Figure 3 shows the element mappings of a Dolime particle about 100 µm reacted for 5 min. While the CaO has transformed to CaS, MgO still remain in the Dolime particle. No appreciable MgS was found.
Similar observations were reported by Nakai et al. They studied hot metal desulfurization with calcinated dolomite and Al-ash. Their microphotograph shows an outer layer of CaS about 50–100 µm covering a bulk of unreacted MgO and CaO.
Figure 2 indicates that using Dolime of brings the sulfur concentration from 460 to 180 ppm. The result compares very well with the results obtained by Nakai et al. They employed dolomite–Al to study desulfurization in a 70 kg induction furnace at 1673 K. An addition corresponding to reduced the sulfur concentration from initially 420 ppm to about 200 ppm after 10 min of stirring.
The fact that only CaO in the Dolime contributes in desulfurization is further confirmed by the experimental result of MgO. As seen in Figure 2, MgO has very little function in desulfurization. Since CaO is responsible for sulfur removal from hot metal, the is calculated using the moles of CaO · MgO in Dolime, which is the same as the number of moles of CaO. Because of this reason, about 70% more mass of Dolime (in comparison with lime) has to be added to reach the same desulfurization level as with commercial-CaO or calcium carbide.
It is well known that CaO is sensitive to calcium hydroxide formation upon interaction with water. Even though the commercial-CaO was stored in a desiccator, it is likely that the surface of CaO particles are subjected to some degree of calcium hydroxide formation and possibly other processes, e.g., adsorption of CO2 when the particles meet the humidity and carbon dioxide during the delivery. Note that this kind of contamination is also inevitable in the delivery and even storage of industrial practice.
A comparison of the thicknesses of the CaS layers on the single particles of the Flucal powder and commercial-CaO powder (Figure 4 and 5) reveals that the layer thickness (30 µm) on Flucal is almost twice of the thickness on the commercial-CaO. While Flucal particle might have superior surface properties, the change of the surface of the particles of the commercial-CaO could also be responsible for the lower growth rate of CaS layer. Obviously, the surfaces of commercial-CaO particles are less reactive. The Flucal powder would perform better than the commercial-CaO when they have the same particle size.
Figure 6 shows that the commercial-CaO particles form agglomerates. The formation of agglomerates would not only reduce the total surface area, but also leads to the formation of calcium silicate. The formation of calcium silicate in agglomerates is in accordance with the observations by Oeters et al. and Takahashi et al., who used CaO rods. The reason for the formation of calcium silicate in agglomerate but not in single Flucal particle is discussed in detail in another paper. This discussion is briefly given below to orient the reader.
The low concentration of dissolved Ca in the metal (<1 ppm) would make the mass transfer of Ca extremely slow, and therefore ruling out practically the possibility of low local calcium activity for reaction (1) to take place
Therefore di-calcium silicate is likely formed by the following reaction.
When CaO is added as the desulfurization agent into hot metal containing high Si, the limiting factor for the formation of calcium silicate through reaction (3) would be the oxygen supply. It is well known that the oxygen level in hot metal is at ppm level, which will not be enough to generate calcium silicate. This argument is supported by the absence of calcium silicate in the case of single small Flucal particles (Figure 4 and 5) and CaC2. The desulfurization reaction,
leads to the formation of oxygen. A tiny CaO particle (usually < 50 µm) would not generate sufficient oxygen locally for reaction (3) to take place, when the particles are well distributed. A particle of less than 80 µm particle is converted to CaS before the calcium silicate layer has formed. This is a direct observation in the experiments as exemplified by Figure 4. The formation of the silicate layer is due to both kinetics and thermodynamics. In the case of bigger particles or CaO agglomerates, the silicate layer is formed when the layer of CaS has become thicker (e.g., 40 µm). The increase of the local oxygen potential would result in the formation of calcium silicate through reaction (3) under the thick CaS layer (see Figure 6).
As seen in Figure 2, an amount of commercial-CaO of brings down the sulfur content to 130 ppm, whereas the same amount of Flucal takes the sulfur concentration down to 20 ppm. The decrease of desulfurization ability is due to three factors: (i) the surfaces of commercial-CaO particles being less reactive (the growth of CaS becomes slow), (ii) the reduced surface area due to the formation of agglomerates, and (iii) the agglomerates leading to the formation of calcium silicate layer, which increases the resistance to mass transfer.
The better performance of Flucal compared to commercial-CaO could be because: (i) the Flucal has been fluidized, which helps to avoid agglomeration of the CaO particles and (ii) the Flucal particles (50–100 µm) has a smaller particles size compared to the commercial-CaO (<300 µm).
While maintaining good surface property of the powder is essential to maintain high reactivity of lime in desulfurization, decreasing the local oxygen potential by addition of other agents would be possible to help even commercial-CaO to improve its performance.
4.3 Lime–Mg Mixtures
One of the alternatives of additives into CaO for desulfurization is Mg granules. To gain an insight of the function of lime–Mg mixtures, it is essential to understand how the two agents work individually. The function of Mg in hot metal desulfurization has been studied and discussed in detail in a previous work. The study shows that when Mg is added into hot metal, most of it rushes out as Mg-gas in less than 2 s. MgS is not formed by homogeneous nucleation but on MgO particles, which originate from the surface of the added Mg-granules. The study indicates that during the Mg addition, 180 ppm of magnesium is dissolved in the sample. This corresponds to 10% of the total addition. Thus, 90% of the Mg-addition went off as gas during the initial 2 s after addition. Because of the escape of large amount of Mg vapor, the use of Mg is not efficient. The study also shows the sulfur content is lowered from 460 to 360 ppm by the formation of MgS on MgO micro particles.
Irons and Guthrie suggested that MgS formation in hot metal takes place at the surface of inclusions by dissolved Mg through reaction (5)
Their finding agrees well with our previous publication. The experimental point shown in Figure 2 indicates that addition of of Mg granules followed by stirring bring down the sulfur content to 120 ppm. As mentioned above, the sulfur content is brought down to 360 ppm after 2 s, and 180 ppm of magnesium is dissolved in the sample after this time period. It is reasonable to expect that reaction (5) continues on the surface of the solid particles in the hot metal after the initial period of 2 s.
A simple calculation reveals that the dissolved Mg quantity (180 ppm) can theoretically remove 237 ppm of sulfur according to reaction (5). Hence, the reaction (5) will bring the sulfur level down further from 360 to 123 ppm. This prediction agrees excellently well with the experimental result in Figure 2, viz. 120 ppm.
A steel industry implemented a desulfurization process of injecting 0.7 kg of magnesium granules (sized 1–1.5 µm) per ton hot metal with heated air at 1633 K. The annual average desulfurization achieved was from 650 to 200 ppm. The results are somehow in line with the present findings.
Jin et al. presented data from an industrial test in a Chinese steel plant using only Mg for desulfurization. The data presented were only a few. But it revealed that the sulfur concentration was lowered to about 100 ppm. Lower sulfur levels were only reached when the Mg addition was accompanied by calcium carbide. In line with Jin's results, Voronova pointed out, based on industrial data that a big increase of Mg consumption is required to reach sulfur content below 100 ppm. Again, the results agree with the present work.
Dyudkin et al. used magnesium bearing wire and lowered the sulfur concentration from 510 to 400 ppm during the 3 min when the wire was inserted. The sulfur concentration then fell to 140 ppm during the subsequent 36 min of floatation under the influence of a slag in a 10-ton ladle. Their results agree also very well with the present work.
Brazzoduro et al. studied hot metal desulfurization by Mg injected with nitrogen gas. They noticed Mg burning off at the surface of the melt. They concluded that the hot metal was not desulfurized to the corresponding degree of theoretical calculation. The escaping of Mg gas observed in the present work is confirmed by their industrial trails.
Shevchenko et al. proposed the use of granular Mg (<0.1 mm) for desulfurization claiming it had superior properties compared to both Mg–CaC2 mixtures and Mg–CaO mixtures. They reported the method was capable of reducing sulfur from 1500 to 10–20 ppm in less than 20 min. Unfortunately, they did not provide any information on the experimental conditions, e.g., the amount of addition. Reading the report carefully reveals that the Mg addition was accompanied by undefined slag additions. Thus, it is difficult to rule out the function of the slag in desulfurization.
The present experimental result of Mg reveals that (i) big amount (90% in the present study) Mg escapes as gas in the initial stage of the addition; (ii) the formation of MgS on the oxide particles brings down the sulfur content from 460 to 360 ppm; (iii) 10% Mg dissolves in the hot metal; (iv) reaction (5) continues on the micro particles, thereby leading to further desulfurization.
Nölle et al. did lab scale desulfurization trials with lime in three different atmospheres (methane, argon, and air) and reached the best desulfurization degree (95%) with methane, 47% using argon, and only 30% desulfurization degree with in air. Kikuchi et al. used a 4-ton scale experimental setup with mechanical stirring and showed 20–70% efficiency improvement of the desulfurization agent (lime–5 mass% Flourspar) when 10–58% propane was added to the nitrogen injection gas. The improvements of desulfurization by using methane and propane are expected, since both gas decrease the oxygen potential in the melt during injection. Lower oxygen potential favors the reaction,
It is well understood that mixing Mg with CaO will have the same effect, since the following reaction (5) takes place parallel to reaction (4)
Lowering the oxygen potential does not only promote CaS formation through reaction (4), but also helps to avoid formation of di-calcium silicate around the lime particles according to reaction (3).
As reported in the previous work, the generation of huge amount of Mg gas stirs the metal bath violently. This study also shows that the Mg gas helps in spreading the lime particles. To help the reader, the main results of lime–Mg mixtures obtained in the previous study are reproduced in Table 3.
Table 3. Distance from the surface of the metal above which reaction products are found
|Sample||Quenched after [s]||Distance from the surface [mm]|
Though the spreading effect depends very much on the reactor and addition position, Table 3 still indicates there is an optimal ratio between CaO and Mg, at which the CaO particles are distributed well in the liquid metal. As the Mg part reduced and the lime part increased, particles are found over a larger area of the sample. Under the present laboratory conditions, 20–30 mass% of Mg seems to be optimal. This optimal agrees very well with the results of desulfurization presented in Table 2 and Figure 2. When the commercial-CaO is mixed with 20 mass% Mg (), the desulfurization reached 80 ppm. The small Mg addition helps the commercial-CaO to perform better, but it is still not as good as the Flucal, which reached 40 ppm at . It should also be mentioned that the violent local stirring of the metal bath by Mg gas could even crush the agglomerates formed during storage. This would further enhance the sulfur pickup by the CaO particles.
The present results and reasoning is in good accordance with the industrial study by Visser and Boom. They desulfurized 283 tons of hot metal at 1673 K from 250 ppm sulfur to 7 ppm in 10 min by injecting 3.1 kg ton−1 CaO–15 mass% Mg mixture (3.1 kg ton−1 steel).
Good results of desulfurization of hot metal by combining CaO and Mg are reported also by other researchers.[5, 15, 17] On the other hand, their explanation of the results is somehow different from the present reasoning.