Silica Lined Converter

[Francisco Raya]

Copper mattecontains Cu2S and FeS. Copper matte is put in asilica-lined converter to remove the remaining FeO and FeS present inthe matte as slag (FeSiO3). Also, some silica isadded to the silica-lined converter. Then, a hot air blast is blown.As a result, the remaining FeS and FeO are converted to iron silicate(FeSiO3) and Cu2S is converted into metalliccopper.

Copper matte mainly contains Cu_2S and FeS. For removing the gangue, FeS, silica present in the lining of the Bessemer converter acts as a flux and forms slag on reaction with FeO.

Therefore, whole of the iron present in matte, is removed as slag.

The engravings represented on the following three pages aremade from photographs taken at the converter plant of the Anaconda Copper Mining Company, where a 45 to 50 per cent, matte is converted into about 99 per cent, metallic copper by means of twelve huge converters running continuously and producing over 225 tons of blister copper in twenty-four hours. Since the erection of the first converter in Butte City, 1884, a new era in the history of copper production can be dated, which together with the increased demand and a great many important and highly interesting improvements of late years have carried this metal to the very front.
Operation : At the Anaconda and other modern converting plants the converters are tilted by means of a pinion and vertical rack attached to the piston-rod of a hydranlic cylinder. This improvement over the old style hand power converter has made it possible to increase the capacity of the converter three or four times. The handling is accomplished by means of two powerful electric traveling cranes, capable of lifting the entire converter from its stand and replacing it with another, in less than five minutes time. When taken into consideration that the silica lining in the coverter only lasts for a certain number of hours, it will readily be seen that the rapid exchange of converters is of vital importance to a successful operation.
The matte is charged into the converter vessel from the cupola (remelting furnace) by means of a moveable spout or runway lined with fire-clay, arranged in such a manner that one cupola and one runway will take care of two converters.
To receive the charge, the converter is tilted in a horizontal position. After charging, a powerful air blast is turned on and the converter raised to a vertical position. The blast, which enters the converter through one of the trunnions at a pressure of

11-12 lbs., causes the sulphur and iron contained in the matte to oxidize. Nearly all the sulphur escapes as sulphuric acid gas, while the iron changes into ferrous oxide and combines with the silica in the acid lining of the converter, forming a basic slag, which is poured off into slag-pots by tipping the converter, leaving a pure cuprous sulphide (white metal) in the vessel.

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The life, reliability and costs of lining in a basic oxygen converter are vital for the smooth operations of the steel melting shop utilizing basic oxygen process for steel production. Higher lining life results into improved availability of the converter which in turn improves its productivity.

Three important factors for achieving higher lining life of the basic oxygen converter (Fig 1) are (i) qualities of refractories and their laying pattern in the converter, (ii) operating practices followed, and (iii) monitoring of the lining wear and practices for the maintenance of the refractory lining. Development of improved refractory materials in combination with improved process control and better maintenance during campaigns make it possible to increase the lining life of the basic oxygen converter.

The causes of wear of refractories in the basic oxygen converter are either due to chemical reasons or due to the physical reasons. Chemical causes for the wear of the converter lining are mainly due to gaseous materials (oxidizing gases, reducing gases, and water vapour), liquid materials (slag. hot metal, and liquid steel melt), and solid materials (fluxes, and carbon disintegration). Physical causes for the wear of the converter lining are excessive temperatures (poor dissipation, and hot spots), static mechanical stresses (spalling, and expansion), and dynamic mechanical stresses (abrasion, impact, and vibrations). The key wear mechanisms of the refractory lining of basic oxygen converter can be summarized as follows.

Large crystallite size is generally considered to be over 140 microns in size. Fused MgO grain can exceed 1000 microns. Large grained crystallite normally outperforms low crystal size due to a reduction in interstitial porosity thereby reducing the chance of slag penetration into the grain boundaries and by lowering the susceptibility of the MgO to reduction by the C present in the brick during high temperature service. The reduction process destroys both the C in the brick and the MgO in the grain producing magnesium metal vapour and CO gas.

Bricks are carbon bonded with the residue of finely divided C remaining after the coking of the binder. This is what holds the brick together. Graphite is non-wetting to steelmaking slags preventing slag penetration into the brick and subsequent dissolution of the magnesia grains. The graphite is also very thermally conductive transferring heat away from the brick surface thereby reducing the kinetics of aggressive reaction. Chemically all graphites are pure carbon but all contain some ash (clay minerals found in the graphite deposits). Impure graphite adds impurities such as silica and alumina to the brick which generates only negative effects. Flake graphite is normally used as it has a higher resistance to oxidation than amorphous graphite and a higher thermal conductivity. Generally the amount of graphite used varies from 5 % to 25 %. Everything else being equal the higher the graphite content the higher is the slag resistance and thermal conductivity of the brick.

Metal powders added to Mag-C bricks act as scavengers for oxygen delaying oxidation of the graphite and C-bond. The powders improve hot strength markedly by forming complex metallic-carbide- oxide bonds in the brick.

Refractories in different zones of basic oxygen converter are subject to different conditions due to which their wear rates vary. Hence different qualities of refractories are required in different zones of the converter to have a uniform rate of wear. This type of lining is known as a balanced lining or zonal lining. In the zonal lining pattern a given segment of lining having lesser wear is assigned a lower quality or less thickness of refractory. Similarly refractories of greater wear resistance and normally having higher cost are assigned to those segments of the converter lining which are having higher wear pattern so as to have longer life of these severe wear areas.

Good control of slag development, oxygen flow and lance practice, and use of bottom stirring and limited use of re-blow practice are key features of the operating practices which influences the lining life of the basic oxygen converter. Knowledge of interactions between process chemistry, blowing dynamics and converter lining wear can achieve both efficient steelmaking as well as long converter lining life.

The most important factors which have maximum effect on the wear rate of the basic oxygen converter refractories are the high bath temperature at the end of the blow and high content of FeO in the slag. Further converter waiting for the tapping for a long time after the end of the blow has a big negative influence on the refractory lining. Other factors which have negative influence on the refractory lining of the basic oxygen converter include (i) high silicon content of the hot metal, (ii) high manganese content of the hot metal, (iii) high frequency of the reblows, (iv) poor reactivity and low quality of lime additions, (v) inadequate addition of lime specially in the initial period of the blow, (vi) converter slag unsaturated with MgO during different period of the blow due to low additions of MgO additives such as calcined dolomite or calcined magnesite, and (vii) low slag basicity.

Also important factors affecting the converter lining life are (i) titanium content of hot metal and titanium oxide content of the slag, (ii) duration of time for which the converter bath has liquid material in it, (iii) high amount of addition of iron ore, and (iv) frequency of cleaning of converter mouth.

Most important factors which have for positive effect on the lining wear rate of the basic oxygen converter include (i) high frequency of slag splashing, (ii) high frequency of slag coating, (iii) appropriate addition of calcined dolomite and/or calcined magnesia, (iv) frequent action for bottom care such as brick patching, and (v) frequent repair measures such as gunning of the worn out areas etc.

A slag saturated with lime is not only important for steelmaking but also to prevent excessive wear of the converter lining. Lime added before and during the blow is to ensure a slightly lime super saturated slag at the end of the blowing process.

The slag development path for different silicon percentage of hot metal show that starting from the high FeO containing initial slag, the SiO2 and CaO contents of the slag rise, as a result of increasing silicon oxidation and lime dissolution. The higher the initial hot metal silicon content, the higher the SiO2 content early on in the blowing process. At the end of the blow slags need to be slightly lime supersaturated in order to avoid excessive refractory wear. In order to achieve this goal a lime addition rate is necessary which is to be adapted to suit the silicon content in the hot metal and to the target slag FeO content.

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