BE1021255B1 - Composite steel lance - Google Patents

Abstract

This invention relates to TSL (top submerged lance) technology. It relates more particularly to a TSL lance comprising a tubular steel element having an upper section and a tip brazed to the upper section, characterized in that the upper section is made of a steel alloy comprising 13-15% Ni and 20-24% Cr, and in that the tip is made of a steel alloy comprising 5% or less of Ni, and 20% or more of Cr. This lance is specially adapted for a TSL process using pure oxygen. The reduced spattering of the slag results in differentiated corrosion conditions along the lance, which provides the opportunity to differentiate the steel alloy of the upper section from that of the tip of the lance. The result is a composite steel lance that is both durable and easy to maintain.

Description

Composite steel lance

This invention relates to non-ferrous metallurgy and in particular to the HSL (top submerged lance) technology as developed at CSIRO.

TSL technology has led to the emergence of a new smelting process to treat a wide range of ferrous and non-ferrous fillers. The process is based on injecting an oxygen-containing gas, usually air, together with a hydrocarbon fuel, directly into a melt by means of a vertically suspended lance. The essential process phenomena, such as the dissolution of the charged material, the energy transfer, the reaction and the primary combustion occur in an intensely agitated slag layer. As a result, the TSL process is extremely efficient with high specific melting rates, and is one of the leading processes for melting and refining metals, particularly lead, copper, and nickel.

In industrial practice, the lance is generally composed of a tubular element or steel pipe with a length of about 10 to 25 m, and a diameter of up to 0.5 m. It is generally mounted vertically, suspended at the top, the upper section passing through the furnace approximately centrally, from top to bottom, the lower section or tip being immersed in the slag phase of a molten bath. The lance is equipped with a coaxial pipe of smaller diameter conveying the liquid or gaseous fuel to the submerged point of the lance. The fuel mixes with the air injected through the lance, ignites, and produces an injection of flames into the melt. The inside of the lance can be provided with a rotating cup rotating the injected air during its descent through the lance, the plating against the inner wall for a more effective cooling effect on the lance.

In the metallurgical process, and particularly during the melting step, the lance is protected to some extent by a solidified slag layer. This is the result of splashes of slag that rise to a considerable height in the oven and on the spear. These projections stick and solidify due to the cooling effect of the gas flowing inside the lance. The tip of the lance also acquires a protective layer of solid slag. Nevertheless, the durability of the lances remains a crucial aspect of the TSL process. Both the top section and the tip may be affected by heat and corrosion, but the tip often wears out before the top section. To avoid throwing the complete spear, the spears are often dismantled and renovated by brazing a new tip in a dedicated workshop.

Point corrosion has been studied by several authors, mainly for slags containing lead. "Verhalten von hitzebeständigen Stählen in Bleischlacken", M. Stelteretcoll., World of Metailurgy - Erzmetall, 61 (2008), No. 4 teaches that steel with a high Cr and Ni content such as steel 1.4841 with 24- 26% Cr and 19-22% Ni is a suitable lance material for temperatures up to 1200 ° C. Less efficient, but cheaper, 1.4301 steel with 17-19% Cr and 8-10% Ni can be used for temperatures up to 1100 ° C. Corrosion of the steel is tested in a slag containing 61% PbO, 6% SiO 2, 10% ZnO, 11% FeO, 0.29% As and 0.16% Cu, which is usual composition in a lead melting furnace.

Another publication titled "Influence of Cr and Ni content in stainless steel on the degradation mechanisms in PbO-CaO-S1O2 slag", A. Malfliet et al., Corrosion Science (2012), 57, 1-10, similarly discusses resistance. corrosion of the tip of the lance in a lead rich slag. Two mechanisms explain the degradation: the oxidation of the steel by PbO, with a simultaneous reduction of the slag in metallic Pb, and the liquid corrosion of the steel by the metallic Pb liquid present in the slag. High levels of Cr tend to protect the steel by forming a film of Cr203 on its surface; High Ni steel is not indicated as corrosion by metallic Pb increases with the Ni content of the steel. The protective effect of a solidified coating is not considered in these publications.

The upper section of the lances is exposed to gaseous emissions leaving the bath and can undergo corrosion at high temperatures. This part must also deal with mechanical stresses when the waves induced in the bath by the intensive bubbling strike and bounce continuously against the spear. It is also subjected to a pronounced thermal cycling, from operating temperature to ambient temperature, whenever the lance is removed or disassembled or when it needs to be renovated.

US Patent No. 5,251,879 addresses the problem of corrosion of the upper lance by providing a coaxial jacket around the upper section of the lance, and blowing a cooling fluid such as air between the lance and the jacket. Cooling solves the corrosion problem, but greatly complicates mechanical design and maintenance operations.

In current practice, and thanks to the solidified coating, good results are achieved by using relatively common steel grades. A medium Ni and Cr grade stainless steel salt such as SAE 316 is usually used for the full lance. Indeed, similar thermal and corrosion stresses apply to both the tip and the upper section, as both are covered with a solidified slag layer and subjected to similar temperatures. The aforementioned steel grade also provides adequate mechanical strength and thermal cycling resistance.

However, the purpose of this paper is to further improve the TSL process by intensifying the metallurgical processes. Intensification can be achieved by blowing enriched air or even pure oxygen instead of air through the lance. The melting capacity of a given furnace can then be increased, for example by at least 50%, particularly when using enriched air containing 80% or even up to 100% oxygen. Fusion costs per tonne are decreasing as fixed costs are more widely distributed. However, the intensification of the process considerably reduces the life of conventional lances, because the tip and the upper section are both subject to additional constraints. The life of the lance then becomes too short for it to be used in practice in an industrial process.

A first reason for the additional constraints is the increase of the temperature of the flame at the point of the lance. The flame temperature of an enriched air burner is indeed much greater than that of a conventional air burner. The slag coating on the tip tends to melt, putting liquid slag in direct contact with the metal of the lance. This entails risks of corrosion in addition to thermal stresses.

A second reason is that the slag coating of the upper section of the lance does not appear. In fact, splashing is greatly reduced when using enriched air or pure oxygen: the gas injection rate in the melt expressed in Nm3 / s is much lower because of the lower prevalence or even the absence of nitrogen. This is especially true when using gas with at least 80% oxygen. Under these conditions, little or no slag is thrown up on the spear. The upper section is therefore directly exposed to higher temperatures and corrosion due to gas emissions such as SO2. It should still remain mechanically elastic and withstand thermal cycling.

Extremely demanding spear conditions are thus created for each part of the spear. It is difficult to cope with using only one spear material. To overcome this problem, two different qualities of stainless steel are proposed, one for each part of the lance. Both qualities must be compatible to allow the brazing of the two parts, thus forming a junction resistant to thermal and mechanical stresses.

More particularly, the invention relates to a TSL lance comprising a tubular steel element having an upper section and a brazed tip on the upper section, characterized in that the upper section consists of a steel alloy comprising 13-15% of Ni and 20-24% Cr, and that the tip is made of a steel alloy comprising 5% or less of Ni, and 20% or more of Cr.

Peak grades are preferred with even lower Ni contents, such as 3% or less, or even 1% or less.

Another embodiment of the invention relates to a furnace TSL equipped with the lance defined above.

Another embodiment relates to the use of the lance defined above in a TSL method in which the gas blown through the lance is pure oxygen. Such a TSL process can be advantageously used to treat metallurgical residues or concentrates comprising any one or more of the Pb, Cu, As, Se and S elements in an amount exceeding 5% by weight on a dry basis.

Another embodiment relates to a method of melting a metallurgical feed, comprising the steps of using a TSL furnace comprising a top-immersed lance; and melting the metallurgical charge by blowing an oxygen-containing gas through the lance; characterized in that a lance corresponding to the aforesaid particular TSL lance is used, and that the oxygen-containing gas is enriched air. The use of enriched air containing more than 80% oxygen is preferred.

By TSL launcher is meant a tubular steel member equipped with a smaller diameter inner tube, usually positioned coaxially, and can convey a liquid or gaseous fuel to the tip. The tip is the part of the spear that is intended to be immersed in the slag. It usually represents a length of 5 to 20% of the total length of the lance. A rotary gas cup, e.g. in the form of a helical insert, can be disposed inside the lance.

For the upper section, an alloy with a high Ni content is used to impart good mechanical strength. Ni is indeed beneficial to avoid the formation of a fragile sigma phase, which would lead to cracks. This is particularly sensible because thermal cycling would accelerate the formation of the sigma phase in this part of the lance which is reused many times.

For the tip of the lance, a lower mechanical resistance is permissible. The use of a lower Ni content is therefore possible. These are quite fortunate circumstances, since Ni would severely damage the corrosion properties of the tip, especially when in contact with a slag containing hot Pb.

The corrosion of the tip is aggravated when As or Cu or S are present in the charge. These elements easily form low melting point liquid alloys such as AsNi, AsCuNi, and PbCuNiS. Part of the Ni is thus leached from the stainless steel, creating porosities and voids, ultimately resulting in mechanical failure.

Cr is useful both in the upper section and the tip of the lance, as it forms a protective film of CrcCh on the surface of the steel. A high Cr content is useful to provide some leeway for some Cr depletion mechanisms resulting from the formation of SCrFe and SeCr when S and Se are present in the slag or furnace offgases. Being able to use compatible stainless steels for both parties is essential in practice. Indeed, one can use the same workshop as that needed to renovate conventional lances.

Figure 1 shows a schematic representation of a known TSL furnace. A single lance material is used (1); strong splashes (2) of the bath are represented, due to the use of air, thus comprising a large amount of nitrogen in the blowing gas; and a slag coating (3) of the upper sections of the lance is shown. In addition, the tip remains covered with solidified slag due to the moderate temperature of the flame at the tip.

Figure 2 illustrates the present invention using a composite steel lance. There is less splashing (2) of the bath due to the use of enriched air as blowing gas; no significant slag coating appears on the upper section of the lance. The tip is subject to extreme temperatures and is not covered by solidified slag. According to the invention, the lance is shown comprising 2 different materials, one for the upper section (1a), and one for the tip (1b).

The hot gas blown through the lance into the bath forms a rapidly developing gas bubble (4), which is schematically shown in Figures 1 and 2. Due to the use of enriched air or pure oxygen, and for the reasons explained above, the volume of the gas bubble under the tip of the lance is smaller in Fig. 2 (according to the invention) than in Fig. 1 (according to the prior art).

Examples

Regarding the requirements for the upper section of the lance, the metal composition is preferably limited to a high Cr content and a high Ni content, otherwise the lance would be quickly unusable due to the formation of cracks and corrosion .

Table 1 shows the cracking and corrosion of the lance material for different types of steel tested in a typical furnace atmosphere and under the mechanical loading conditions encountered by the upper section of the lance.

Table 1: Cracking and corrosion of different steels used as upper lance material

A low corrosion limit of 0.05 mm / h or less is required because the top section of the lance is reused many times by refurbishing it with a new tip.

Regarding the requirements for the lance tip, the corrosion resistance of steel alloys in the usual Cu / Pb slag is demonstrated by laboratory experiments where the conditions of the lance are simulated. Table 2 shows the corrosion in mm / h for a high Cr / high Ni content steel and a high Cr / low Ni content steel in a PbO / CuO slag at 1200 ° C.

Table 2: Corrosion of different steels used as advanced material

Table 2 demonstrates that alloys with high Cr / low Ni content are recommended to minimize slag corrosion. This applies especially to the tip of the spear.

A corrosion limit of up to 0.8 mm / h is acceptable for the tip, as it is acceptable for regular renovation.

Claims (7)

claims Lance comprising a tubular steel element having an upper section and a brazed tip on the upper section, characterized in that the upper section consists of a steel alloy comprising 13-15% Ni and 20-24% of Cr, and in that the tip is made of a steel alloy comprising 5% or less of Ni, and 20% or more of Cr. 2. TSL furnace equipped with a lance according to claim 1. 3. Use of a lance according to claim 1 in a TSL process in which the gas blown through the lance is oxygen-enriched air or pure oxygen. 4. Use of a lance according to claim 3 in a TSL process for treating metallurgical residues or concentrates comprising Pb and / or Cu, in amounts exceeding 10% by weight on a dry basis. 5. Use of a lance according to claim 4 in a TSL process for treating metallurgical residues or concentrates comprising one or more of the elements As, S and Se in amounts exceeding 10% by weight on a dry basis. 6. A process for melting a metallurgical charge, comprising the steps of: - using a TSL furnace comprising a lance immersed from above; and - melting the metallurgical charge by blowing an oxygen-containing gas through the lance; characterized in that the lance of claim 1 is used, and that the oxygen-containing gas is enriched air. 7. Process for melting a metallurgical charge according to claim 6, characterized in that the enriched air contains more than 80% oxygen.