Large-scale tunnel kiln DRI process for industrial practice
Wang Shouping Wang Lixin Wang Xiangyu
（Tangshan Outstanding Science & Technology Co., LTD，Tangshan 063020）
Abstract：In order to improve the industrial scale degree for coal based tunnel kiln DRI process, and achieve the single line capacity for 100,000 to 200,000 TPY DRI the large-scale tunnel kiln and auto loading machine must be used as the key equipment. This can change the status of tunnel kiln process for DRI producing.
Keywords：Coal based reducing process; large-scale tunnel kiln; industrial scale
One．The origin of coal based tunnel kiln DRI process
In 1908, the Hoganas E. Sieurin of Sweden invents the tunnel kiln DRI process. This process firstly be used in power metallurgy industrial. For one hundred years this technology has been dominant in the field of powder metallurgy of reduced iron powder. For the past few years, this process be used in DRI process for steel making in China, but for the reasons of market, technology and limitation of concept, the capacity of DRI for this process is only 20,000TPA in China until 2007. It is widely believed that the tunnel kiln DRI process with low industrialization level, big labor intensity, environmental pollution and high production cost, it directly restricts the scale development of DRI tunnel kiln reduction method.
Two．The development of coal based tunnel kiln DRI process
In order to develop the coal based tunnel kiln DRI process, OTSK established large-scale tunnel kiln DRI process, this production line have high level automation. The tunnel kiln is 260m for length and 2.68m for inner width. The capacity of the kiln is 50,000 to 70,000 TPY DRI. The come out of large-scale tunnel kiln open the development of the large scale industrial coal based tunnel kiln process.
Three．Large-scale tunnel kiln DRI process for industrial practice
3.1 Raw material processing
3.1.1.Iron ore：
Feeding the iron ore with moisture 8-10% into the dryer for drying, the dryer heat source is the residual heat of tunnel kiln. The moisture of the iron ore after drying should be under 2%. The dry iron ore be put into the hopper for standby.
3.1.2.Reducer：According to the process requirement to ingredient, crushing. The particle size should be under 3mm.
3.1.3.Desulfurizer：The desulfurizer is limestone should according to the process requirement to ingredient, crushing, the particle size should be under 3mm.
3.2 Auto loading & unloading section
3.2.1. Use auto loading machine to load the iron ore and reducer into the SiC container on the kiln car.
3.2.2.After the reduction and cooling in tunnel kiln, the DRI is unloaded by unloading machine.
3.3 Reduction section——260m gas tunnel kiln
3.3.1Kiln design
Design of loading
SiC reducer container（φ=350mm, H=380mm）
90kg DRI in each group container.
The working day is 335days and reducing time is 36~28 hours.
Size of kiln car: length 3400mm×width2600mm. Within the height, set 5 layers can; within the width set 6 suits can; within the length set 8suits can. There are 48 group cans in each kiln car, and 4.32 tons in each car.
3.3.2The main technical parameter of the tunnel kiln
3.3.2.1 Main parameter
A. the type of tunnel furnace： OTSK260/2780 type
B. structure of furnace：Masonry-type flat roof gas tunnel kiln
C. baking product and throughput：
① production：DRI （material: pellet iron ore or magnetic iron ore）
②producing capacity：50,000T/year/kiln
③working days for one year: 335 days
D. the basic size in the kiln
length ：260m
width ： 2.78m
height：1.942m（car surface to roof）
E. kiln heating system
reducing cycle：20~40hrs (adjustable)
reducing temperature：1140～1180℃(adjustable)
the highest temperature using：1220℃
fuel using：gasifier gas（Qlow=1450kcal/nm3）
setting Burner：152pairs
kiln atmosphere ： Deoxidization
shipment method：SiC container
F. kiln vehicle and loading size：
Size：Length 3400mm×width 2600mm×height 932mm
quantity of kiln vehicle：100 cars
vehicle in the furnace：76 cars
The load of one vehicle：48units can
G. heat measure and auto control:
Temperature checking points：25points
Pressure checking points：4 points
Control mode：Auto display of temperature meter and pressure
3.3.2.2. Kiln section function description the and structure design
The iron ores in the SiC container, after entering the tunnel kiln will go through preheating, reducing and cooling the three stages, so the tunnel kiln should be designed to follow the reduction of physical and chemical changes in the charging principles, the tunnel kiln is divided into three sections with each realization of preheating, reducing, cooling functions.
——Preheating section is 51.10 meters length, divided into 15 car spaces. The main role of the section: use thermal gas of the reducing section to preheat the iron ore, to complete the water evaporation and decomposition of hydrates, to promote the escape of volatile in reducing agent and the decomposition of the desulfurization agent calcium carbonate (i.e., limestone) , the heat of generated emissions comes from the branch smoke channel can be used for combustion by heat exchanger , after the exchange the exhaust gas is pumping out of the plant by exhaust fan; set electric lift door in the head of the kiln to prevent cold air outside the kiln and the kiln heat flow out.
The material reaction mechanism at preheating section includes the following aspect. The water evaporation and hydrate decomposition for the iron ore material, the VM escape from the reducer and the decomposition of the limestone.
Absorption water evaporation escapes from the material above the temperature 100℃. At the temperature 200℃ crystal water began to decompose, and between 300℃ and 400℃ the decomposition come to an end.
H2O+CO=H2+CO2 （≈500℃）
2H2O+C=CO2+2H2 （≈500℃）
H2O+C=H2+CO （≈1000℃）
If the moisture of the material is high, during the preheating section will consume too much heat and CO or carbon, so it is not good for the reduction section. During the raw material processing, the moisture of the material should be strictly controlled.
As the material be warmed to 400℃，the VM of the reducer will escape from the material intensively and will come to the end at 800～1000℃. The limestone will decomposition at 750℃。
CaCO3=CaO+CO2↑
This is endothermic reaction and at 910℃ will reaction intensively. The decomposition product CaO will reaction with the Sulfur.
CaO+FeS=CaS+FeO
——Reducing section is 153meters length, divided into 45car spaces.
The main role of the section: As the mineral is gradually heated to high temperature, iron oxide is reduced by CO, until all the metallic iron is reduced, this process requires a more stable temperature curve and the reducing time. This section is designed 76 pairs of gas burner on both sides of the kiln, by auto adjusting the ratio of gas and air to control the kiln atmosphere into reducing and get an ideal temperature curve, so that products stay in the area longer to make a good reducing effect;
The material reaction mechanism at reduction section includes the following aspect.
As the temperature increase to high temperature the iron oxide be reduced by CO till to all the iron ore be reduced. The carbon will reaction at carbon layer for C+CO2=2CO and the FeO+CO=Fe+CO2 reaction at iron ore layer.
At the end of the reduction of the FeO, as the disappear of the FeO the following reaction will start. 2CO=[C]+CO2 or 3Fe+2CO=Fe3C+CO2
——Cooling section is54.40 meters length, divided into 16 car spaces.
The full use of indirect cooling wall and the water cooling structure to make the product cooling indirectly, in the temperature region below 500℃, can use fans to cool the products directly, to lower the temperature of the product out of the kiln. Use the waste heat from preheating and cooling parts to heat the combustion air.
3.4 Briquetting section
The cast iron ingot is sling by unloading machine, the ingot be put into the feeding machine and go to the jaw crusher for rough crushing and fine crushing in the hammer crusher, and then use the magnetic machine to separate and sent to barn, then come to briquetting machine through material distributor. After the briquetting the CBI is transported to the product storage by belt. In the Storehouse there is a travel crane with electromagnetic chuck for easy loading shipments.
The sponge ingot go through rough breaking，fine breaking and then transported to the briquetting machine by belt machine. The sponge iron was pressed into CBI, and the density is 4.0~4.5t/m3
(This article was published in the Chinese Society Metals, "non-blast furnace Iron-making 2008 Annual Conference Proceedings")