Blast Furnace Tap Hole Mass


Blast Furnace Tap Hole Mass

Blast furnace tap hole mass is a prepared ready to use refractory product, made of a bond of aggregates, additives, and plasticizers. It is used to close the tap hole of a blast furnace after tapping so that no material can leak out, and to keep it plugged until the tap hole is opened for next tapping. It is applied to ensure periodical and stable tapping from the blast furnace and also to protect the inner surface of tap hole bricks. Its functions are (i) to enable smooth operation of the tap hole, (ii) to maintain constant tap hole length,  (iii) to control the liquid flow out of the blast furnace, and (iv) to ensure separation of hot metal and slag. Blast furnace tap hole mass is normally developed and designed to suit the operating parameters of the blast furnace and to maintain stable tapping time and tap hole depth even under severe operational conditions with high productivity coefficients (ton/cu m/day) or high hot metal temperatures.

Requirements of blast furnace tap hole mass

Typical requirements from the blast furnace tap hole mass include the following.

  • It should be soft and plastic (workable) enough to inject when pushed by mud gun, but ‘hard’ enough to effectively displace tapping liquid and to deliver a substantial quantity of tap hole mass only to plug to the required depth in the tapping channel.
  • It should be curing to the required strength (often described as sinterability) and without shrinkage to ensure a tight seal within the tap hole (not prematurely in the mud gun), in the required mud gun dwell time and plug to next tap time.
  • It should effect safe tap hole closure (i.e., without subsequently self opening) and without tap hole and furnace lining damage (e.g., limited gas evolution and associated turbulence) and with ‘mushroom’ stabilization. This requires consideration of both effective tap hole mass displacement in the injection direction, and ‘good spreading ability in the direction perpendicular to the injection direction’ to maintain a stable ‘sedimentary deposit that is gradually and stably grown’ and exhibiting good high temperature adhesion to the constituents already present in the tapping channel.
  • It should be sufficiently soft to be readily drilled straight down the middle of the tapping channel without deviation, in an acceptable time (especially important where productivity constraints exist).
  • It should allowing a stable, controlled tapping stream flow without surging or splash (often associated in iron making with blast gas tracking and gas entrained with ‘viscous fingering’ to above the critical value that induces a deleterious splashing casting stream, even to the extent of slag flows).
  • It should be ideally ‘hard’ and durable enough to withstand penetration, corrosion and erosion by the tapped metal/matte and/or slag and so preserving a protective and adhesive annulus between the tap stream and tap block refractory (without additional corrosive reaction with tap hole refractory), to extend the useful tapping channel lifetime of acceptable controlled diameter, shape (i.e., minimal long term fluting) and length.

In order to get the optimal quality of the blast furnace tap hole mass, there are some additional requirements which include (i) stand alone blast furnace tap hole mass storage building, (ii) maintenance of a 10 day maximum storage of blast furnace tap hole mass with temperature controlled at around 25 deg C to  30 deg C, and (iii) maintenance of temperature at around 25 deg C to 30 deg C for the blast furnace tap hole mass stored at the cast house. Prolonged storage of resin bonded blast furnace tap hole mass, especially at temperatures exceeding 40 deg C, is detrimental to its performance. However, especially for tar bonded blast furnace tap hole mass, a minimum of 15 days ageing is usually essential to obtain adequate tap hole mass loss in plasticity and increased hardness.

Materials used for BF tap hole mass

The high productivity (5000 to 10000 tons/day of hot metal), combined metal and slag duty, high pressure (around 10 kg/sq cm at tap hole) and long tap hole length (2.5 m to 4 m) at the modern blast furnaces impose high demands on the quality of blast furnace tap hole mass for smooth operation of blast furnace and simultaneously  for the lowering of specific consumption of the mass (kg/t HM). Early developments have shown a progression from coke, to alumina, to silica and back again to pitch impregnated alumina and high alumina fine matrix (less than 45 microns and more than 50 % by mass) and/or coarser aggregates (around 20 % by mass  of size 1mm to 3 mm),  with different matrix addition of zirconia, kyanite, SiC, and silicon, aluminum and ferrosilicon and/or nitride  as fine powder. This matrix addition is being made to lower porosity, shrinkage, decrease volatiles, increase antioxidant action, lower wettability by slag and improve extrudability, corrosion, sintering and erosion performance. There is also a trend towards  smaller particle size for improved compaction and improved tapping channel sealing against gas egress. This trend is even towards ultra fines (less than 10 microns) for improved strength, corrosion and abrasion resistances. Pure silica and pure alumina sources helps in the improved corrosion resistance and higher positive residual expansion coefficient performance when compared with alumino-silicates.  A ‘swelling’ characteristic, is important for sealing of tap hole subject to temperature fluctuation from the extreme of superheated tapping temperatures to cold closure conditions in water cooled tap blocks. A somewhat more empirical approach has similarly led to convergence on use of blast furnace tap hole mass of high alumina content for high intensity operations.

Binder for blast furnace tap hole mass

Traditionally coal tar pitch was used as the binder ( around 20 % by mass for the blast furnace tap hole mass. This was followed by a period when phenolic resin binder became more preferred. Then came the period when some operators moved back to tar bonded blast furnace tap hole mass, while some other operators preferred, resin and resin-tar binder combinations.

Tar bonded blast furnace tap hole mass is generally thermoplastic, hard (often requiring pre heating of the tap hole mass in the mud gun barrel by gas heaters or hot water/steam to become pliable, and slower curing (2 hours cast time is deemed insufficient for full curing and sintering, although only 20 min to 30 min is frequently encountered in practice). The latter necessitates the mud gun to remain in position for an extended time after plugging to avoid subsequent unintended tap hole self opening. Unlike resin binders, tar bonded tap hole mass is reported to have an advantage of forming a transition free union with carbon based refractory resulting in a monolithic tap hole lining and improved adhesiveness under high temperature conditions.

Resin bonded blast furnace tap hole mass cures at a faster rate and has the benefit of shorter mud gun dwell time and quicker tap hole turn around. Occasionally, though, the tap hole mass can cure too fast, leading in hotter tap holes to the tap hole mass curing before injection is complete, or, in the extreme, to blocking prematurely in too hot a mud gun barrel with risk to delay in effective tap hole closure.

Resin bonded blast furnace tap hole mass can be prone to greater volatility upon heating and more undesirable gas evolution. Some varieties of resin bonded tap hole mass have tendency to cure too hard for acceptable drill times ( less than 15 min).

Small changes in binder can  change substantially the dimensional and strength properties of blast furnace tap hole mass. This emphasizes the need for close control of conditions during the manufacture of blast furnace tap hole mass for getting a consistent quality. Generally, using cold crushing strength (greater than 7.6 MPa) and workability (18 % to 28 %) as quality criteria, tap hole mass rejection rates up to 40 %, or even more, are not uncommon for the manufacturers of blast furnace tap hole mass. However, rejection rates below 15 %  are more acceptable.

Different materials used in the production of blast furnace tap hole mass and their main functions are shown in Tab 1.

Tab 1  Tap hole mass materials and their main functions
1 Fused alumina, calcined bauxite, chamotte, and fire clays Main aggregates, filling materials
2 Clay minerals Plasticity, injection
3 Pyrofillite Plasticity, expansion
4 Silica, fine sand Expansion
5 Kyanite Expansion
6 Fine calcined alumina / fume silica Sintering/mullite formation/expansion
7 Zirconium/chromites Special additives / spalling / corrosion resistance
8 Silicon carbide, carbon Corrosion resistance
9 Silicon nitride Erosion resistance
10 Metallic powders Erosion resistance/sintering
11 Coke, ultra fine carbon Drilling
12 Tar/resin/special oils Binders

Important characteristics of blast furnace tap hole mass

The function of the blast furnace tap hole mass in a blast furnace is to enable smooth operation of the tap hole, maintain constant tap hole length and ensure separation of hot metal and slag. Table 2 depicts the various functions and property matrix for the blast furnace tap hole mass.

Tab 2 Function vs. property matrix of blast furnace tap hole mass
1 Closing of the tap  hole Easy plugging and drilling (i) Plasticity of fireclay, (ii) Good gas permeability, (iii) Proper sintering
2 Hearth protection Constant length (i) Resistance to hot metal and slag, (ii) High stickiness to the furnace wall
3 Constant delivery Erosion resistance (i) Expansive nature, (ii) Resistance to hot metal and slag

The plugging performance of the blast furnace tap hole depends on (i ) the quality of tap hole mass, (ii) the capabilities of the mud gun and drill machine, (iii) the use of the tap hole mass, (iv) the gas sealing around the tap hole, and (v) the blast furnace driving. (Fig 1).

Factors affecting tap hole mass performance

Fig 1 Factors affecting the plugging performance of blast furnace tap hole mass

The blast furnace tap hole mass must have properties to consistently and safely perform the following functions.

  • Flow when pushed by the mud gun, to plug the tap hole. Tap hole mass must be pliable enough to be correctly injected.
  • Cure within the tap hole during the plug to tap time but without shrinkage to ensure a tight seal.
  • Harden fast enough to withstand pressure from the inside of the furnace.
  • Be drilled in an acceptable time.
  • Allow a stable, controlled melt stream at tap without spray.
  • Withstand erosion and chemical attack by hot metal and slag during tapping.
  • Form a stable substrate for the next plug.
  • Provide a stable and controllable tap hole length.
  • Porous enough to allow distillation gases to escape.
  • It must also, in certain cases, be suitable for maintaining the tap hole and restoring it to its normal state if it has deteriorated.

The requirements of blast furnace tap hole mass are the following.

  • Improved plasticity – To provide better workability
  • High corrosion and abrasion resistance – To provide constant tap hole diameter and tap hole length
  • Good sinterability – To provide good strength
  • Adhesive strength – It is required for proper adhering of new tap hole mix with old one
  • Good gas releasing capacity – To reduce the formation of gas pressure inside the blast furnace
  • Easy drilling and tapping

To fulfill the plugging, the tap hole mass must be plastic. The temperatures of the tap hole mass during the injection and the maturity have an influence on the plasticity of the tap hole mass. When plugging the tap hole, compatibility is necessary between the old and the fresh mass. This compatibility is improved by the carbon network and the expansion of the fresh mass during curing. During the plugging process, it is essential to ensure the formation of a mushroom at the back of the tap hole. At each plug, enough tap hole mass must be pushed to maintain this mushroom, which wears due to abrasion by the liquids exiting the blast furnace during the tapping.

A high tap hole mass feed  does not provide any benefits, but  it results into (i) a safety risk of tap hole splashing at the start of a tap, (ii) higher consumption of the mass, (iii) longer curing time of the mass, (iv) risk of the oxygen lancing may be needed for opening of the tap hole, and (v) risk of shorter durations.

Typical compositions of some blast furnace tap hole mass

The following are the typical compositions of some of the varieties of blast furnace tap hole mass generally being used.

  • Type 1 – Silicon carbide + carbon – 40 %, silica + crude silicon – 15 %, alumina (bauxite) – 29 %, and binder – 16 %.
  • Type 2 – Refractory aggregates – 60 % to 85 %, graphite + silicon carbide – 5 gm to 30 gm, clay – 5 gm to 15 gm, binder – 15 gm to 25 gm, and organic fiber – 0.01 gm to 0.75 gm.
  • Type 3 – Silica – 65 % (grain size – 0 mm to3 mm), density – 2.2 gm/cc, total carbon – 20 %, alumina – 10 %, and loss of ignition – 10 % to 20 %.
  • Type 4 – Clay – 16.7 %, ground coke – 50 %, coal tar pitch – 16.7 %, grog powder – 16.6 %.
  • Type 5 – Alumina = 7.5 %, silica = 90 %, iron oxide – 0.8 %, titanium oxide – 0.5 %, and graphite powder = 2 %.

Typical properties of some blast furnace tap hole mass is given in Tab 3.

Tab 3 Typical properties of blast furnace tap hole mass
Property Unit Type A Type B Type C
Apparent porosity % 25-35 25-35 25-35
Bulk Density gm/cc 2.3 2.28 1.54
Permanent linear shrinkage % 1 (-) 0.8 at 900 deg C, (-) 0.1 at 1400 deg C 2
Cold crushing strength kg/sq cm 61.5 75 35

Health issues associated with blast furnace tap hole mass

A minimum blast furnace tap hole mass curing time of 45 minutes before drilling and tapping is needed for a lower emission of fumes. Tar binder poses health risk through release of polycyclic aromatic compounds such as benzopyrene that are carcinogenic. Similarly release of undesirable formaldehydes and phenols is associated with the resin binders.