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DC Furnace Technology - BATEMAN Engineering NV

• Processing fine materials and dusts
  
  
• High metal recoveries
  
  
• Low-cost reductants
  
  
• Low electrode consumption
  
  
• Low electrical flicker
  
  
• Accurate process control through independent voltage and current regulation
  
  
• Extended sidewall life from high-intensity copper cooling panels

BATEMAN offers

• Turnkey DC-furnace plants tailored to your specific needs
  
  
• Feasibility studies and financial and commercial arrangements
  
  
• A comprehensive range of pyrometallurgical equipment and services.

Typical applications

Smelting of

• Chromite
• Nickel Laterite ores
• Ilmenite.

Recovery of metals from

• Steel-plant dust
• Stainless steel-plant dusts
• Waste slags - lead blast-furnace slags (LBFS), cobalt.

BATEMAN's DC-furnace technology

• Provides a robust and reliable bottom-anode design
• Minimises electromagnetic arc-deflection which causes sidewall hot spots
• Accommodates high sidewall and roof heat loads associated with open-arc and open-bath
  operations
• Minimises stray arcing caused by typical high operating voltages.

BATEMAN has designed, built and commissioned plants or executed feasibility studies for numerous
ferrous and nonferrous pyrometallurgical processes utilising AC and DC furnaces. These include
smelting of ferrochrome, ferrosilicon, ferromanganese, ilmenite, and recovery of metals such as
zinc and cobalt from waste dusts and slags.

DC FURNACE PAPER

ABSTRACT

Effective pyrometallurgical process vessel design requires accurate assessment of the heat fluxes
through the walls of the furnace. This is particularly important for freeze lining operation which is
designed to protect refractory materials exposed to chemically corrosive molten contents, or
facilitate high temperature operation when the refractory materials are used at conditions
close to their service limits.

Numerical modelling of fluid flow and heat transfer in process vessels is often used to aid in the
design of process vessels. Sophisticated models are used to analyse the three dimensional flow
and heat transfer predicting the effects of electrical heating, magnetic stirring, buoyancy, shear
forces, various cooling effects and ultimately heat fluxes at the walls of the furnace and refractory.
Traditionally these models are applied to the separate single fluid systems in a vessel such as the
freeboard region including the arc, the slag region and the metal bath. Boundary conditions such
as shear forces and heat fluxes between connecting regions such as the slag and metal bath are
either estimated or carried over from separate solutions.

Shortcomings in these traditional approaches include the estimation of sometimes critical
boundary conditions leading to unreliable heat flux calculations. Also when boundary conditions
are carried over between solutions, the process is difficult to set up, time-consuming and finally
not fully coupled.

In this paper the definition and results of a fully integrated numerical model of a complete arc
furnace are presented. The most important mechanisms acting in an arc furnace were considered,
including the fields of electrical potential, current, magnetism, momentum, heat transfer and radiation.

Temperature dependant properties included electrical conductivity, density, viscosity, and thermal
conductivity. The geometry consists of the freeboard, the arc, slag, metal baths and different
refractory regions. Although the combined model of air, slag and metal would be defined as
a multi-phase problem it is not solved as such. Instead the different fluids are separated by
sets of special solid baffles. These baffles allow the implicit transfer of current, magnetism,
heat transfer and shear forces between the different fluids and disallow mixing of the
separate fluids.

The strengths of the integrated model are threefold: Firstly, it provides robustness in defining the
geometry and boundary conditions for the overall model. Secondly, it provides the capability to switch
on and off individual mechanisms such as buoyancy, magnetic stirring and shear forces in order to
observe their individual importance. Finally, it provides a useful tool in the design process through
its ability to obtain results of parameter changes in short time scales.

1. INTRODUCTION

High intensity pyrometallurgical smelting and melting processes employing DC plasma arcs as
the energy input source have increased the demands on the performance of the containment
vessel refractory lining. DC furnaces normally operate with open baths in which molten process
liquids are in direct contact with the refractory lining. The high reaction kinetics associated with
vigorously stirred baths reduces black top formation of un-reacted material floating on the slag
surface, thereby increasing freeboard-operating temperatures by radiation and convection. This
combined with processes involving chemically aggressive 565 slag and superheated phases
have made understanding the influence of the arc on velocities and temperature gradients within
the molten bath critical to successful vessel design.

The open arc offers significant additional operating flexibility, as the total effective furnace resistance
can be adjusted by operating with different arc lengths, thereby ensuring maximum power input over
a wide range of bath resistance. This change in distribution of total power between the arc and the
bath results in a tradeoff between excessive radiation loading in the freeboard and increasing forced
convection in the bath. Bath convection has a direct influence on freeze lining thickness and
mechanical stability, and it also affects the erosion of protective partially reacted raw material
side wall banks. Higher bath convection film coefficient increases the refractory hot face temperature.
The goal of this research was to develop a parametric model in a suitable Computational Fluid
Dynamics (CFD) package, which would enable both design and operating parameters to be
assessed and optimised.

The development plan included:

• Applying fundamental boundary conditions of current, anode potential and external temperatures
  to limit the number of assumptions required for model generation.

• Inclusion of temperature dependant properties (if published).

• Analysis within a single model without the need to transfer partial results between sub-analysis
  steps.

• Conjugate heat transfer1 to obviate the need for assumptions of convection heat transfer
  coefficients.

• Joule (resistance) heating estimated directly from current distribution, and local resistivity.

• Parametric mesh generation to enable the geometry and boundary condition to be
  conveniently varied without the need to start each model from fundamental input commands.

• Control of relaxation factors and other numerical methods to ensure stability during the
  solution, which would be applicable to a wide range of geometries and operating parameters.

• Post processing to allow convenient interpretation and comparison of results.

• Furnace design parameters include:

  • Overall furnace geometry.

  • Cooling systems.

  • Refractory composite selection (based on thermal conductivity).

• Furnace operating parameters include:

  • Furnace inventory levels and tapping cycle management.

  • Selection of furnace electrical operating parameters (volts and amps to achieve a certain
    input power, arc length proportional to operating voltage).
    
    

AN IMPROVED DC-ARC PROCESS FOR CHROMITE SMELTING

ABSTRACT

Mintek together with Middelburg Steel & Alloys, developed the DC arc furnace process for the
production of ferroalloys with the expressed objective of smelting ore fines (<1mm) without the
need for agglomeration. In the case of ferrochrome production, the DC arc furnace, being a
semi-open bath operation, loses radiant energy directly from the bath to the furnace roof and
sidewalls above the melt. This energy loss is reflected in a higher electrical energy usage per
ton of alloy than for a submerged-arc operation smelting lumpy ores or sintered pellets. The
difference increases still further when pre-heated sintered pellet feed is used in a submerged-arc
operation. This shortcoming of the DC arc furnace can however be overcome by preheating the
feed to the furnace in a fluidized bed or flash reactor fuelled by cold furnace off-gas. In addition to
fluidization and preheating testwork, various preheating flowsheets with associated mass and
energy balances have been developed and compared from a techno-economic viewpoint.

The three major advantages identified were:

1. fluidized beds can preheat ores and fluxes up to 950°C without sticking and defluidizing,

2. the open bath DC arc furnace does not require expensive coke or char to maintain the burden
   porosity and,
   

3. the capital cost associated with the milling, pelletising and sintering plant falls away since ore
   fines are used directly.

DC Furnace Technology Brochure - PDF

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