How to prolong life of the 5100M3 blast furnace?

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By Yu Guohua, Li Qingyang, Chencheng, Zhang Xiangguo & Wang Bin

 

The capacity of the 5,100m3 blast furnace at ShanSteel Group Rizhao Co Ltd is designed for 15,000t/m3 of extended BF life. In order to achieve a long campaign life of the blast furnace, a series of advanced technologies is used. These include a reasonable blast furnace profile, scientific selection of lining structure, selection of high quality refractories, full cooling structure of the BF body and closed loop soft water cooling system, advanced and perfect monitoring system for the furnace body.

Two New BFs

ShanSteel Rizhao Steel Plant plans to build two sets of 5,100m3 blast furnaces with an annual output of 8.1 million tonnes  (MnT) of hot metal. It will apply the design concept of “high efficiency, low consumption, high quality, long campaign life and cleanness”, and reliable, energy saving, environmentally friendly, high efficiency and long campaign life technology and equipment.

 

The key to high efficiency and long campaign life design lies in the organic combination of the blast furnace’s inner profile, lining structure, cooling system and automatic detection. The capacity of the 5100m3 blast furnace of ShanSteel Group Rizhao Co Ltd is designed for 15,000 tonne/m3 of campaign life. To achieve a longer operating life, the blast furnace design closely follows the above aspects, and adopts a large number of advanced and reliable technologies.

Reasonable BF Internal Profile

Reasonable blast furnace profile is the basis for reasonable and stable blast furnace gas flow distribution, and is the precondition for realising “stable, smooth, high-yield, low-consumption and long campaign life” of a blast furnace.

 

Determination Of BF’s Internal Profile Parameters

The design of the furnace is based on the accumulation of operational experience data. The furnace burden structure, raw material and fuel conditions are determined by empirical formula, depending on a blast furnace profile with similar furnace capacity, similar raw material conditions and operating conditions, and advanced production indexes. A thin stave lining is applied to fix the index of the blast furnace to achieve optimal period of production, so that the blast furnace productivity is optimal throughout its entire production life.

 

The comparison of a BF’s internal profile design parameters at ShanSteel Rizhao Steel Plant’s 5100m3 blast furnace and the same grade domestic and overseas blast furnace are as follows:

Project Code Unit ShanSteel  Shasteel Cao Feidian Bao Steel KoreaTangjin Russia
Cherepovets5#
Japan
Kashima 3#
Effective volume Vu m3 5192 5867 5576 5047 5250 5549 5020
Furnace hearth dia. D mm 14600 15300 15500 14500 14850 15100 15000
Furnace bosh dia. D mm 16800 17500 17000 16400 17000 16500 16300
Furnace throat dia. D1 mm 11000 11500 11200 10800 11100 11400 10900
Blast furnace height Hu mm 32000 33200 32800 32100 32400 34300 31800
Salamander height h0 mm 3600 3200 3200 3672 3700 1500
Furnace hearth height h1 mm 5400 6000 5400 5500 4900 5200 5100
Furnace belly height h2 mm 4000 4000 4000 4400 4800 3700 4000
Furnace bosh height h3 mm 2400 2400 2500 2400 2500 1700 2800
Furnace shaft height h4 mm 18000 18600 18400 17800 17700 21200 16900
Furnace throat height h5 mm 2200 2200 2500 2000 2500 2500 3000
Furnace belly angle ? ° 74.624 74.662 79.421 77.82 77.376 79.287 80.770
Furnace shaft angle ? ° 80.848 80.879 81.085 81.06 80.538 83.141 80.923
Tuyere number No 40 40 42 40 42 40 40
Taphole number No 4 3 4 4 4 4 4
Height diameter ratio Hu/D 1.905 1.897 1.930 1.960 1.906 2.079 1.951

 

Main Design Characteristics Of BF’s Internal Parameters

  • The furnace bosh is a soft melt forming zone, where the porosity is at its lowest, and gas permeability the weakest. Therefore, the furnace bosh area gas flow rate needs to be relatively low; from the actual production point, the larger the furnace bosh diameter is, the easier the blast furnace can accept the air volume, and better the breathability. Therefore, it is more advantageous to have a larger diameter of the furnace bosh, which is 16.8 m.

 

  • If there is appropriate, strong gas permeability ability, it is possible to improve the permeability of the blast furnace, and reduce coke quality  dependence. Designed effective height Hu=32.000m, D=16.8m, Hu/D=1.905.

 

  •  Lesser the furnace belly angle ?=74°37?25??the better it is to improve gas flow distribution, stabilise slag scale and prolong furnace belly service life. At the same time, gas permeability burden is improved, the gas flow rate is reduced, the friction of burden swelling on the inner liner and slag scale is reduced, and the working environment of the tuyere cooling stave is improved.

 

  • The height of the furnace hearth is 5.4m, which ensures that there is enough space in front of the tuyere to facilitate full combustion of fuel and higher gas (liquid) permeability of the lower central part of the blast furnace, which is important for improving aerodynamic conditions.

 

  •  The diameter of the furnace hearth is 14.6m, and 40 tuyeres are provided. The chord length between adjacent tuyeres in the hearth is 1,146 mm, thus ensuring the continuity of the combustion zone. Four tapholes are set up, and the inclined angle between the tapholes is 81 degrees.

 

  • There is reasonable and deep salamander depth. The depth of the salamander is 3,600 mm, and the ratio between it and the furnace hearth diameter is 24.7%, which is important for reducing the erosion rate of the hot metal flow to the furnace hearth refractory. If the depth of the salamander is too large, it will cause the infiltration of hot metal to intensify and increase the difficulty of forming a stable protective layer in the lower part of the furnace hearth and furnace bottom.

Scientific Selection Of BF Lining Structure

The choice of refractory must be combined with the working environment of each part, and whether it is able to withstand erosion and damage of each part.

 

Furnace Bottom And Furnace Hearth Refractory Configuration

At present, the main refractory structure of the blast furnace bottom and  hearth is “heat transfer method” and “heat insulation method”. The concept of “heat transfer method” is to use the high thermal conductivity of carbon bricks to form a “self-protecting” slag iron shell with low thermal conductivity between the hot metal and carbon bricks, and to isolate slag iron and carbon brick by using slag iron shell.

The concept of the “insulation method” is to directly contact hot metal with a ceramic cup with low thermal conductivity, and to isolate the slag iron and carbon brick through anti-hot metal erosion performance. Two design systems that seem to be quite different, but the essence of the technical principle is the same: ie, by controlling 1,150 °C isotherm distribution in furnace hearth, and to make carbon bricks avoiding 800~1100 °C embrittlement temperature range as much as possible. The “heat transfer method” is to control 1,150 °C isotherm in the “self-protecting” slag iron shell; and the “insulation method” is to control the 1,150 °C isotherm in the ceramic cup.

 

The blast furnace production practice shows that both refractory structurals can achieve a long campaign life of the furnace bottom and hearth. However, for “heat insulation method” refractory configuration, anti-hot metal erosion performance of the ceramic cup cannot fully meet the theoretical requirements, the life of the ceramic cup cannot reach the design life, and finally the blast furnace long campaign life still depends on a “self-protecting” slag shell formed by high thermal conductivity of carbon bricks.

 

Therefore, after investigation and research, the 5100m3 blast furnace of ShanSteel Rizhao Iron and Steel Plant finally chose the refractory structure form of the “heat transfer method”. The mathematical model is used to analyse the temperature field, optimise refractory configuration, adopt carbon bricks with high thermal conductivity and excellent resistance to hot metal penetration, optimise heat transfer system of the furnace hearth, to ensure that the 1,150 °C isotherm is away from the refractory material and forms permanent slag iron protection shell, so as to achieve a long productive life of the blast furnace.

 

Furnace Bottom Refractory Configuration

The furnace bottom adopts large carbon brick + ceramic mat from Sigrid, Germany. From bottom to top, the first layer of the furnace bottom adopts 400 mm thick SGL high thermal conductivity graphite brick RN-X, the second layer adopts 600 mm thick SGL microporous carbon brick 3RDN, the third and fourth layers applies 600 mm thick SGL ultra-microporous carbon brick 9RDN. The ceramic layer is built on the fourth layer of the carbon brick, the lower layer of the ceramic pad is plastic phase composite corundum brick ZSG-2. The upper  edge of the ceramic mat is plastic phase composite corundum brick ZSG-3, and central part is mullite brick ZYM-1, and high alumina bricks are placed on top of the ceramic pad. For the entire furnace bottom refractory, the thickness of carbon brick is 2,200 mm, and the thickness of the ceramic pad is 1,000 mm.

 

Furnace Hearth Refractory Configuration

The furnace hearth adopts full carbon brick structure. The side walls of the furnace hearth are all bricked with German SGL, and the material is ultra-micro-porous carbon brick 9RDN. High-aluminum bricks are laid below the taphole area inside the furnace hearth carbon brick, and the dense clay protective brick is laid above the taphole area till the lower edge of the furnace bosh.

Taphole area: The carbon brick area adopts ultra-microporous large-combined carbon brick; furnace hearth carbon brick is thickened 450 mm at the taphole area and the transition is smooth; aluminum-silicon carbide silicon gel combined self-flowing castable is used in the taphole frame and runner.

The ring seam between carbon brick (except the uppermost layer) and cooling stave are filled with SGL carbon ramming mass RST16 ECO, the ring seam between carbon brick and ceramic pad are filled with SGL carbon ramming mass RST18 ECO, and between the top layer carbon brick and cooling stave,  silicon carbide silicon gel combined with self-flowing castable is used.

Furnace bottom and hearth refractory configuration
Furnace bottom and hearth refractory configuration

Furnace bottom and hearth carbon brick adopts dry masonry method, and masonry brick joint is ?0.5mm, which can avoid the influence of the masonry brick joint on the refractory life.

Other Area Refractory Configuration

The tuyere area adopts a new corundum composite brick that combines heat conduction and erosion resistance, and adopts a composite brick at the bustle pipe area. The gap between the tuyere combination brick and tuyere cooling stave, the gap between the upper part of the tuyere sleeve and tuyere combination brick adopts silicon carbide silicon gel combined with self-flowing castable high thermal conductivity, which could balance thermal conductivity and ensure filling compaction, so as to prevent the generation of air gaps. Between the lower half of the tuyere and tuyere combination brick, refractory buffer mortar is applied to absorb refractory expansion of the furnace hearth.

The upper part of the blast furnace adopts thin stave lining structure with in-laid brick cooling stave.

 

The high heat load area of the furnace belly, bosh as well as middle and lower parts of the furnace shaft are mainly eroded by slag iron and gas flow. The 7th to 12th copper cooling staves are in-laid with silicon nitride combined silicon carbid bricks, and the in-laid thickness is 100 mm.

 

The upper part of the furnace shaft is mainly eroded by furnace burden and gas flow. The 13th to 17th SG cooling staves are in-laid with phosphate impregnated clay bricks, and the in-laid bricks are 150 mm thick.

 

The transverse and vertical joints between the staves have silicon carbide ramming mass; between the cold side of the cooling stave and furnace shell, low thermal conductivity silicone gel is applied combined with self-flowing castable. In order to allow interaction of construction with cooling stave installation, one has to allow for segmented installation and casting to maximise the sealing performance of the furnace body and reduce temperature of the furnace shell.

 

Spray the gas-resistant paint on the hot side of furnace belly, bosh and furnace body cooling staves (7th to18th). The inner lining spraying thickness of the furnace bosh is 120 mm, and other parts spraying are adapted to the blast furnace inner profile.

 

The gas sealing hood is sprayed with 200-mm thick gas-resistant coating. The anchor is made of “Y” stainless steel anchor bolt and reinforced with hexmetal to cure bonding between the spray coating and furnace shell.

Furnace Shaft Full Cooling Structure

The purpose of the blast furnace cooling is to conduct out the heat of the lining, improve the working conditions of the masonry, extend the service life of the lining, maintain a reasonable internal profile, and protect the cooling equipment and furnace shell. The quality of the blast furnace cooling system is directly related to the service life of various cooling equipment and furnace linings, thus affecting campaign life of the entire blast furnace.

 

The design is to realise that there is no cooling blind zone in the blast furnace proper and the cooling equipment and cooling system are reasonably selected to realise the synchronous long campaign life of various parts of the furnace body.

 

Furnace Shaft Cooling Equipment Selection

Cooling staves of different structure and materials are used according to different working conditions and heat load in each area of the furnace shaft.

 

  • Furnace Bottom Cooling Structure

The furnace bottom adopts the direct-buried stainless seamless steel pipe with diameter of ?89×8 as the cooling equipment, and is arranged in parallel below the furnace bottom sealing plate. The spacing between steel pipes is 300 mm, with a total of 56 pieces.

  • Furnace Cooling Stave Type

Totally, there are 18 cooling staves from the furnace bottom to the lower edge of the furnace throat’s steel brick.

The 1st~6th tuyere and under tuyeres (except taphole area) adopt heat-resistant cast iron smooth cooling staves.

In order to better protect the taphole and strengthen the cooling effect of the taphole area, put the cast copper cooling stave in the taphole area, four pieces for each taphole, and cooling stave thickness should be 120 mm.

For the 7th to 12th furnace belly, bosh as well as furnace shaft lower part and middle part high heat load area, put six sections of rolling drilling full-covering in-laid copper cooling staves (in which BF shaft copper cooling stave height is 6.7 metres). The copper cooling stave thickness is 125 mm, in-laid brick is silicon nitride combined with silicon carbid brick with a thickness of 100 mm. The copper cooling stave can meet the requirements of blast furnace intensification smelting, and has sufficient cooling strength. It is easy to form slag scale on the belly, bosh, lower part and middle of the furnace shaft, to protect the cooling stave and furnace shell.

The 13th~17th form the upper part of the furnace shaft, which offers full covering of in-laid bricks in the SG cast iron cooling stave. The cooling stave’s thickness is 240 mm, and in-laid brick is phosphate mixed with clay bricks with thickness of 150 mm.

The furnace shaft in the 18th section provides an inverted “C” SG cast iron cooling stave.

The furnace throat steel brick provides one section with water cooling structure, the material of which is resistant alloy cast steel.

Except the tuyere, taphole cooling stave, cooling staves in other positions offer four-in four-outwater pipes. The cast iron cooling stave water pipe spec. is ?80×6, rolling copper cooling stave is a double round compound hole channel. The cold hot area ratio of the furnace shaft (cooling water pipe specific surface) could reach 1.15, to ensure sufficient cooling strength of the furnace shaft.

 

A soft closed circulating cooling system can hasten the furnace body cooling.

The 5100m3 BF of ShanSteel Rizhao Steel Plant adopts the combined soft water closed circulation cooling system to combine the cooling stave (including furnace throat steel brick), furnace bottom, tuyere small sleeve, tuyere middle sleeve, direct blow tube and hot blast stove valve through series and parallel methods into one system, the total circulating water volume of the system is ~7200m3/h.

The combined soft water closed circulation cooling system has the advantages of no scaling, high cooling strength, good cooling effect and low operating cost.

The soft water coming out of the soft water pump station is divided into two circuits at the cast house, wherein the cooling water at the furnace bottom is ~840m3/h, and cooling water in the series connection for the cooling stave and furnace throat steel brick is ~6360m3/h.

In order to ensure intensified cooling effect of the soft water closed circulation cooling system, the slag iron condensation protection layer is formed in the high heat load area, the campaign life of blast furnace is prolonged, and the safe operation of the system is ensured. The soft water closed circulation cooling system has the following characteristics:

  •  Select reasonable water flow rate based on the cooling stave material and heat load difference. The water flow rate of the cast iron cooling stave pipe in the upper part of the furnace hearth and furnace shaft is 2.0m/s; for the copper cooling stave of furnace belly, furnace bosh, and lower part of furnace shaft, the water flow rate is 2.65m/s to strengthen the cooling in the high heat load area.
  •  Calculate head loss of each parallel branch in detail, and take measures to optimise pipeline configuration to ensure that the parallel cooling loop resistance is basically equal, which is beneficial for normal operation of the system, and the pump boost value is not too high, avoiding energy consumption waste.

 

  • The main safety measures: Water supply pumps are provided with two independent power sources. The water supply pump group is provided with the standby pump, medium pressure soft water supply pump and high pressure soft water supply pump for added security, and a main pump for cooling the stave is set up with diesel pump, and the diesel pump can drive the whole cooling water cycle when power failure occurs.

n Perfect furnace body monitoring system: The perfect furnace monitoring system allows the operator to fully understand the furnace conditions, the state of the furnace, and correspondingly guide the normal production process of the blast furnace and prolong its life in the process.

BF lining temperature monitoring

Around 868 temperature detection points are set up at different longitudinal and radial positions at the base of the blast furnace, bottom and hearth to monitor the refractory temperature of different parts on real time basis, and track erosion of the furnace hearth. Through this temperature data, one can establish the furnace bottom and hearth erosion rate, to deduce furnace lining erosion conditions. The temperature sensor uses an armoured flexible thermocouple, which offers centralised extraction of the furnace shell to ensure the service life of the thermocouple.

 

BF Cooling Stave Temperature Monitoring

One way to judge the blast furnace condition is to track the temperature detection point in the cooling stave of the furnace belly and along different longitudinal and radial directions for real-time monitoring of the cooling stave temperature.

 

Water Temperature Difference & Thermal Load Monitoring

Set BF cooling water temperature difference and heat load monitoring system, install temperature and flow rate monitoring points on corresponding inlet and outlet of water branch pipes, to track online monitoring of water temperature difference in the cooling stave of the furnace bottom, furnace hearth, furnace bosh, furnace belly, lower part of the furnace shaft as well as the tuyere’s small sleeve, to deduce the heat load, operating furnace profile, slag thickness, and slag peeling frequency. This system sets up 818 temperature detection and 200 flow rate detection points. The temperature sensor uses a special block to replace the temperature sensor at any time without affecting normal productivity of the blast furnace. The high-precision wireless digital water temperature difference sensor is used in the non-high temperature environment to reduce the amount of field cable and thus maintenance.

 

Furnace Shaft Static Pressure Monitoring

On furnace belly and furnace shaft, totally set up four layers of static pressure detection, which are arranged at the 8th, 11th, 13th, 16th cooling staves, four points for layer, in total 16 points. Through the change of pressure value at each point, the change of gas flow pressure at different positions in the longitudinal direction of the blast furnace is monitored, which provides a basis for judging the change of local gas flow in the furnace in advance, and reduces the abnormality of furnace condition.

Conclusion

Reasonable BF design is the precondition for extending the working life of a BF. For the furnace body design of the 5100m3 BF of ShanSteel Rizhao Steel Plant, one needs to adopt a series of advanced, mature and practicable  technology, to realise an organic combination of BF’s internal profile, lining structure, cooling system, automatic detection, so as to provide superior conditions for stable operation and long life.

Based on the advanced design concept, the 5100m3 blast furnace EPC project of ShanSteel Rizhao Steel Plant has laid a solid foundation for a long campaign life of the blast furnace through excellent material selection, equipment quality and strict installation and construction processes.

 

 

Disclaimer: Please note that any information offered in this article is expressly the opinion of the author of that article and does not (necessarily) reflect the views of Steel 360 magazine. No part of this publication may be copied, reproduced or stored in a retrieval system or transmitted in any form or by any means, mechanical, electronic, photocopying, or otherwise without the prior written permission of the author and publisher.

 

The authors: Yu Guohua is Vice President; Li Qingyang, Chief Engineer; Chencheng, Deputy Director of the Iron-Making Department; Zhang Xiangguo, Deputy Director of Iron Making Department and Wang Bin is the Deputy Director Of the Iron Making Department, at Shandong Province Metallurgical Engineering Co Ltd.