Irregularities in Blast Furnace during Operation
Irregularities in Blast Furnace during Operation
For a stable and efficient operation of a blast furnace, smooth and uniform movement of burden materials downward and movement of furnace gases in the upward direction is very important. To ensure this a lot of work has been done in the recent past. This includes improvement in the characteristics of the burden materials, improvement in the furnace charging system, adequate automation of the furnace operation to eliminate human errors and improvement in the furnace operating procedures. In spite of these improvements, the blast furnace does not run as smoothly as one is led to believe. Furnace upsets are not as frequent as they were in earlier years but irregularities still do occur. These irregularities are the main concern for a blast furnace operator and often need quick thinking and timely good judgment on the part of the operator to prevent serious trouble. The major irregularities in the blast furnace operation are described below.
Furnace hanging and slipping
When the materials charged at the top of the blast furnace do not move continuously towards the hearth of the furnace, the phenomenon is called ‘hanging’ of the burden. Hanging of the burden material in the blast furnace stack occurs when the material below the hang continues to move downward, forming a space that is void of materials but filled with gas at very high pressure. This space continues to grow unless hang finally collapses. The collapsing of the hang is a phenomenon called ‘slipping’ during which the charged materials fall uncontrollably toward the hearth of the furnace in a thermally unprepared state which leads to the furnace getting cold. It also forces the hot gases upward with the force of an explosion. The sudden rush of gases opens the furnace top gas bleeder and sometimes can cause top equipment damage.
Hanging conditions can develop due to a variety of reasons. Due to these reasons the permeability of the burden materials is decreased because some of the material plugs up the voids or openings between the charge materials and bonds them loosely together. This happens due to the following reasons.
- Loose bonding of burden materials happen specially when there are a high percentage of fines in the burden and the velocity of furnace gas is relatively high.
- Furnace slag that has been melted is blown upward in droplets. These molten slag droplets when subsequently contacts with colder burden material gets re-solidify and plugs up the openings between the particles and tends to cement them together.
- Sometimes the carbon monoxide decomposition reaction (2 CO = CO2 + C ) will be catalyzed and the carbon deposited as soot will plug up the openings between the particles and will hold the particle together
- In some cases where the alkali content of the burden is high, the alkali compounds gets reduced in the furnace to alkali vapours which ascends with the furnace gases and condenses in the cooler portion of the charge material and thus cause plug up of the openings between the particles and results into hanging condition of the furnace.
- Sometimes hanging conditions occurs in those furnaces which are being run very efficiently and being pushed for higher production. Under these conditions if there is a slightly unfavorable change in the gas distribution, coke strength or the size variation of the burden in the furnace, then the iron oxide will not reduce to metallic iron rapidly enough, and will melt and run down as a liquid on the coke particles. If this happens the coke will reduce the liquid iron into solid iron and will consume considerable heat during reduction. This will cause cementing of the coke particles together resulting into significant reduction of the permeability and hence hanging.
- Hanging may also be caused if the blast furnace is run at too high a flame temperature for the quality of the burden material. When the high temperature isotherms expand far enough above the furnace, they can begin to melt unreduced material. As this material descends into a more reducing environment, it reduces and depending on the temperature it may solidify and plug the gaps between the materials. This happens since the melting point of FeO (1370 deg C) is lower than of pure iron (1535 deg C)
When the furnace movement is not proper and there is sluggish movement of the burden material through the furnace, the furnace operator must take corrective measures immediately to avoid a major slip which can be very disastrous. Under very extreme condition this can lead to a frozen furnace. Every hanging and slipping is to be properly investigated to determine the causes of the hanging so that changes can be made in the operating procedure to prevent the hanging from recurring.
The term scaffolding is used when accretions or scabs build up on the furnace walls and cause a decrease in the cross sectional area of the stack of the blast furnace. Scaffolding can occur relatively at the higher level of the stack of the blast furnace or relatively low in the stack, near the top of the bosh.
The scaffold formation near the top of the bosh often results because of excessive fines in the burden material and a higher than normal lime chemical composition of the slag (Reflected by the higher basicity of the slag). The solution of lime into the slags formed in the furnace stack increases the melting point of the slag. Since the slag often carries some of the fines particles from the burden in suspension, the increase in the melting point can cause this mixture of fines and slag to adhere to the upper bosh walls. This build up in the upper bosh wall deflects the hot furnaces gases farther to the centre of the furnace. With lesser volume of hot gases along the walls, the accretions tend to cool down and solidify completely. These scabs then may grow until they block a large percentage of the cross section area of the blast furnace.
Alkali or zinc compounds are reduced to metallic vapours near the bottom of the blast furnace. These vapours rises with the furnace gases to the cooler top portion where they are reoxidized to very fine solid particles. These fine particles adhere to the furnae wall along with other fine materials entrapped in it. This is also the another cause of starting of the formation of a scaffold.
The blockage due to the scaffolding reduces the area available for smelting of the iron bearing materials. Scaffolds distorts the gas flow inside the furnace and increases the fuel rate while promoting hanging and slipping of the furnace. It also decreases the furnace productivity. Due to higher fuel rate lower furnace fuel efficiency results. When the scaffolds dislodge from the walls, it descend into the hearth. This causes serious furnace upsets and reduces the uality of hot metal. In case the size of scab is too big, it can cause the furnace to freeze.
The phenomenon of channeling happens when the ascending gases in the furnace does not properly get uniformly distributed both radially and circumferentially in the furnace and find a passage of least resistance. The different causes for channeling to occur in the blast furnace are charging of excessive fines, improper distribution of the burden material inside the furnace and high level of liquid iron and liquid slag in the hearth. In case of fines charging, the channeling leads to the increase of the heat load at the walls of the blast furnace. Due to the fines, the ascending gasses gets diverted from the area and channel around the fines. This diversion of the ascending gases upset the preheat of the materials and the reduction process. It causes unscheduled bleeder opening, off chemistry of the hot metal, unstable production of the blast furnace and reduction in the furnace productivity.
A ‘breakout’ is the term used to denote the conditions and results of the escape of gas and coke, or slag, or iron, from the bosh, tuyere breast, or hearth of a blast furnace. Breakouts may occur at any point below the fusion zone in the furnace, but the most of the severe breakouts are of liquid slag and of liquid iron. Liquid iron breakout takes place at a level below the surface of iron lying in the hearth, and are either through the hearth walls and jacket or into the hearth bottom and out under the hearth jacket. Typical cross section of the lower part of a blast furnace showing hearth and bosh is at Fig 1. These are the places where breakouts usually happens.
Fig 1 Typical cross section of the lower part of a blast furnace
Breakouts are caused by failures of the walls of the hearth, with the result that liquid iron or liquid slag or both may flow in an uncontrolled way out of the furnace and surrounding auxiliaries. Slag break outs are usually not as serious as iron break outs, because there are not as much danger from explosions as in the case where molten iron and water come into contact.
With either type of breakout, it is necessary, if at all possible, to open the tap hole and drain out as much liuid material as possible, and to take the furnace off blast.
In case of a slag breakout, the breakout may be chilled by stream of water, and the hole where the breakout occurred may be closed bhy replacing the bricks, or pumping fireclay grout in the opening or ramming a plastic cement or putting asbetos rope into it.
In case of iron breakout, there is practically no control. The hot metal is to run out of the hole until the furnace is dry. After the accumulated iron has been cleared away, a suitable refractory may be used for closing the hole. In iron break out is severe then a complete hearth repair will be needed. In case of non severe break outs it may be ncessary to change the damaged hearth cooling staves.