tapping of metallurgical silicon furnaces – a brief ... · silicon in submerged arc furnace is...

Furnace Tapping 2018, Edited by J.D. Steenkamp & H. Kotzé, Southern African Institute of Mining and Metallurgy, Kruger National Park, 14–17 October 2018

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Tapping of metallurgical silicon furnaces – a brief comparison between continuous and discontinuous

processes

V.D. de Oliveira1 1 Viridis.iQ GmbH, Konstanz, Germany

Abstract – The efficiency of the standard carbothermic reduction process for producing f metallurgical silicon in submerged arc furnace is highly dependent of the tapping process. The liquid metalloid generated at high temperatures in the furnace hearth needs to be tapped from the crucible in such a way that the upstream processes are not affected (i.e. stoking process in the top of the furnace, instability of the electrical conditions in the reaction zones, and electrode positions in the charge). There are two basic processes that are used in this industry with proven results and good efficiencies: continuous and discontinuous tapping. The aim of this article is to compare the advantages and disadvantages of these two tapping processes for metallurgical silicon furnaces and provide some guidance for the new entrants, during the process engineering design for greenfield projects, and also to share with operators some important aspects to be considered in their actual operations or eventual Brownfield expansions. The authors will explore different aspects like process control, thermal efficiency of the equipment, silicon recovery, slag generation, losses in the process, environmental emissions and operational complexity, safety risk assessment, and engineering details that allow higher efficiencies to be attained in both cases. The general idea is to provide sufficient information to be used in decision-making, improvement analysis, or optimization of existing operations, expansions, or new projects.

Keywords: Silicon production, continuous/discontinuous tapping, slag viscosity,

environmental emissions, thermal efficiency, silicon recovery.

INTRODUCTION In silicon production, electricity is fundamental to the reduction process. The overall reaction for the silicon process is the following: SiO2 (s) + 2C (s) → Si (l) + CO (g) (T > 2000oC) [1]

The reaction requires a considerable input of electrical energy to occur, which is brought to the reaction zone through the electrodes. The typical path of the electricity is shown in Figure 1.

Figure 1. Overview of the process path of the electricity until the reaction zone (Viridi.iQ GbmH).

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Figure 2 shows a schematic importance of the electrodes

During the production process, the main reactions occur in different zones furnace crucible, as Figure may react and lower the productivity of the furnace, impacting the tapping process (Schei

The general reactions are shown in Figure Under lower part of furnace hearth, Tuset, and The main product


Figure 2. Typical raw materials and electricity path through electrodes in Si process (Viridis.iQ GmbH)

During the production process, the main reactions occur in different zones furnace crucible, as

Figure 3. Distribution of may react and lower the productivity of the furnace, impacting the tapping process (Schei

The general reactions are shown in

Figure 4. Diagrammatic

Under the standard operational conditions, the liquid lower part of furnace hearth, Tuset, and Tveit

the balance between the most exothermic reactions occurring inside the furnace. The main product


Typical raw materials and electricity path through electrodes in Si process (Viridis.iQ GmbH)

During the production process, the main reactions occur in different zones furnace crucible, as illustrated in F

Distribution of phases and elements inside a silicon furnace. Liquid may react and lower the productivity of the furnace, impacting the tapping process (Schei

The general reactions are shown in

Diagrammatic view - reactions in

the standard operational conditions, the liquid lower part of furnace hearth,

Tveit, 1998). These high temperaturesthe balance between the most exothermic reactions occurring inside the furnace.

The main products, as shown in

Figure 2 shows a schematic of the silicon importance of the electrodes and tapping process


During the production process, the main reactions occur in different zones illustrated in Figure

phases and elements inside a silicon furnace. Liquid may react and lower the productivity of the furnace, impacting the tapping process (Schei

The general reactions are shown in Figure 4.

reactions in a submerged arc furnace for silicon production

the standard operational conditions, the liquid lower part of furnace hearth, termed

These high temperaturesthe balance between the most exothermic reactions occurring inside the furnace.

as shown in Figure 3,

silicon production process,and tapping process


During the production process, the main reactions occur in different zones igure 3.

phases and elements inside a silicon furnace. Liquid may react and lower the productivity of the furnace, impacting the tapping process (Schei

igure 4.

a submerged arc furnace for silicon production

the standard operational conditions, the liquid termed the lower reaction zone, above 2000

These high temperaturesthe balance between the most exothermic reactions occurring inside the furnace.

igure 3, are silicon

production process,and tapping process.


During the production process, the main reactions occur in different zones

phases and elements inside a silicon furnace. Liquid silicon in contact with carbon or SiOmay react and lower the productivity of the furnace, impacting the tapping process (Schei

a submerged arc furnace for silicon production

the standard operational conditions, the liquid silicon is produced mostly in the lower reaction zone, above 2000

These high temperatures are achieved due to the electrical arc the balance between the most exothermic reactions occurring inside the furnace.

are silicon metal and slag in

production process, with the emphasis on the


During the production process, the main reactions occur in different zones

ilicon in contact with carbon or SiOmay react and lower the productivity of the furnace, impacting the tapping process (Schei, Tuset, and Tveit, 1998

a submerged arc furnace for silicon production (Bateman

ilicon is produced mostly in the lower reaction zone, above 2000

are achieved due to the electrical arc the balance between the most exothermic reactions occurring inside the furnace.

metal and slag in the

with the emphasis on the


During the production process, the main reactions occur in different zones in

ilicon in contact with carbon or SiOTuset, and Tveit, 1998

(Bateman Engineering)

ilicon is produced mostly in the lower reaction zone, above 2000°C (Schei,


the liquid state.

with the emphasis on the

Typical raw materials and electricity path through electrodes in Si process (Viridis.iQ GmbH).

in the

ilicon in contact with carbon or SiO2 Tuset, and Tveit, 1998).

ngineering).

ilicon is produced mostly in the Schei,


liquid state.

Based on measurein the past 30 years, the tapped temperature of this mix of liquid normallydepending on the process and the operational conditions of electrodes length and useful operati(>1800In some cases, containing some SiC, but th Figure 5 till the refractory ladle (liquid material) and the tapsecondary deextraction system tapping are Figure depicted In the bottom of the furnace hearth, the liquid silicon (brighter material) is produced the lower reaction zone at high temperatures and refractory ladles with capacitiesconsidered as base case scenario a submerged arc furnace with MW. The main targets that furnace operators should consider at this stage are the following:

1 Viridis.iQ GmbH Metallurgy and Engineer2 Viridis.3 Reference: measure4 Viridis.iQ GmbH Metallurgy & Engineering project design

Based on measuren the past 30 years, the tapped temperature of this mix of liquid

normally1 in the range of 1600depending on the process and the operational conditions of electrodes length and useful furnace area available.operating conditions, some gases (mostly SiO(>1800°C)2 and high speed (up to 65In some cases, containing some SiC, but th

Figure 5 shows where the process take place, inside the furnace towartill the refractory ladle (liquid material) and the tapsecondary de-dusting system. extraction system tapping area, within the main furnace building.

Figure 5. Cross-section of tapdepicted (Viridis.iQ GmbH)

In the bottom of the furnace hearth, the liquid silicon (brighter material) is produced the lower reaction zone at high temperatures and refractory ladles with capacitiesconsidered as base case scenario a submerged arc furnace with MW.

The main targets that furnace operators should consider at this stage are the following: Guarantee all

(including cleanness and availability of the tools in the best condition) Usage of the best standard operational procedures in order to avoid

the tap-hole productionwhich result in temperatures, producing reaction:

Viridis.iQ GmbH Metallurgy and EngineerViridis.iQ GmbH Metallurgy & Engineering estimatesReference: measurements done at Elkem plant published in Tveit, et all paper in 1992 Viridis.iQ GmbH Metallurgy & Engineering project design

Based on measurements in different furnaces worldwide, where the author n the past 30 years, the tapped temperature of this mix of liquid

in the range of 1600depending on the process and the operational conditions of electrodes length and

furnace area available.conditions, some gases (mostly SiOand high speed (up to 65

In some cases, the tapping stream may contain containing some SiC, but th

shows where the process take place, inside the furnace towartill the refractory ladle (liquid material) and the tap

dusting system. extraction system is important to minimize the general gas emissions

, within the main furnace building.

section of tap- hole and furnace hearth(Viridis.iQ GmbH).

In the bottom of the furnace hearth, the liquid silicon (brighter material) is produced the lower reaction zone at high temperatures and refractory ladles with capacitiesconsidered as base case scenario a submerged arc furnace with

The main targets that furnace operators should consider at this stage are the following:Guarantee all safety and environmental

luding cleanness and availability of the tools in the best condition)Usage of the best standard operational procedures in order to avoid

hole by materials generated inside the furnace (this could lead to production, due mainly

result in liquid temperatures, producing reaction:

Si (l) + SiO2

Viridis.iQ GmbH Metallurgy and Engineering thermodynamic simulationsGmbH Metallurgy & Engineering estimates

s done at Elkem plant published in Tveit, et all paper in 1992 Viridis.iQ GmbH Metallurgy & Engineering project design

in different furnaces worldwide, where the author n the past 30 years, the tapped temperature of this mix of liquid

in the range of 1600–1650°Cdepending on the process and the operational conditions of electrodes length and

furnace area available. While the tapconditions, some gases (mostly SiOand high speed (up to 65 m/s

the tapping stream may contain containing some SiC, but these instances would


dusting system. Although is important to minimize the general gas emissions


hole and furnace hearth

In the bottom of the furnace hearth, the liquid silicon (brighter material) is produced the lower reaction zone at high temperatures and refractory ladles with capacities4 up to 10 t. considered as base case scenario a submerged arc furnace with

The main targets that furnace operators should consider at this stage are the following:safety and environmental


materials generated inside the furnace (this could lead to mainly to the accumulation of liquid

liquid silicon comtemperatures, producing silicon monoxide

+ SiO2 (s) → 2SiO

ing thermodynamic simulationsGmbH Metallurgy & Engineering estimates

s done at Elkem plant published in Tveit, et all paper in 1992 Viridis.iQ GmbH Metallurgy & Engineering project design


1650°C. These temperatures can depending on the process and the operational conditions of electrodes length and

While the tap-hole is open conditions, some gases (mostly SiO gas and CO

m/s)3 will be expelled the tapping stream may contain

instances would not


Although some plants do not collect those gases, is important to minimize the general gas emissions


hole and furnace hearth. The ladle, shell, furnace

In the bottom of the furnace hearth, the liquid silicon (brighter material) is produced the lower reaction zone at high temperatures and

up to 10 t. considered as base case scenario a submerged arc furnace with

The main targets that furnace operators should consider at this stage are the following:safety and environmental


materials generated inside the furnace (this could lead to to the accumulation of liquid

ilicon comingilicon monoxide

2SiO (g) (>1800°C)

ing thermodynamic simulations. s done at Elkem plant published in Tveit, et all paper in 1992


se temperatures can depending on the process and the operational conditions of electrodes length and

hole is open , depending on gas and CO gasbe expelled during the tapping process.

the tapping stream may contain unreacted material and slurry not occur during

shows where the process take place, inside the furnace towartill the refractory ladle (liquid material) and the tap-hole chapel; also called

some plants do not collect those gases, is important to minimize the general gas emissions

ladle, shell, furnace

In the bottom of the furnace hearth, the liquid silicon (brighter material) is produced the lower reaction zone at high temperatures and flows through the tap

up to 10 t. For this specific paper, the authconsidered as base case scenario a submerged arc furnace with

The main targets that furnace operators should consider at this stage are the following:safety and environmental conditions in


materials generated inside the furnace (this could lead to to the accumulation of liquid

ing into contact with quartz at high ilicon monoxide according to the following

C)

s done at Elkem plant published in Tveit, et all paper in 1992 – “The Tapping

in different furnaces worldwide, where the author n the past 30 years, the tapped temperature of this mix of liquid silicon and slag is

se temperatures can vary by about ±depending on the process and the operational conditions of electrodes length and

, depending on gas) at higher temperatures during the tapping process.

unreacted material and slurry occur during normal operation.

shows where the process take place, inside the furnace towarhole chapel; also called

some plants do not collect those gases, is important to minimize the general gas emissions

ladle, shell, furnace platform, and tapping tools are also

In the bottom of the furnace hearth, the liquid silicon (brighter material) is produced through the tap

For this specific paper, the authconsidered as base case scenario a submerged arc furnace with an active power of 24

The main targets that furnace operators should consider at this stage are the following:conditions in the


materials generated inside the furnace (this could lead to to the accumulation of liquid silicon inside the furnace

contact with quartz at high according to the following

“The Tapping process” – NTNU Conference paper.

in different furnaces worldwide, where the author has worked ilicon and slag is

vary by about ±depending on the process and the operational conditions of electrodes length and

, depending on the furnace ) at higher temperatures

during the tapping process. unreacted material and slurry

normal operation.

shows where the process take place, inside the furnace towards the tap-hole chapel; also called

some plants do not collect those gases, is important to minimize the general gas emissions in the post

and tapping tools are also

In the bottom of the furnace hearth, the liquid silicon (brighter material) is produced through the tap-hole into

For this specific paper, the authactive power of 24

The main targets that furnace operators should consider at this stage are the following:the tapping area

luding cleanness and availability of the tools in the best condition). Usage of the best standard operational procedures in order to avoid clogging of

materials generated inside the furnace (this could lead to lossilicon inside the furnace

contact with quartz at high according to the following

[ 1]

NTNU Conference paper.

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worked ilicon and slag is

vary by about ±2%, depending on the process and the operational conditions of electrodes length and

furnace ) at higher temperatures

during the tapping process. unreacted material and slurry

normal operation.

-hole, hole chapel; also called the

some plants do not collect those gases, a gas the post-

and tapping tools are also

In the bottom of the furnace hearth, the liquid silicon (brighter material) is produced in to the

For this specific paper, the author active power of 24

The main targets that furnace operators should consider at this stage are the following: tapping area

clogging of loss of

ilicon inside the furnace contact with quartz at high

according to the following back-

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Over time, the accumulated silicon may also react with carbon and hence disturb electrical conditions in the reaction zones, leading to instability of electrode positions and high volumes of gas (blows) on top of the furnace. These conditions will also significantly increase the temperature on the top of the furnace, putting the operation and equipment at risk.

During operation, be assured of the dimensional integrity of the tap-hole, and the best usage of the main tools used during the tapping procedure.

Avoid potential cross-contamination (most likely by iron) during the tapping process, caused mostly by incorrect usage of the steel tools during tapping.

Carry out important support and control procedures during tapping (level of liquid silicon into the refractory ladle, thermal control between 1500 and 1700°C, based on the author’s experience), liquid silicon sampling, etc.).

Evaluate and maintain all equipment and tools in this area and the post-tapping area (platform position and conditions, tapping stinger, mud gun, secondary de-dusting valves and the chapel, tap-hole refractories, as well as the carbon blocks) as a preventative action.

Problems related to the tapping process will affect furnace performance and in general also the stoking and control room operations. Quality and environmental problems could also arise due to improper tapping operation. The tapping area is one of the most risky areas for operators and safety procedures and condition of equipment MUST to be observed to avoid any injuries or accidents. Based on the author’s experience in silicon operations worldwide, good tapping procedures will help to obtain optimum silicon yields in the process. The tapping process includes direct costs and hence influences the overall operational expenditures of the plant. Tapping in silicon operations normally is done continuously, keeping the tap-hole constantly open and exchanging the ladles time to time based on the tapping cycle. In a discontinuous tapping process, the tap-hole is open for a certain period of time and closed at the end of that cycle. Table 1 is a simplistic qualitative overview of the pros and cons of continuous and discontinuous tapping of silicon furnaces.

Table 1 Comparison (qualitative) on continuous and discontinuous tapping. Item Continuous tapping Discontinuous tapping Stoichiometry +++ + Thermal stability + +++ Electrical stability +++ + Environmental emissions + +++ Tapping consumables +++ + Labour intensity + +++ Cross-contamination (quality) +++ + For continuous tapping the stoichiometric balance inside the furnace is favourable, due to the lower risk of reverse reaction; this also impacts positively o the electrical stability due to the lower risk of liquid metalloid accumulation inside the crucible. Since the frequency of opening and closing of the tap-hole is much lower than with discontinuous tapping, much fewer consumables are needed. The risk of quality cross-contamination is also lower.

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With discontinuous tapping, thermal stability is more easily attained, due to the fact that during the part of the cycle while the furnace is closed, the liquid silicon is accumulated and the reaction zone loses no energy through the tap-hole. Keeping the furnace closed during the discontinuous tapping cycle will also reduce the environmental emissions. In terms of labour intensity, discontinuous tapping has an advantage due to the fact that during the part of the cycle while furnace is closed the furnace operators (tappers) are not required to work specifically in this region. A general aggregated versus non-aggregated value for both cycles is presented in the next section. Nowadays, there are several silicon operators that are using continuous casting in their operations, for example the Ferroglobe furnaces in the USA, Europe, and South Africa; Brazilian producers such as RIMA, Liasa, and Minas Ligas; and RW Silicon in Germany (AMG Group). On other hand, a few producers in the Western world are using the discontinuous tapping process; i.e. Mississippi Silicon and PCC Bakki will implement the discontinuous tapping process. In the Eastern world, most of the Chinese operators and SIMCOA (Australia) are running their furnaces with discontinuous tapping. SIMCOA utilizes discontinuous tapping most of the time and a continuous regime part of the time. GENERAL ANALYSIS OF THE TWO ROUTES IN AN ESTABLISHED BASE CASE

SCENARIO In this section, a base case scenario is presented and used as premise for a brief comparison between continuous and discontinuous tapping. All the advantages and disadvantages described here are related to that specific case. In general, the ideal tapping process needs to be carefully evaluated case by case for an optimum decision in terms of results to be achieved. The type of equipment, raw materials used in the process, skills level of the operational staff, as well as the post-tapping layout area are fundamental considerations in this analysis. The base case scenario is a 24 MW, 33 MVA furnace operating with three monophasic oil-forced-water-forced (OFWF) transformers, with separate tapchangers for the different levels of electrical parameters that the furnace is operated at. The electrode diameter is 1272 mm (50 inches) and is considered an alternative electrode technology to Södeberg paste. The pitch circle diameter (PCD) used for the analysis was 3050 mm and furnace hearth dimensions 6900 mm (bottom diameter) and 3200 mm (total height). The secondary operation range was 76–84 kA, and the active power range between 23 and 24 MW. The range C3 Westly factor was 9.6 to 10.2 and the operational resistance 1.1 to 1.2 mΩ. The optimum distance between the bottom of the crucible (hearth) and the tip of electrode recommended by the author as a key operational parameter was 400 mm. The mass balance considered was 108% (with 8% of excess carbon considering the stoichiometric balance of the overall reaction (Equation [1]). The raw material considered in the reaction zone is depicted schematically in Figure 6 (carbon source: 50% from charcoal, 30% from low-ash coal, 20% from woodchips) –

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excess from carbon)

Figure V.D.Oliveira Based on the scenario defined, the energy consumption considered was 11process

Figure

Based on the parameters mentioned above, the silicon produced in the reaction zone at 2000

whereSi output = PW =t = Time Utilizatione.c. = 6) 11.

excess from lowarbon).

Figure 6. Reducing agents usage in V.D.Oliveira benchmarking

Based on the scenario defined, the energy consumption considered was 11 232 kWh/t of process equipment

Figure 7. Silicon recovery vs

Based on the parameters mentioned above, the ilicon produced in the reaction zone at 2000

=here:

Si output = hourly production PW =Active power

ime (h) – considered 1Utilization=%timee.c. = Energy consumption

.232 MWh/t.

low-ash coal addition in the normal

Reducing agents usage in benchmarking data, 2013)

Based on the scenario defined, the energy consumption considered kWh/t of silicon (considering a

equipment of 90% and 80% respectively, as shown in

Silicon recovery vs thermal efficiency


× × . .× ,

ourly production ower (MW) – considered 23considered 1 h (hourly production)timeinoperation (1

nergy consumption (kWh/t.

coal addition in the normal

Reducing agents usage in silicon and FeSi furnaces worldwide (Myrvågnes, 2008data, 2013).

Based on the scenario defined, the energy consumption considered ilicon (considering a

90% and 80% respectively, as shown in

thermal efficiency in metallurgical


[2]

ourly production (t/h) considered 23h (hourly production)

operation (1 – %downtime) kWh/t) – calculated based on Shei

coal addition in the normal

and FeSi furnaces worldwide (Myrvågnes, 2008

Based on the scenario defined, the energy consumption considered ilicon (considering a silicon


in metallurgical silicon

Based on the parameters mentioned above, the author determineilicon produced in the reaction zone at 2000°C:

considered 23.33 MW (PWh (hourly production)

%downtime) – considered 100% calculated based on Shei

coal addition in the normal feeding and batching


Based on the scenario defined, the energy consumption considered silicon recovery and


silicon manufacturing process (

author determine

MW (PW=ficos0.707×

considered 100% calculated based on Shei, Tuset,

feeding and batching


Based on the scenario defined, the energy consumption considered in recovery and thermal efficiency

90% and 80% respectively, as shown in Figure 7).

manufacturing process (

author determined the volume of liquid

×App.power

considered 100% (hourly production)Tuset, and Tveit

feeding and batching process (+8% of

and FeSi furnaces worldwide (Myrvågnes, 2008; updated with

in these estimates thermal efficiency

.

manufacturing process (Schei, et all, 1998

the volume of liquid

ower33MVA)

(hourly production) Tveit (1998) (F

process (+8% of

updated with

these estimates thermal efficiency of

1998).

the volume of liquid

Figure

7

Based on these parameters, the calculated hourly production of the reference submerged arc furnace is approximately 2.077 t of silicon. Considering the stationary product generation in normal conditions in the reaction zone, as well as the temperature of 2000°C; is possible to calculate the liquid silicon density inside the furnace. This data will be important in determining the metalloid static height increase due to silicon accumulation inside the furnace. Considering the liquid silicon density as a function of temperature:

( ) = . − . − . [3]

the densities of the tapped liquid silicon (t) (1650°C) and reduced silicon (r) inside the hearth (2000°C) are respectively ρSi(t) = 2.453 g/cm3 and ρSi(r) = 2.427 g/cm3. In terms of silicon accumulation, 0.855 m3/h accumulation inside the furnace hearth and 0.847 m3/h for the tapping process are used. The furnace hearth volume, for didactic purposes was calculated based on three different diameters (referring to the upper, middle, and lower reaction zones. It is important to mention that this is very dynamic parameter and depending on the operation of the furnace, these reactions zones could be mixed and disturbed along the furnace crucible hearth. The premises are based on a design project for a 33 MVA submerged arc furnace for silicon production (Figure 8).

Figure 8. Schematic view of a submerged arc furnace shell used as a base case scenario (Viridis.iQ GmbH). Table 2 summarize the dimensions of the upper, middle, and lower reaction zones, considered for the useful volume calculation (n = U, M or L):

Table 2 Dimensions considered for utile volume calculation. Reaction zone Diameter (Ф nZ) Height (h nZ) – from bottom to the top cumulative UZ (upper reaction zone) 9000 mm 3200 mm MZ (middle reaction zone) 7800 mm 2800 mm LZ (lower reaction zone) 6900 mm 1000 mm Based on previous studies (including technical papers and excavation works done in several plants in Norway), in normal operations is appropriate to target the tip

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electrode position close to thereasons for that aim Figure Based also in some references and image analysis, the operational considered by the author

Figure This represents normal conditions of this actual base case scenario here described, considering the operational furnaces conditions and of the production, until the ’reverse

electrode position close to thereasons for that aim

Figure 9. (a) Representative behaviour for the bottom region (0.3

Based also in some references and image analysis, the operational considered by the author

Figure 10. Schematic of a silicon furnace and

This represents normal conditions of this actual base case scenario here described, considering the operational furnaces conditions and

the density of production, until the ’reverse reaction

electrode position close to thereasons for that aim.

(a) Representative behaviour for the bottom region (0.3

Based also in some references and image analysis, the operational considered by the author is

Schematic of a silicon furnace and


density of silicon, approximately 2production, until the silicon inside the hearth

reaction’ shown in

Figure 11

electrode position close to the hearth bottom, at 0.3

(a) Representative behaviour for the bottom region (0.3

Based also in some references and image analysis, the operational is in the range of 10 to 20% of the design

Schematic of a silicon furnace and photograph


ilicon, approximately 2ilicon inside the hearth

shown in Equation [

11. Thermodynamic parameters of the

hearth bottom, at 0.3

(a) Representative behaviour for the bottom region (0.3 m) and (b) for the top region (0.6

Based also in some references and image analysis, the operational in the range of 10 to 20% of the design

photograph of crust build


ilicon, approximately 2 hilicon inside the hearth

[2] starts to

Thermodynamic parameters of the

hearth bottom, at 0.3 m. The graphs

m) and (b) for the top region (0.6

Based also in some references and image analysis, the operational in the range of 10 to 20% of the design

of crust build-up around g

This represents normal conditions of this actual base case scenario here described, considering the operational furnaces conditions and previously

hours (1.98ilicon inside the hearth accumulates

starts to take place.

Thermodynamic parameters of the reverse

m. The graphs in Figure 9

m) and (b) for the top region (0.6

Based also in some references and image analysis, the operational in the range of 10 to 20% of the design useful

up around gas cavity (Elkem report)

This represents normal conditions of this actual base case scenario here described, previously calculated parameters

98 hours) of liquid accumulates to a height

reverse reaction.

in Figure 9 show the

m) and (b) for the top region (0.6 m).

Based also in some references and image analysis, the operational useful volume useful volume.

as cavity (Elkem report)

This represents normal conditions of this actual base case scenario here described, calculated parameters

) of liquid silicon a height where the

show the

volume

as cavity (Elkem report).

This represents normal conditions of this actual base case scenario here described, calculated parameters

ilicon where the

The (Figure 11) liquid SiOkilogram of From the process standpoint, the difference between continuous and discontinuous tapping is that the cycles of discontinuous tapping must to be carefully the accumulation of material inside the furnace losses of silicon due to the backwards reaction risk is much lower, Based on a the tapping area of a similar furnace for this analysis:

The correlated energy outputs to the tapping area were calculated considering the SiO and CO gas The energy loss calculation considered the diffusion flux of the main species (CO, SiO, slag, The estimated loss based on the kJ/s (or 591(14%), This data is importmethods in metallurgical

5 The enthalpy of a system is equal to the system's6 The Gibbs free energy reversibleand pressure Table 2

The enthalpy5 and Gibbs free energy(Figure 11) confirm that above 1800liquid SiO2; this represents 6kilogram of silicon converted via

From the process standpoint, the difference between continuous and discontinuous tapping is that the cycles of discontinuous tapping must to be carefully the accumulation of material inside the furnace losses of silicon due to the backwards reaction risk is much lower,

Based on a mass and energy balance the tapping area of a similar furnace for this analysis:

The correlated energy outputs to the tapping area were calculated considering the SiO and CO gas flows

The energy loss calculation considered the diffusion flux of the main species (CO, SiO, and Si), heat losses, species ent

The estimated loss based on the kJ/s (or 591 KW) distributed (14%), and heating flux (8%).

This data is importmethods in metallurgical

The enthalpy of a system is equal to the system'sThe Gibbs free energy

eversible work that may be performed by apressure (isothermal

Table 2I.- Times simulation for the three tapping cycles (estimates for the base

and Gibbs free energyconfirm that above 1800

; this represents 6con converted via

From the process standpoint, the difference between continuous and discontinuous tapping is that the cycles of discontinuous tapping must to be carefully the accumulation of material inside the furnace losses of silicon due to the backwards reaction risk is much lower, since the level

mass and energy balance the tapping area of a similar furnace for

Figure 12. The energy streams in the tapping area (Kamfjord

The correlated energy outputs to the tapping area were calculated considering the SiO flows into the tapping process.

The energy loss calculation considered the diffusion flux of the main species (CO, SiO, and Si), heat losses, species ent

The estimated loss based on the KW) distributed eating flux (8%).

This data is important for methods in metallurgical silicon production.

The enthalpy of a system is equal to the system'sThe Gibbs free energy is a

that may be performed by aisothermal, isobaric).

Times simulation for the three tapping cycles (estimates for the base

and Gibbs free energyconfirm that above 1800°

; this represents 6 kWh endothermic consumption and 1.6con converted via the reverse

From the process standpoint, the difference between continuous and discontinuous tapping is that the cycles of discontinuous tapping must to be carefully the accumulation of material inside the furnace losses of silicon due to the backwards reaction

since the level of accumulation is considerably smaller.

mass and energy balance the tapping area of a similar furnace for

The energy streams in the tapping area (Kamfjord

The correlated energy outputs to the tapping area were calculated considering the SiO into the tapping process.

The energy loss calculation considered the diffusion flux of the main species (CO, SiO, and Si), heat losses, species enthalpy

The estimated loss based on the tap-hole design and process base case scenario is KW) distributed between reating flux (8%).

for comparingilicon production.

The enthalpy of a system is equal to the system's internal energy

is a thermodynamic potentialthat may be performed by a


and Gibbs free energy6 related to the °C liquid silicon can react endothermically with

kWh endothermic consumption and 1.6the reverse reaction.

From the process standpoint, the difference between continuous and discontinuous tapping is that the cycles of discontinuous tapping must to be carefully the accumulation of material inside the furnace losses of silicon due to the backwards reaction will

of accumulation is considerably smaller.

mass and energy balance study (Kamfjorthe tapping area of a similar furnace for silicon production


The correlated energy outputs to the tapping area were calculated considering the SiO into the tapping process.

The energy loss calculation considered the diffusion flux of the main species (CO, SiO, halpy, and effective conductivity.

hole design and process base case scenario is between radiation (51%),

ing the energy losses between the two tapping ilicon production.

internal energy thermodynamic potential that can be used to calculate the maximum of

that may be performed by a thermodynamic system


related to the reverse ilicon can react endothermically with

kWh endothermic consumption and 1.6reaction.

From the process standpoint, the difference between continuous and discontinuous tapping is that the cycles of discontinuous tapping must to be carefully the accumulation of material inside the furnace (liquid silicon)

will occur. For continuous tapping, this of accumulation is considerably smaller.

Kamfjord, 2012), ilicon production


The correlated energy outputs to the tapping area were calculated considering the SiO

The energy loss calculation considered the diffusion flux of the main species (CO, SiO, and effective conductivity.

hole design and process base case scenario is adiation (51%), air convection (27%),

the energy losses between the two tapping

plus the product of its pressure and volumethat can be used to calculate the maximum of

thermodynamic system


reverse reaction (ilicon can react endothermically with

kWh endothermic consumption and 1.6 Nm

From the process standpoint, the difference between continuous and discontinuous tapping is that the cycles of discontinuous tapping must to be carefully

ilicon) reaches a level where For continuous tapping, this


d, 2012), the energy streams from ilicon production were used as reference for

The energy streams in the tapping area (Kamfjord, 2012).


The energy loss calculation considered the diffusion flux of the main species (CO, SiO, and effective conductivity.

hole design and process base case scenario is ir convection (27%),


plus the product of its pressure and volumethat can be used to calculate the maximum of

thermodynamic system at a constant

Times simulation for the three tapping cycles (estimates for the base case scenario).

reaction (Equation [ilicon can react endothermically with

Nm3 of SiO(g)

From the process standpoint, the difference between continuous and discontinuous tapping is that the cycles of discontinuous tapping must to be carefully handled

reaches a level where For continuous tapping, this


the energy streams from used as reference for


The energy loss calculation considered the diffusion flux of the main species (CO, SiO,

hole design and process base case scenario is ir convection (27%), purge gas


plus the product of its pressure and volume that can be used to calculate the maximum of

at a constant temperature

case scenario).

9

Equation [2]) ilicon can react endothermically with

(g) per

From the process standpoint, the difference between continuous and discontinuous handled once

reaches a level where For continuous tapping, this

the energy streams from used as reference for


The energy loss calculation considered the diffusion flux of the main species (CO, SiO,

hole design and process base case scenario is 591 urge gas


that can be used to calculate the maximum of temperature

10

In order to evaluate the production steps of thused

To achieve an are supposedly similar in terms of overall equipment efficiency and the silicon yield is considered the same. a furnace losses negatively Considenon-tapping, the activities considered as aggregated value for the stream mapping process are aaggregated value activities in that value stream mapping. This difference is Considering the same performance, kg/min mention that ththe beginning of the tapping cycle at the end of the process is lower than the average. These oscillations shows much lower amplitude for continuous tapping, because the furnace is kept open all the time. From the safety standpoint, thihigh volume of liquid hot metalloid (important to mention that the base case scenario was considered moveable furnace operator platform with proper refractory walls and metallic mirror protection.

In order to evaluate the production steps of thd as reference the tapping cycles

Table 3 -

achieve an impartial analysis, the author started from the point that both process supposedly similar in terms of overall equipment efficiency and the silicon yield is

considered the same. furnace that run

losses from furnaces running with continuous tapping could benegatively impacting

Considering the -aggregated value

tapping, the activities considered as aggregated value for the stream mapping process at a level of

aggregated value activities in that value stream mapping.

This difference is Considering the same performance, kg/min and during mention that ththe beginning of the tapping cycle at the end of the process is lower than the average. These oscillations shows much lower amplitude for continuous tapping, because the furnace is kept open all the time.

From the safety standpoint, thihigh volume of liquid hot metalloid (important to mention that the base case scenario was considered moveable furnace operator platform with proper refractory walls and metallic mirror protection. Non

In order to evaluate the production steps of thas reference the tapping cycles

Tapping cycle

impartial analysis, the author started from the point that both process supposedly similar in terms of overall equipment efficiency and the silicon yield is

considered the same. On the runs under

furnaces running with continuous tapping could beimpacting the silicon yield recovery for those furnaces, for example.

ring the three tapping cycles for a similar furnace, the initial aggregateaggregated values for both cycles are quite different. In the case of continuous

tapping, the activities considered as aggregated value for the stream mapping process level of 96%, while for discontinuous tapping, this value is only 23% of


This difference is compensatConsidering the same performance,

during discontinuousmention that these averages will oscillate, mostly for discontinuous tapping where in the beginning of the tapping cycle at the end of the process is lower than the average. These oscillations shows much lower amplitude for continuous tapping, because the furnace is kept open all the time.

From the safety standpoint, thihigh volume of liquid hot metalloid (important to mention that the base case scenario was considered moveable furnace operator platform with proper refractory walls and metallic mirror

Nonetheless, higher volumes of liquid

In order to evaluate the production steps of thas reference the tapping cycles shown in

cycles parametrize


On the one hand, the under the discontinuous tapping regime;

furnaces running with continuous tapping could bethe silicon yield recovery for those furnaces, for example.

tapping cycles for a similar furnace, the initial aggregatefor both cycles are quite different. In the case of continuous

tapping, the activities considered as aggregated value for the stream mapping process while for discontinuous tapping, this value is only 23% of


compensated by the liquid flow of Considering the same performance, for

continuous tapping the average is 90se averages will oscillate, mostly for discontinuous tapping where in

the beginning of the tapping cycle the flow is at the end of the process is lower than the average. These oscillations shows much lower amplitude for continuous tapping, because the furnace is kept open all the time.

From the safety standpoint, this high oscillation is a risk for the operation due to the high volume of liquid hot metalloid (simportant to mention that the base case scenario was considered moveable furnace operator platform with proper refractory walls and metallic mirror

, higher volumes of liquid

In order to evaluate the production steps of thshown in Table 2

s parametrized (compari


one hand, the reverse discontinuous tapping regime;





by the liquid flow of for continuous

tapping the average is 90se averages will oscillate, mostly for discontinuous tapping where in

the flow is substantially at the end of the process is lower than the average. These oscillations shows much lower amplitude for continuous tapping, because the furnace is kept open all the time.

s high oscillation is a risk for the operation due to the silicon) and liquid slag

important to mention that the base case scenario was considered moveable furnace operator platform with proper refractory walls and metallic mirror

, higher volumes of liquid

In order to evaluate the production steps of these two tapping methods, the author Table 2I.

d (comparison discontinuous vs continuous)


reverse reaction could impact negativeldiscontinuous tapping regime;





by the liquid flow of silicon during both regimes. continuous tapping

tapping the average is 90se averages will oscillate, mostly for discontinuous tapping where in

substantially higher than the average and at the end of the process is lower than the average. These oscillations shows much lower amplitude for continuous tapping, because the furnace is kept open all the time.

s high oscillation is a risk for the operation due to the ilicon) and liquid slag

important to mention that the base case scenario was considered moveable furnace operator platform with proper refractory walls and metallic mirror

, higher volumes of liquid silicon lead to

tapping methods, the author

son discontinuous vs continuous)


reaction could impact negativeldiscontinuous tapping regime; on other hand the heat

furnaces running with continuous tapping could be the silicon yield recovery for those furnaces, for example.



ilicon during both regimes. tapping the average flow is 36

tapping the average is 90 kg/min. Itse averages will oscillate, mostly for discontinuous tapping where in

higher than the average and at the end of the process is lower than the average. These oscillations shows much lower amplitude for continuous tapping, because the furnace is kept open all the time.

s high oscillation is a risk for the operation due to the ilicon) and liquid slag that must be

important to mention that the base case scenario was considered to incorporate a moveable furnace operator platform with proper refractory walls and metallic mirror

ilicon lead to a higher operational


son discontinuous vs continuous)


reaction could impact negativelother hand the heat

slightly higher, the silicon yield recovery for those furnaces, for example.

tapping cycles for a similar furnace, the initial aggregatedfor both cycles are quite different. In the case of continuous


ilicon during both regimes. the average flow is 36

kg/min. It is crucial to se averages will oscillate, mostly for discontinuous tapping where in


s high oscillation is a risk for the operation due to the that must be handled

to incorporate a moveable furnace operator platform with proper refractory walls and metallic mirror

higher operational



reaction could impact negatively on other hand the heat

higher,

d and for both cycles are quite different. In the case of continuous

tapping, the activities considered as aggregated value for the stream mapping process while for discontinuous tapping, this value is only 23% of the

ilicon during both regimes. the average flow is 36.01

s crucial to se averages will oscillate, mostly for discontinuous tapping where in


s high oscillation is a risk for the operation due to the ed. It is

to incorporate a moveable furnace operator platform with proper refractory walls and metallic mirror

higher operational

11

risk that needs to be mitigated by appropriate operational standard procedures, handbooks, and visual control. sAnother important point related to the process is the heat exchange correlated to continuous and discontinuous tapping. Considering the tap-hole design for the base case scenario with an inclination of 6 degrees (internally) and 15 degrees (in the tap lip) and a tap-hole 150 mm in diameter, the total energy lost per minute in the base case scenario, based on the author’s calculations, is around 591 kWh. Considering that for continuous tapping the furnace is open during all the time used in the calculus basis (390 minutes – Table 2I) while for discontinuous tapping the tap-hole is open for 90 minutes, the estimated losses in the base case scenario are quite different. While for discontinuous tapping the energy loss on tapping is 0.6%, in continuous tapping the loss could reach up to 2.5%. Considering also the higher risk of the reverse reaction in discontinuous tapping, this will contribute significantly to the risk of blows on top of the furnace in the stoking area (1.6 Nm3 SiO gas per kilogram of silicon converted back to SiO (Kamfjord, 2012); this would lead to higher temperatures in the stoking area and a higher volume of gases in the primary de-dusting system. The situation described above could also impact negatively on the electrical conditions inside the furnace hearth, causing considerable difficulty in maintaining electrode penetration into the submerged positions in the charge. In terms of consumables, in the short term, discontinuous tapping will result in higher consumption of tap-hole clays and an increase in tapping mouth maintenance and repairs, which will increase maintenance costs because of the much more frequent processes set-ups (opening and closing of tap-holes). In continuous tapping, on the other hand, the consumption of the tapping stinger is higher. The risk of mushroom formation inside the tapping channel is greater for discontinuous tapping. Therefore, from this perspective the continuous tapping regime has a slight economical advantage. The skills level of operational staff is a fundamental factor for this analysis. Although discontinuous tapping requires fewer operational staff in the tapping area (most likely, the tappers), the furnace control operators (controllers) are required to have much higher skills to operate the furnace due to the greater electrical and thermochemical instability caused by increased liquid accumulation inside the crucible. For new entrants in greenfield operations, this aspect will favour the adoption of a continuous regime. In terms of quality, for the same raw materials used in the smelting process, with continuous tapping, the time required for refining by oxidization and slagging in the refractory ladle is much greater than with discontinuous tapping. This could lead to low OPEX and higher assertiveness hit levels; increase the overall equipment effectiveness for the plant (OEE7). Based on actual scenarios in industry, there are different references so far. The equipment and layout of the tapping area will also influence the changes that the operator will implement to increase tapping efficiency. The ladle car design, layout of

7 Overall equipment effectiveness (OEE) is a term coined by Seiichi Nakajima[1] in the 1960s to evaluate how effectively a manufacturing operation is utilized. It is based on the Harrington Emerson way of thinking regarding labor efficiency

12

the post-tapping area, and the tools used for tapping are fundamentally important regarding the best regime to implement.

CONCLUSIONS Although each tapping regime, both discontinuous and continuous, has advantages and disadvantages, it is important to evaluate the best option on a case-by-case basis. However, in general, considering the post-tapping installations that allow ladles to be exchanged with no losses and movement in the furnace area, continuous tapping looks to be a better option from the quality and environmental standpoints. From the operational standpoint, discontinuous tapping involves lower energy losses in the tapping process and less exposure of workers to high-temperature materials and gases (although greater volumes of molten materials must to be handled in this type of regime). The operational risks are lower in continuous tapping, and process stability is easier to achieve. The level of operational staff skill required for discontinuous tapping is higher than for continuous tapping. Therefore, new entrants building greenfield silicon plants could encounter fewer difficulties if the plant design is a flexible one, allowing operation under either continuous or discontinuous tapping. The learning curve for operators will be less steep if continuous tapping is the chosen regime for star-up. For brownfield operations, the installations and operational staff skills will affect the decision as to which regime is the best. As an example, for a brownfield expansion in a plant that is running with good results using discontinuous tapping, as long as the knowhow of the staff will be used to run the expansion, that regime will probably be the best choice for running the ’new’ furnace. In terms operational excellence, the best regime to reach the highest efficiency levels will depend on several factors such as operational staff and planned installations (mainly the tapping and post-tapping equipment). In discontinuous tapping the risk of the reverse reaction (producing silicon monoxide) due to the high levels of liquid silicon accumulated while the tap-hole is closed could cause problems for the electrical stability in the reaction zones and electrode penetration. This could result in several issues during operation, which will adversely affect furnace output. On the other hand, the higher thermal losses during continuous tapping could adversely affect the thermal balance in the reaction zones, which will also reduce the process efficiency. To mitigate this, a hybrid approach could be used as to balance the positive and negative aspects of both regimes. It should be stressed that there is no ’definitive best solution’ regarding a choice between the two regimes – continuous and discontinuous tapping. This should be a ’case-by-case’ decision with due consideration of the project type (greenfield/brownfield); plant installations, operational skills of staff, and engineering design. Last but not least, some companies are nowadays operating with a kind of ’hybrid’ mode that uses either continuous or discontinuous tapping, depending on the furnace and plant conditions. This is quite reasonable and in some cases the optimum operational level could be found with this approach.

This paper is published by permission of colleagu

Domingos de Oliveira, V

Schei Ciftja, T.A.

Tveit, production, ELKEM ASMay https://www.saimm.co.za/Conferences/FurnaceTapping/147 Ravary, B. and Laclau, J

Kadkhodabeigi,

Domingos de Oliveira,

Gunnewiek,

Johansen,

Kojima Endo, R

Kamfjord, N.E. Trondheim,Westly J.

This paper is published by permission of colleagues are gratefully

Domingos de Oliveira, VMetallurgical Industry Silicon and Ferrosilicon plantsConference (CFAIC)

A., Tuset, J., Ciftja, T.A., Engh,

DepartmentTveit, H., Andersen, production, ELKEM ASMay 2014. Southern African Institute of Mining and Metallurgy, Johannesburg. https://www.saimm.co.za/Conferences/FurnaceTapping/147

Ravary, B. and Laclau, Jflow in theProducers Research Association (FFF), Trondheim

Kadkhodabeigi, M. (2011). NTNU, Trondheim,

Domingos de Oliveira, to operations

Gunnewiek, L., Ravary, fugitive emissions control121-129. https://www.pyrometallurgy.co.za/InfaconXII/121

Johansen, S.T., Tveit, aspects of ferroProceedings of https://www.pyrometallurgy.co.za/InfaconVIII/059

Kojima Endo, R.,silicon by molecular dynamics simulation505-511. https://pdfs.semanticscholar.org/914e/7ea67bd5ec8c7be943df0edbc4b1f2d3234c.pdf

Kamfjord, N.E. (2012). Trondheim, NorwayWestly J. (1979). Dimension des fours de reduction pour Fe

du Four Electrique

This paper is published by permission of es are gratefully acknowledged.

Domingos de Oliveira, V.. (2016). Metallurgical Industry Silicon and Ferrosilicon plantsConference (CFAIC) - IOM3

and Tveit H. (1998).Engh, M., and Tangstad

Department of MaterialsAndersen, V., Berget,

production, ELKEM AS. Proceedings of Southern African Institute of Mining and Metallurgy, Johannesburg.

https://www.saimm.co.za/Conferences/FurnaceTapping/147

Ravary, B. and Laclau, J-C. (1999the charge and out

Producers Research Association (FFF), TrondheimM. (2011). Modeling of tapping processes in submerged arc furnaces

NTNU, Trondheim, https://core.ac.uk/download/pdf/52103741.pdfDomingos de Oliveira, V. (2014).

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fugitive emissions controlhttps://www.pyrometallurgy.co.za/InfaconXII/121

Tveit, H., Gradahl, aspects of ferro-siliconProceedings of Infacon VIII, Beijing, China, 7https://www.pyrometallurgy.co.za/InfaconVIII/059

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du Four Electrique, 1, 14-

Valdiney Domingos de OliveiraDirector of Metallurgy & Engineering, ViridisMr. Domingos has more than 16 years of

engineering and operations. he worked on Experience with different SAF technologies (SMS, Tenova, Elkem, Tagliaferri, BrazilianChinese equipment); operational experience with from 10 MW to 32process, quality

ACKNOWLEDGEMENTSThis paper is published by permission of

acknowledged.

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Journal

industrial experience in silicon and ferroalloys pecialist in Metallurgy Thermodynamics and Chemical Engineer,

electrode technologies. Experience with different SAF technologies (SMS, Tenova, Elkem, Tagliaferri, Brazilian, and

tapping in furnaces Responsible for operations,

tapping of metallurgical silicon furnaces – a brief ... · silicon in submerged arc furnace is...

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