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CLU vs AOD for FeCr refining

What is the best process for oxygen refining of high carbon ferrochrome (HC FeCr) to medium carbon (MC) and low carbon (LC) FeCr? As with most things – there is not a simple answer. UHT metallurgists Annie Lundberg and Kristina Beskow break down the key differences between the CLU and AOD refining processes with calculations and heat simulations made with the process control system UTCAS®. They conclude that the main areas where the processes differ are argon consumption, process time, and slag amount and FeSi consumption.

With a growing demand of MC and LC FeCr, more and more FeCr producers are looking into the possibility to install a converter refining unit in their existing production facility. The most common oxygen converter processes are AOD (Argon Oxygen Decarburization) and CLU (Creusot-Loire-Uddeholm), which are used for stainless steel and ferroalloy refining. Both AOD and CLU are based on injection of oxygen and inert gases, typically argon and nitrogen, into a converter vessel. However, one crucial difference between them is that the CLU process also uses superheated steam as an additional process gas.

– When steam is injected into the converter during processing, it decomposes into hydrogen and oxygen in an endothermic, heat-consuming, reaction, Annie Lundberg, process engineer at UHT, explains.

She continues to describe that the heat-consuming reduction of steam adds a cooling benefit to the CLU process. The formed hydrogen gas also acts as an inert gas to replace argon while oxygen takes part of the carbon oxidation process.

Process design

In order to make a comparison between the two processes, Annie decided together with Kristina Beskow, manager of Process & Control at UHT, to calculate and simulate HC FeCr refining scenarios with the UTCAS® process control system. The system, developed in-house by UHT, has a design tool for process engineers to define and simulate process plans and standard operation practises that thereafter can be used by the system for processing.

– We wanted to create process designs for both AOD and CLU that are as close to actual conditions as possible, Kristina says. Therefore, all calculated cases are designed to finish on similar end carbon content and temperatures to give a representative comparison.

The starting composition was set to 65% chrome and 9% carbon, with end carbon levels of 1.5 and 0.5% and a maximum process temperature of 1750 °C. The heat balance was controlled in the simulation by adjusting the tuyere gas blowing mix of argon, oxygen, and in the case of CLU also steam.

– The UTCAS® system also takes into account the impact of metal chemistry and the composition of added raw materials in its calculations. To find the best processing practices, it is therefore important to try to be as close to actual conditions and raw material contents as possible, Kristina explains.

Kristina Beskow, Manager Process & Control

Annie Lundberg, Process Engineer

Short process time and low argon consumption with CLU

Annie and Kristina noted some interesting differences between the CLU and AOD refining processes in terms of:

  • Process time
  • Slag amount and FeSi consumption
  • Argon consumption

– A large advantage of the CLU process in comparison with AOD is the shorter process time, Annie says. The process time is 42 minutes shorter for the 1.5 %C grade when refining with CLU compared to AOD. For the 0.5 %C grade the process time was 67 minutes shorter.

– The short CLU process time is primarily due to the added steam. The decomposition of the H2O molecule into oxygen and hydrogen results in a cooling effect and provides the process with extra oxygen that speeds up the decarburisation. One of the main reasons to keep the process time as low as possible is that you will have significantly less refractory wear in the converter, Kristina adds.

Due to the addition of more oxygen per time unit in the CLU compared to the AOD, there will be a higher total amount of slag in the CLU cases. The argon consumption however, is notably lesser in the CLU process.

– Argon is an expensive gas, and something you should try to keep as low as possible to keep costs down. Nitrogen, which is used e. g. in stainless steel refining, is not an option due to the high solubility of nitrogen in refined FeCr. Therefore, obtaining hydrogen from steam as an inert gas significantly decreases the need for argon and gives the producer a low-cost option.

In summary, the results show that from a cost-effective point-of-view, the CLU process provides the best option with low argon consumption and process time, while the AOD process could be an option if there is an incentive to keep the slag amount down and if there is a tolerance for longer processing time and more refractory wear.

CLUAOD
FeCr ≤ 1.5 %CFeCr ≤ 0.5 %CFeCr ≤ 1.5 %CFeCr ≤ 0.5 %C
O2 (Nm3)1136151213731982
H2O (Nm3)670208600
Ar (Nm3)14113915383276
Slag (kg)41911158034517146
FeSi (kg)40920232581078
Process time (min)93165135232

Calculated process data from HC FeCr refining with starting conditions of 15000kg FeCr (65 %Cr, 9 %C) and 1600 oC. The processes are designed for an end temperature of 1750 oC.

CLU vs AOD for FeCr refining

What is the best process for oxygen refining of high carbon ferrochrome (HC FeCr) to medium carbon (MC) and low carbon (LC) FeCr? As with most things – there is not a simple answer. UHT metallurgists Annie Lundberg and Kristina Beskow break down the key differences between the CLU and AOD refining processes with calculations and heat simulations made with the process control system UTCAS®. They conclude that the main areas where the processes differ are argon consumption, process time, and slag amount and FeSi consumption.

With a growing demand of MC and LC FeCr, more and more FeCr producers are looking into the possibility to install a converter refining unit in their existing production facility. The most common oxygen converter processes are AOD (Argon Oxygen Decarburization) and CLU (Creusot-Loire-Uddeholm), which are used for stainless steel and ferroalloy refining. Both AOD and CLU are based on injection of oxygen and inert gases, typically argon and nitrogen, into a converter vessel. However, one crucial difference between them is that the CLU process also uses superheated steam as an additional process gas.

– When steam is injected into the converter during processing, it decomposes into hydrogen and oxygen in an endothermic, heat-consuming, reaction, Annie Lundberg, process engineer at UHT, explains.

She continues to describe that the heat-consuming reduction of steam adds a cooling benefit to the CLU process. The formed hydrogen gas also acts as an inert gas to replace argon while oxygen takes part of the carbon oxidation process.

Process design

In order to make a comparison between the two processes, Annie decided together with Kristina Beskow, manager of Process & Control at UHT, to calculate and simulate HC FeCr refining scenarios with the UTCAS® process control system. The system, developed in-house by UHT, has a design tool for process engineers to define and simulate process plans and standard operation practises that thereafter can be used by the system for processing.

– We wanted to create process designs for both AOD and CLU that are as close to actual conditions as possible, Kristina says. Therefore, all calculated cases are designed to finish on similar end carbon content and temperatures to give a representative comparison.

The starting composition was set to 65% chrome and 9% carbon, with end carbon levels of 1.5 and 0.5% and a maximum process temperature of 1750 °C. The heat balance was controlled in the simulation by adjusting the tuyere gas blowing mix of argon, oxygen, and in the case of CLU also steam.

– The UTCAS® system also takes into account the impact of metal chemistry and the composition of added raw materials in its calculations. To find the best processing practices, it is therefore important to try to be as close to actual conditions and raw material contents as possible, Kristina explains.

Kristina Beskow, Manager Process & Control

Annie Lundberg, Process Engineer

Short process time and low argon consumption with CLU

Annie and Kristina noted some interesting differences between the CLU and AOD refining processes in terms of:

  • Process time
  • Slag amount and FeSi consumption
  • Argon consumption

– A large advantage of the CLU process in comparison with AOD is the shorter process time, Annie says. The process time is 42 minutes shorter for the 1.5 %C grade when refining with CLU compared to AOD. For the 0.5 %C grade the process time was 67 minutes shorter.

– The short CLU process time is primarily due to the added steam. The decomposition of the H2O molecule into oxygen and hydrogen results in a cooling effect and provides the process with extra oxygen that speeds up the decarburisation. One of the main reasons to keep the process time as low as possible is that you will have significantly less refractory wear in the converter, Kristina adds.

Due to the addition of more oxygen per time unit in the CLU compared to the AOD, there will be a higher total amount of slag in the CLU cases. The argon consumption however, is notably lesser in the CLU process.

– Argon is an expensive gas, and something you should try to keep as low as possible to keep costs down. Nitrogen, which is used e. g. in stainless steel refining, is not an option due to the high solubility of nitrogen in refined FeCr. Therefore, obtaining hydrogen from steam as an inert gas significantly decreases the need for argon and gives the producer a low-cost option.

In summary, the results show that from a cost-effective point-of-view, the CLU process provides the best option with low argon consumption and process time, while the AOD process could be an option if there is an incentive to keep the slag amount down and if there is a tolerance for longer processing time and more refractory wear.

CLUAOD
FeCr ≤ 1.5 %CFeCr ≤ 0.5 %CFeCr ≤ 1.5 %CFeCr ≤ 0.5 %C
O2 (Nm3)1136151213731982
H2O (Nm3)670208600
Ar (Nm3)14113915383276
Slag (kg)41911158034517146
FeSi (kg)40920232581078
Process time (min)93165135232

Calculated process data from HC FeCr refining with starting conditions of 15000kg FeCr (65 %Cr, 9 %C) and 1600 oC. The processes are designed for an end temperature of 1750 oC.

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