaving the calciner is between 900 – 950oC. The exhaust gas carries a substantial amount of very fine CaO as well as a large quantity of CO2 generated during the process of calcination. The ratio between the carbon dioxide and calcium oxide is very high and the only thing that stops the reverse reaction is the high temperature. During the heat recovery process the temperature of the off gas has to drop so in a sense, the process of recombining CaO with CO2 in unavoidable. To eliminate or reduce to a minimum scaling of the equipment we decided to introduce a sort of fluidized bed or rather suspended bed in which the major temperature drop and possible reconversion would occur on the particles of the limestone itself.

Figure 2. Fluidized bed lime calciner

The feed of limestone is introduced into the center of the preheater where hot gas from the calciner suspends the particles in fast flowing steam creating a very turbulent bed. The velocity required for the suspension of the particles, ranging between 1 – 6 mm, was calculated and simulated using Computational Fluid Dynamics software.

From the preheater the cooled exhaust gas is directed to the further heat recovery whilst the preheated limestone is directed to the fluidized bed calciner. The major challenge with the design of the calciner was proper selection of the fluidizing velocity and the velocity of the gas in the expansion or disengagement section of the reactor. The material in the bed that consists of fresh limestone, coal, partially calcined limestone and calcined limestone (CaO). Each of those has adifferent density and varying particle size distribution and therefore each has a different minimum fluidizing velocity. Another factor equally important is the volume of the fluidizing

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media. In the process, preheated air from the recuperator is used for fluidization and for combustion of coal that is injected into the bed. Another gas that plays a very important role in the fluidization is the carbon dioxide liberated from the limestone. On top of that is the temperature that greatly influences the volume of the fluidizing media. All those factors needed to be carefully considered when selecting the size of the equipment and establishing the operating parameters. At each stage of the operation of the calciner different conditions exist in the bed as well as above the bed in the disengagement zone. Different conditions prevail at the start-up of the calciner when cold, and are different when it is hot. On the initial cold start, the calciner is loaded with finer limestone and only after the bed gets hot and properly fluidized a coarser stone should be introduced.

After the fluidization of the bed is stabilized and the calcination of the limestone begins the particles break and when fully calcined are entrained with the stream of the off gas and carried away to the cyclone. To achieve that, the velocity in the disengagement section is sufficiently high but not too high as only light CaO particles should leave the bed. The particles entrained with the exhaust gas from the calciner are directed into the primary cyclone. The cyclone off gas is directed into the preheater (previously described) and the solids are directed to the fluoseal at the bottom of the cyclone.

The fluoseal is a simple fluidized bed apparatus that provides lock between two vessels with different pressure. In this instance, the pressure in the cyclone is smaller than in the calciner. In order to return some of the product collected in the cyclone back to the calciner an air lock isrequired. The same fluoseal also serves another function. At the bottom of the fluoseal is a water-cooled screw conveyor that removes the product from the system. At the start-up of the process all cyclone product is returned back to the calciner in order to build a re-circulating load. After the re-circulating load is sufficient, the product screw conveyor may start.In order to achieve the best performance of the equipment at all conditions we conducted very extensive theoretical calculation for all possible scenarios that could exist. Some of the most important parameters are listed below:

� Different operating set points are provided for different stages of operation.� Finer particle size distribution is used for cold start-up.� The fluidizing blowers are selected to deliver additional volume of air at the start-up of

the system.� The start-up burner is designed to deliver an additional heat at the start-up.� Product partial re-circulation is designed into the system to improve the quality of the

product.

Double Stage Calciner With Direct Feel Injection And Water Cooled Cooler

Very often in industrial processes where high temperatures are required heat recovery plays very important role in the design of the equipment. Such a process is illustrated in Figure 3. The objective of the process is to calcine the silica sand in temperature of 850oC in order to remove as much as possible of organic matter contained in the crystals. After the calcination the product is cooled to 100oC. The feed into the calciner consists of fine silica sand with particle size between 150 to 350 μm and contains in average between 8-12% moisture. The designed equipment consists of FB preheater, FB calciner with direct injection of the Heavy Fuel Oil and

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FB cooler with water tubes submerged in the bed. The wet feed is introduced to the preheater where the hot gas from the calciner evaporates all the moisture and preheats the sand before it enters the calciner. The preheater is placed directly over the calciner and therefore the calciner fluidizing blower has to overcome the pressure drop across two distributors and two layers of the bed. The dry and preheated sand enters the calciner through a loop seal. The volume and the temperature of gases leaving the calciner are relatively constant and therefore the fluidizing velocity on the preheater also remains constant. As the feed rate and the moisture into the system varies, the temperature of the sand reporting to the calciner also varies but the effect on the calciner bed temperature control system is relatively small. The calcined product from the calciner is discharged via a water cooled screw feeder into the FB cooler with water tube bundle submerged in the bed. The screw feeder is also used to maintain and control the bed level in the calciner and at the same time to control the retention time of the sand in the calciner.

Figure 3. Two stage calciner

Conclusion

The few examples of processes described above illustrate different ways the fluidized bed technology may be used. The understanding of the principles of fluidization and theory of mechanisms observed during such processes is of most importance when designing a new system. However experience gained from practical observations in real conditions where equipment is exposed to harsh reality of industrial applications and operating conditions very often plays the most critical part.

References

1. D. Geldart, Powder Technol. 7 (1973) 285-2922. R.H. Perry, D.W. Green, Perry’s Chemical Engineer’ Handbook, 1997, 17, 2-19

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