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Formation of Scaffold in Blast Furnace Shaft

• satyendra
• April 6, 2020
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• Alkalis, BF burden, blast furnace, Build up. BF volume, coke, refractories, Scaffolds, zinc,

Formation of Scaffold in Blast Furnace Shaft

The term scaffold is used when there is accretion or scab formation on the blast furnace (BF) wall which causes a decrease in the cross sectional area of the shaft of the BF. Scaffold can occur relatively at the higher level of the BF shaft or relatively low in the BF shaft (near the top of the bosh). It is difficult to generalize the types of scaffolds since there is very little in common between the structure and location of scaffolds from different BFs. However, scaffolds can be generally arranged in two groups. These groups are (i) laminated scaffolds, and (ii) non laminated scaffolds. Scaffolds with laminated structure consist of alternate layer of metallic iron (Fe) and burden rich in alkalis. Typical formation of a large scaffold in a BF is shown in Fig 1.

Fig 1 Typical formation of a large scaffold in a BF

Scaffolds can cause hanging in the BF. Hanging originates when the burden, on its way down, meets a very high resistance resulting into the stoppage of the movement of the burden. The hanging can rupture, and then the material fall down because of the gap which has been created below the hanging. After, the rupture, when the slipping takes place, then normally large quantity of materials fall down and results into irregular working of the BF resulting into a non-uniform gas distribution with its implications on the BF parameters.

Prerequisites for stable and harmful scaffold formation are (i) presence of suitable material in the BF burden to build the scaffold (e.g. fines, poorly screened burden, sinter with inferior low temperature reduction degradation characteristics, use of long time stored, wet and cold sinter, or small size coke etc.), (ii) presence of agglomerating (cementing) material for the agglomeration of the burden material, (iii) presence of a fixing (anchoring) mechanism to build the scaffold on the shaft wall of the BF which can be a chemical bond with the lining material, physical anchoring around the cooling plates, arch building towards the bosh walls, or simply condensation of the agglomerating  material on the wall, (iv) continuous supply taking place of the adhering components, and (v) formed scaffold is strong enough to withstand the wearing forces of the descending materials.

The place where the scaffold is located depends on the agglomerating material, adhering material, burden materials, furnace operation, and furnace constructional features such as cooling elements and lining material. It can be located at various levels in the BF such as the shaft, the bosh, or thee belly. Fig 2 gives some views of scaffolds in a BF.

Fig 2 Views of scaffolds in a BF

Typical chemical composition of the root and the hard crust of the scaffold samples from some of the BFs of Europe are given in Tab 1.

Scaffolds in the bosh and belly 

In an ideal BF, the build up and peeling of scaffolds happens continuously so that no large variations in the cooling losses or slag chemistry are observed. In older BFs with cooling plates and ceramic lining, this balance is difficult to achieve especially when the lining has become eroded. In recent BFs with stave or copper plate-graphite lining, there is a regular build up and peeling sequence of scaffolds alternating. Peeling of the scaffolds normally takes place two or more times in a week.

The scaffold consists of coke breeze, reduced ore components, slag, dust components like soot, and condensed alkali etc. This kind of scaffold is brittle and it peels off quite easily. If this kind of scaffold remains for a longer time on the bosh wall, especially when it covers the bosh circumference, it is mechanically stable and gains in strength with time. Iron oxides are reduced to metallic Fe, sintered together and carbonized. Coke carbon (C) is gasified by solution loss reaction leaving space for dust and condensing materials, e.g. potassium oxide (K2O). When the scaffold has stayed for a longer time at higher temperatures in the bosh the scaffold is compact and contains mostly Fe and slag. The longer time the scaffold stays, the more it gains in dimensions and the risk of serious disturbances grows.

A typical scaffold formation proceeds due to the reasons and the steps consisting of (i) there are present in the burden a lot of fines and poorly screened sinter with inferior low temperature reduction degradation characteristics, (ii) The fine material gets segregated to the wall side where the gas flow is weak, the temperature low and reduction rate slow, (iii) the root of the cohesive zone is located low in the bosh and it can be also mixed burden, (iv) the normal descent of the burden stops due to a hanging or a maintenance stop, (v) the decrepitation of sinter continues due to its reduction, (vi) softened or half molten material sticks to a cooling element and solidifies, (vii) when the burden starts to descend again (the hanging slips or the furnace is started again after the stop), this adhered material stays on its place, (viii) zinc (Zn) and alkali compounds condense from the gas in the stagnant material agglomerating the fines together, (ix) formation of the root of the scaffold, (x) on the surface of the stagnant material layer, towards the burden, the condensation of agglomerating compounds and dust goes on building a hard crust of Fe, ZnO (zinc oxide), K2O, and slag components, (xi) continued growth of upwards and slightly towards the furnace centre. If this kind of scaffold is allowed to grow, it can grow for some time without creating any significant troubles, but after that it causes serious disturbances in the operation of the BF, e.g. chilled hearth, tuyere breakdowns etc.

Scaffolds in middle and lower shaft

Traditionally alkalis are being considered as the reason for the formation of scaffolds in the BF shaft. At temperatures above 1100 deg C, alkalis are reduced and vaporized from the molten slag and ascend with the gas. Alkali cyanides are formed at temperatures ranging from 900 deg C to 1000 deg C and at temperature range of less than 750 deg C to 880 deg C these vapours are condensed and oxidized by CO2 to carbonates. Alkalis can also react with burden materials, dust particles and the lining if the temperature is high enough. In the Na2O-K2O-MgO-CaO Al2O3-SiO2 system, there are many compositions with low melting point with some of them having melting point as low as 700 deg C. At fluctuating temperatures these melts can dissolve more solid material when the temperature is rising and agglomerate particles together when the temperature goes down again. These low melting mixtures decompose to more stable phases during solidification and it is difficult to fix a certain melt composition as agglomerating phase. The crystallized phases have usually higher melting points than the initial molten phase. That is why e.g. KAlSiO4 (kalsilite) is often found in the scaffolds, even though it has a very high melting point (greater than 1700 deg C). The influence of K on the scaffold formation is shown in Fig 3.

Fig 3 Influence of K on the scaffold formation

Zinc is reduced at around 800 deg C and is evaporated at 907 deg C. It is oxidized back to ZnO in the colder parts of the burden where the temperature goes down below 800 deg C and where the ratio of CO2 / (CO2+CO) is higher than 0.2.

The scaffold can grow very fast if there is no wearing mechanism such as the movement of the descending burden. During the movement of the descending burden, the coke lumps with hard edges act as grinding material. The growth of the scaffold is a balance between adhering and wearing phenomena. There are many contributing factors are described earlier. It is generally difficult to point out only one factor. Normally a sum of many factors triggers the growth of a scaffold.

Scaffolds in the upper shaft

Scaffolds in the upper shaft are very common when the burden materials are not thoroughly screened or the values of their low temperature breakdown properties are poor. The fines in the burden are segregated towards the walls or they are formed by decrepitation of sinter. Excessive amounts of fines give rise to high pressure drop and can cause so called ‘dust hangings’. Stagnant fine material forms a base for agglomerating compounds like ZnO and K2CO3. Sometimes moisture in wet and cold burden can also condense in the fine materials at the wall side.

The root of the scaffold is located in the middle of the shaft, preferably anchored around outstanding cooling plates. Because the root of the scaffold is located so high in the shaft, it is difficult to destroy it by melting with the slag. Besides, a scaffold in the upper shaft makes it difficult or even impossible to control the ore / coke distribution along the furnace radius.

Causes for the formation of scaffolds

There are several theories regarding how the formation scaffolds is initiated, but all the theories accepts that alkalis and / or Zn are involved in the process of scaffold formation. The basic difference between the theories is that whether alkalis and Zn act only as initiators for scaffold formation, or whether they are responsible for whole of the process. However, it is not very certain that whether the presence of alkalis and Zn is a prerequisite for the formation of scaffolds, or whether they merely aggravate the whole of the process of the formation of scaffolds.

Based on the chemical and mineralogical studies carried out on the samples collected for the scaffolds of several BFs, it is evident that the formation of scaffolds in The BF can take place either due to a single cause or due to the multiple of the causes. The various causes are summarized below.

Zinc – Zn and Zn containing compounds are found in several samples. The buildup of Zn takes place due to cycling of Zn in the BF. Zn containing compounds such as ZnO enter the BF with the ferrous burden. While the Zn is melting at temperatures of 419.7 deg C and evaporating at 906 deg C, the melting point of ZnO is 1975 deg C. Depending on the partial pressure of Zn and the CO / CO2 and H2 / H2O contents, ZnO is reduced at high temperatures and under conditions found at the tuyere level as per the reversible reactions ZnO + C = Zn + CO, ZnO + CO = Zn + CO2, and ZnO + H2 = Zn + H2O. Gaseous, metallic Zn rises up into the shaft, where conditions change and the Zn is oxidized again and condenses on the burden material. The precipitation forms a white-greenish layer on top of the burden material. Due to the high melting temperature, ZnO remains stuck to and moves down with the burden. Together with the new Zn, entered with the burden, Zn accumulates in the BF as long as it is not brought out.

Oxidation and further reactions produce various Zn compounds which have been established during the mineralogical studies of the samples. ZnO is most frequently found. Some of the other compounds found are zinc silicate (Zn2SiO4), gahnite (ZnAl2O4), hardystonite (Ca2ZnSi2O7), and franklinite (ZnO.Fe2O3). ZnO can agglomerate different burden fines and dust, forming a scaffold in the shaft. Such a scaffold needs not necessarily be attached directly to the entire wall it covers.

In a typical case of a BF operation where the burden material has a very high Zn load upto 45 kg per ton of hot metal (kg/tHM), the Zn quantity in the BF accumulates more quickly than the BFs with lower Zn concentrations of the burden. However, scaffolds caused by Zn are not a specific problem of a particular BF. Zn concentrations can reach high amounts in every BF, if Zn is not brought out of the process by specific action such as high top gas temperatures.

Sintering – If part of the ferrous burden is kept in an accumulated state over a long period of time then it can be reduced even at temperatures and atmosphere existing in the shaft. This accumulation becomes rich of metallic Fe grains, found close to slag phases. Due to the pressure of the burden and temperatures above 900 deg C, the metallic Fe grains can be sintered together. A solid build up of thick Fe takes place. Because of its stability, it only has to be attached to the wall at the bottom of the accumulation in the shaft causing formation of scaffold.

Alkalis – Alkalis normally enter the BF with the ferrous material and with the coke in the form of silicates. Build up of alkalis can take place similar to Zn build up in the BF. Reduced at the tuyere level, alkalis rise up in the shaft and condense at temperatures of 882 deg C (sodium, Na) and 779 deg C (potassium, K).

The alkali vapours which ascend with the surrounding gas condense in the upper part of the BF where a part leaves with the top gas, while the remaining condenses on the inner walls or on the feed material. Because of the volatilization and condensation of the alkali in the different thermal zones, alkali tends to cycle within the BF, leading to an accumulation and interactions with other feed materials. This can have a significant impact on the process, even when the alkali is charged in small amounts, generally less than 5 kg/tHM. A simplified view of alkali circulation in the BF is shown in Fig 4. Studies of excavated BFs have shown that the alkali level is highest where the temperature is above 1000 deg C, which means that there is an increased alkali concentration in the lower part of the BF.

Fig 4 Simplified view of alkali circulation in the BF

Several reactions which are taking place to form the oxides of alkali having melting points above the shaft temperature level. Alkalis can also agglomerate the burden and dust. If precipitated at the shaft wall, alkalis can stick burden parts together and form a scaffold.

It is not essential that the scaffolds mainly made up of alkalis are formed in every BF. However, in BFs, thin layers, containing kalsilite (KAlSiO4) and other K2O compounds are present. They are formed directly at the wall. The behaviour of KAlSiO4 is important. It can stick burden parts together and form a scaffold. Ferrous parts are then reduced and sintered together, forming a solid scaffold. As KAlSiO4 is not a stable composition, it can react away so that no or very little amounts of alkalis are later found in the samples of scaffold.

Mushy zone – At temperatures around 1100 deg C, ferrous burden material starts to soften. Soft structures within this mushy zone are forced by the weight of the burden to stick together. During a stoppage of the furnace or once colder wall near regions are reached, this mushy zone solidifies, attaches to the wall and forms the front layer of a scaffold. The scaffold disturbs the gas flow through the shaft and forces the flow into another direction. If due to the changed gas flow, the temperatures stay below their original levels, the scaffold can hardly melt away. Reduction and sinter processes then generate a layer of metallic Fe grains in slag phases which are even harder to remove. Very big scaffolds can be found in BFs where temperatures have reached more than 1100 deg C. These scaffolds are made up of slag phases and metallic Fe grains sintered together. Very small amounts of Zn or alkalis are present. Hence, in this area, it is estimated that not these elements, but a solidification process of mushy material is responsible for the initial sticking. Further indicators for this formation process are the low carburization of the Fe and the tightly enclosed coke particles.

Water – Water enters the BF in different ways. The most important are (i) wet sinter and coke, especially sinter received from the open storage which is not roof protected  from environmental conditions such as rain, (ii) water vapour injection on tuyere level, and (iii) liquid water injection on top of the burden, in case top gas temperatures exceed a certain value. Also, near the top of the burden, in the cold wall near regions, water vapour can condense. Along the shaft wall, liquid water can flow down and reach deeper levels of the shaft. Water accumulates dust and fine particles of the burden and can stick these agglomerates to the wall. Over a long time period, sinter and reduction processes form a solid build scaffold.

Water flowing down at the shaft wall, reduces the temperature in wall near regions. Due to this temperature drop, Zn and alkalis condense much faster and the corresponding scaffold formation processes are speeded up. Once water reaches hotter regions, it evaporates, while the liquid water phase runs further down in the BF shaft. The water vapour reduces the H2 / H2O content in these areas. As a result, the equilibrium of the reversible reaction in the equation ZnO + H2 = Zn + H2O is changed towards or further into the direction of ZnO.

In the predominance area diagram for the system Zn-O-H2 (Fig 5), three different phases are shown as a function of the H2 / H2O content and the temperature. The phases are (i) Zn liquid, Zn(l), (ii) Zn vapours, Zn(v), and (iii) solid ZnO, ZnO(s). The diagram has been calculated out of thermo-chemical data for pure substances. The solid lines with a bend at the boiling point are coexistence lines where Zn and ZnO exist in equilibrium together. Zn is stable above those lines and ZnO is stable below. Coexistence lines are a function of the activity of the Zn, a(Zn). For ideal gases, the activity of Zn vapours, a(Zn) equals the partial pressure of Zn(v). Under conditions normally found at BF, activity values between 0.1 and 0.01 are expected for Zn vapours. The further conditions are away from the coexistence lines, the more of a product is created before the reaction reaches equilibrium again. If more ZnO is formed, the chances to bring Zn out of the process by high top gas temperatures decline.

Fig 5 Predominance area diagram for the system Zn-O-H2

The black circle in the diagram characterizes a H2 amount which is three times the amount of water vapour at the temperatures a little above the boiling point of Zn. Depending on the partial pressure of Zn, the thermo-mechanical equilibrium is close, probably even in favour of Zn(v). If water is added, it evaporates and the H2 / H2O content declines into the direction of the arrow. Conditions are now further away from the lines of coexistence. As a result, the amount of ZnO formed is raised. After precipitation, the scaffold forms much faster as the scaffold formation process accelerates.

Lime together with water – If too much burnt lime is added to the sintering process, it can force the sinter to split and shatter, if it gets in contact with water and reacts. As a result, the amount of fine burden parts is increased in the BF. In general, fine parts of the burden are much easier to agglomerate and support the formation of scaffold. Inside the BF, burnt lime and water can react to form cement. The cement agglomerates the burden and forms a very stable, solid build scaffold. If ferrous burden parts are reduced and sintered together, an even more stable scaffold is formed.

Process of forming scaffolds

Scaffolds are normally made up of a solid shell on the inner side of the BF and a layer of loose burden material between this shell and the wall of BF. Studies have shown that the solid shell develops along an isotherm. During formation, this isotherm is located at the position of the solid shell. There are two different structures forming the solid shell on the inner side are possible namely (i) metallic Fe grains in slag phases, and (ii) burden glued together by ZnO. Two formation processes seem possible.

In the first process, soft, ferrous burden solidifies along an isotherm, if the temperature inside the BF drops. The reason for a temperature drop can be a stoppage or a fluctuation in the process. Over a time interval long enough, the Fe containing burden is reduced and further sinters together at shaft temperature and atmosphere. At the end of the process metallic Fe grains in slag phases have formed.

In the second process, Zn, alkalis and their compounds precipitate and form once the temperature falls below a certain point. Only at a certain temperature, reaction and precipitation processes create enough Zn or alkali compounds to bond the burden together. The coke and sinter structures bonded together by ZnO are the product. During further reduction and sinter processes of the Fe bearing burden, Zn can react away and gasifies. In this case, the metallic Fe grains in slag phases are the final product of the scaffold, which in the first process is made up of coke and sinter structures bonded together by ZnO.

The scaffold formed by the any of the above processes explains the shape of the solid shell, the layer of loose burden material and why the scaffold is formed within a few days. Once the solid shell exists, it disturbs the gas-flow through the shaft and forces the flow into another direction. As a result of the changed gas-flow, temperatures can stay below their original levels. In this case, the scaffold can hardly melt away. Condensation and precipitation in the layer of loose burden material later accumulates Zn and alkali compounds. These accumulation processes can also lead to the formation of a new layer in front of the already existing shell. The existing shell is a structural support for the further growth of a new layer and protects it from abrasion.

Scaffolds are normally continuously formed. The alkalis and Zn contents of the ascending gas are deposited on the burden or refractory as the temperature decreases up in the shaft. This phenomenon creates slowly growing scaffolds. The speed of growth of the scaffold layer on the refractory is generally of the order of a few millimetres per week. This kind of formation of scaffolds is mainly influenced by the alkali and Zn load of the burden and the temperature distribution in the shaft.

Disturbances or interruptions of the process in the BF can cause the burden to start sintering. If this occurs in the lower region of the furnace, the descending burden can remove partly or all the agglomerated material. If the agglomeration occurs in the uppermost part of the shaft, the partly agglomerated burden can become stagnant. Material beneath this zone is also to be stagnant. All this material gets reduced slowly with time and become reduced Fe. During the time of reduction and depending on the temperature distribution in the BF, the alkali and Zn fumes can deposit on the surfaces of the Fe oxide particles and form compounds with alumina and magnesia silicates. The speed of growth of this phenomenon can be considerably larger compared with scaffold formation due to the deposition of alkali and Zn on the refractory. The speed of growth depends on parameters such as the extension of the initial agglomerated burden zone, temperature distribution, and the amount of fine particles in the ascending gas.

Measures for avoiding the formation of scaffolds

If scaffolds grow very big they disturb the process in the BF and reduce the BF efficiency. Solid-build scaffolds can sometimes only be removed by blasting. Such radical counter measures sometimes cannot be prevented. However, it seems possible to intervene with less extreme measures for avoiding the formation of scaffolds. When the scaffolds are still comparatively small in size, especially during the formation process, steps against their formation are effective. Some of these measures are given below.

Cooling capacity – To initiate the melting process of solidified burden material at the shaft wall and to prevent a solidification of the mushy zone in cold, wall near regions, the capacity of the shaft cooling-system can be reduced. The number of cooling-boxes in the upper part of the shaft can be minimized. Less cooling capacity keeps the inside temperature high enough, to prevent a possible mushy zone from solidification during the stoppages of BF. Alternatively it seems possible, to charge high amounts of coke near to the wall, before a stoppage. The coke is to be placed in such a way, that during the stoppage, it stays at the region, where the lower part of the scaffold normally sticks to the wall. That way a solidification and wall-sticking of soft burden material can be prevented, because coke does not get soft at shaft temperatures.

Near to the wall charging of coke – High amounts of coke, preferably charged toward wall near regions raise the temperature and can melt up scaffolds. The temperature rise also prevents or reduces Zn and alkali condensation and as a result, the development of new scaffolds. The first results of a coke charging near to the wall are generally visible two or three days after its initiation. The disadvantage of this counter measure is of course a higher wear process of the shaft wall refractory and reduced gas utilization.

Silica – Silica can react with the material of scaffold and can form eutectic silica compounds with low melting points. Hence silica can be added to wall near regions, to melt scaffolds off the shaft wall.

Centre -charging of fine parts of the burden – Fine parts of the burden are much easier to agglomerate and are to be reduced. If at all charged to the BF, these are to enter the BF to the centre region. This way the chances of sticking of the fine burden agglomerates to the shaft wall are minimized.

Balancing of alkalis and acid slag – Input and output balance provides the valuable information about the actual amount of alkalis in the BF process. Alkalis can then bring out of the BF by an acid slag. Acid slag also raises the amount of sulphur in the hot metal (HM). Hence, balancing is important to operate the BF with the acid slag for a limited period and hence the disadvantages as less as possible.

Forced slipping – Forced slipping takes place when there is a stoppage of the blast pressure for a few minutes. It forces the burden to descent suddenly in the shaft and the top of the burden sinks down. Due to the sudden force initiated by the weight of the burden, scaffolds can break off. Forced slipping is to be given only at the end of tapping, to prevent a quality loss and contamination of the HM by the sliding down material.

Optimization of constructional features of BF – Especially at their cold top, cooling boxes act like an anchor for future scaffolds. In contrast to staves, cooling boxes generate cold spots distributed over the shaft wall. These cold spots are ideal starting-points for condensation and solidification. Staves with the same cooling-effect as cooling-boxes produce a steady temperature field, without the extreme temperature minima. Hence, staves are less prone to generate scaffolds.

High top gas temperature – Zn can be brought out of the BF process with high top gas temperatures, not giving the Zn vapours enough time to condense or to react. To reach this goal, an optimum top gas temperature needed is around 350 deg C. Together with the high amounts of coke, charged to the wall area, Zn amounts of 45 kg/tHM can be handled at the BF.