VINCENT PIRET1, MARCOS LOPES2

ENHANCEMENT OF THE EAF DUST RECYCLING IN SELF-REDUCING BRIQUETTES BY CONTROLLING THEIR HYDRATION

BEHAVIOUR

Abstract

The steel production via the EAF route produces free lime containing dust. In the case of carbon steel production, the main dust recycling process is the Waelz furnace, allowing the recovery of zinc but not of Fe. In the case of stainless steel production, the recovery of valuable alloying elements (a.o. nickel, chromium, and molybdenum) can be achieved by recycling the dust in the form of self-reducing briquettes into a liquid steel bath. A hydration step is mandatory before the briquetting of free lime bearing dust in order to avoid swelling and degradation of the produced briquettes.

The amount of added water must be well controlled in order to ensure the full hydration and also to limit the residual humidity to avoid the need for a subsequent drying step. A straightforward methodology for the determination of the free lime content and of the water amount to be added has been developed and validated. Hydration trials have been carried out to study the respective impacts of the dust nature, the temperature and added water on the reaction kinetics. Industrial trials have confirmed that the behaviour is the same at pilot and industrial scales.

Keywords: EAF dust recycling, stainless steel, hydration, self-reducing briquettes, free lime, hydrated lime.

List of symbolsEAF: Electric Arc FurnaceCaO: Calcium Oxide, Free LimeCa(OH)2: Calcium Hydroxide, Hydrated Lime

DTA Differential Thermal AnalysisXRF: X-Ray FluorescenceXRD: X-Ray DiffractionCatot: Calcium TotalSEM: Scanning Electron MicroscopeEDX: Energy Dispersive X-ray

1 CRM Group, Avenue du Bois Saint Jean 21, 4000 Liège, BELGIUM

2 Aperam R&D, BP15 Rue Roger Salengro, 62330 Isbergues, FRANCE

1. Introduction

The steel production via EAF route produces free lime containing dust [1]. In the case of carbon steel production, the main dust recycling process is the Waelz furnace [2], allowing the recovery of zinc (in the off gas) but not of Fe which is fully oxidized in slag phase. In the case of stainless steel production, the presence of valuable alloying elements (a.o. nickel, chromium, and molybdenum) in the dust is a driver to treat them in another process in which those elements can be recovered.

Since 2010, Recyco (an Aperam SA subsidiary located in France), has developed and operates a recycling pyrometallurgical process [3]. First, by-products from the stainless steel industry (dust and sludge) are mixed with a reducing agent and then roll pressed to produce self-reducing briquettes. After curing at room temperature, the briquettes are charged in an EAF to produce ferroalloys which are sent to the by-products producers and the slag valorised as construction material. In 2015, the plant has successfully treated 50 000 tonnes of stainless steel by-products. Co-processing dust and sludge, if mixed in controlled proportion with binder, reducing agent and flux, allows reaching nearly 100% of the EAF input in the form of self-reducing briquettes. The amount of each material in the recipe is chosen depending of the targeted final products composition, availability of the material, mechanical requirements for the briquettes …

In order to briquette a mix containing free lime bearing materials, a hydration step is mandatory before the compaction. The free lime contained in the dust reacts with water (Equation 1) to form hydrated lime, the reaction produces heat and the swelling on the dust.

CaO + H2O Ca(OH)2 + heat (1)

As the briquetting process of oxidised materials requires a binder witch is often dispersed with water [4] and as the mix contains water from non-dried sludge, the hydration reaction and consequently the swelling behaviour cannot be avoided. The Figure 1 shows the detrimental effect of the use of non-hydrated dust on mechanical properties. The briquettes seem of good quality at the output of the press but swell and crack after a few hours due the lime hydration inside the briquette.

Fig1: briquettes at the output of the press (left), after hydration within the briquettes (right)

Therefore, the hydration reaction must be completed before compaction. Industrially, the free lime bearing dusts are mixed with water in order to obtain after reaction a wet powder that

can safely be introduced in the mixer together with the other materials. The amount of water added must be well controlled in order to ensure the completion of the hydration reaction and avoid a costly drying step.

The work presented in this paper presents a straightforward methodology to determine the amount of free lime contained in the dust, which can be directly related to the minimal amount of water needed for the completion of the hydration. The main part of this paper is focussed on the key factors influencing the kinetics of the free lime hydration reaction within the dust. Tests have been performed at pilot and industrial scale. 2. Experimental setup

Free lime measurement

The most used techniques for the determination of “free lime” (ASTM C114) are the Franke method using acetaocetic ester and the Lerch and Bogue method using glycerol-alcohol [5]. Those methods were developed for the determination of free lime within Portland cement and do not distinguish the calcium oxide (CaO or free lime in this paper) from the calcium hydroxide (Ca(OH)2 or hydrated lime in this paper) [6]. Other authors use DTA [7-8] analysis to determine the amount of Ca(OH)2 or XRD [9] for CaO determination. In EAF dust, another method consists in a XRF analysis for dosing Catot [10].

As those techniques were mainly devoted to cement analysis and as the CaO content is not determined by a direct measurement, CRM has decided to develop a straightforward methodology to evaluate the available free lime amount in a dust. For the free lime hydration study, the amount of water needed for finishing the reaction between water and the dust is the parameter of interest. The water is mainly needed for the reaction with CaO, but also with MgO or other compounds of the dust.

The measurement principle is the following: the dust is mixed with a defined excess of demineralised water, after defined mixing time, the solution is dried at 105°C. The weight gain of the sample is accounted for water chemically linked with the dust, which is mainly in the form of Ca(OH)2.

Setup of the hydration reaction monitoring

Two parameters have been monitored during the hydration trials, the temperature (thermocouple measurement) and the electrical resistance of the dust. The follow up of the temperature allows indirect measurement of the heat released by the reaction and the electrical resistance is related to the water repartition. The resistance is obtained by the measurement of the current circulating in the dust under a continuous potential difference of 25V. The Figure 2 presents a schematic view and a picture of the used device. For practical reasons, the current flowing through the device will be used instead of the electrical resistance. In this system, one variable can be easily obtained from the other by using the Ohm’s law (Equation 2).

U [V] = R [Ω] I[A] (2)

Fig2: scheme of the electric resistance monitoring device (left), picture of the device (centre), device in use during a hydration trial (right)

For the pilot scale trials, the dust was mixed with the desired amount of water then placed in a 200 litres metallic barrel.

3. Results

In the frame of this work, three different EAF dusts from two plants have been studied at lab and pilot scale. Industrial measurements have been performed in order to assess the relevance of the measurements performed at CRM.

Free lime content of the studied dusts

Table 1 shows the minimal water amount needed to fully perform hydration and the free lime content of the studied dusts. Those results were obtained by using the method described in the previous section.

Table 1: free lime content of the studied dusts

Typical results and curves interpretation and utilisation

Figure 3 presents typically obtained results during the hydration trials. For instance this figure was plotted from a test on the dust B. The temperature curve gives an indirect indication about the reaction speed; the heat flow generated by the reaction is directly related to the reactants consumption but cannot be directly transposed to the temperature evolution because of the specific heat variation of the material (Cp water+CaO ≠ Cp Ca(OH)2) and the heat losses through the barrel. The current curve gives information about the different steps of reaction and allows a more accurate calculation of the reaction duration.

The temperature evolution (Figure 3) shows that the hydration reaction accelerates with temperature. The temperature plateau around 100°C is obviously related to the water evaporation process. For long reaction times (~3500s in the example), a slight temperature decrease is observed meaning that the heat losses become more important than the residual heat generated by the ending reaction.

Fig3: characteristic curves of temperature and current obtained from a hydration trial

From the current curve, the different steps of the reaction can be extracted. The first increase is due to the reorganisation of the material around the stainless steel bars. The first decrease is the consequence of the water migration from the surface to the core of the particles. Then the current slightly decreases due to the water consumption by the reaction until a peak appears when the reaction runaway (strong increase of the temperature). The time difference between the peaks of the current curve will be used in the rest of this paper as “the reaction time”.

In the EAF dust, the free lime is spread in the material in consequence of what the reaction is evolving simultaneously everywhere in the barrel. This is confirmed by Figure 4 which presents the temperature evolution at different positions in the barrel, far and near the walls. In the centre of the barrel, the thermal losses are low and almost constant: the temperature evolution is similar for the three studied positions which means that the reaction is occurring at the same time and speed.

In the positions “near the wall” the runaway of the reaction is delayed in comparison to the “far from wall” but also stronger. This can be explained by taking into account the fact that the closer of the wall the bigger are the thermal losses, leading to lower temperature and therefore lower reaction acceleration. In those positions, the increase of temperature is not only a consequence of the heat coming from the hydration reaction but also from the diffusion heat coming from the “far from wall” material, the combination of both explaining the step-like runaway. This particular experiment is an additional proof of the temperature impact on the hydration kinetics.

Fig4: simultaneous temperature measurement at different positions in the barrel

In order to properly interpret the information given by those curves, a sampling of the material during the reaction (trial on dust B) was performed to assess the evolution of the dust swelling tendency. The sampled material was compacted in the form of small cylinders and then cured under air at room temperature. The results are shown at Figure 5.

Fig5: evolution of the dust swelling tendency during the hydration reaction

Before the runaway (red curve), the dust cannot be used for briquettes production because the swelling due to free lime hydration is not fulfilled. Minutes after the runaway, the dust can be used but with a risk of small swelling. Fifteen minutes after the reaction time the dust can be safely briquetted (for dust B).

Industrial measurements

Measurements have been performed at industrial scale in the Recyco plant (Figure 6), in order to compare with pilot scale results.

Fig6: setup for industrial measurement

Figure 7 presents a set of curves obtained during the industrial trials. The lack of temperature data until 1800 seconds is due to a technical issue that has been solved during the test. The conclusion is that the behaviour of hydration is the same at pilot and industrial scales, therefore the result obtained at CRM should be easy to transpose for industrial use.

Fig7: current and temperature curves obtained after industrial trials

Impact of the dust nature

The main characterisation parameter of the hydration trials is the “reaction time”. The impact of the dust nature on the reaction time is shown on Figure 8.

Fig8: reaction time for the three different studied dusts

The reaction is very dependent on the dust nature; the comparison of this result with table 1 could give the wrong impression that the reaction is inversely proportional to the free lime content. A test on another sample of dust B (but another production date) containing only 11.8% of free lime gave a reaction time of the same order of magnitude.

The explanation of this difference between the reaction times lies in the thickness of the protecting layer around the bigger particles of the dust (lime, silica, alumina …). A paired SEM-EDX analysis (Figure 9) has put in light the existence of such layer composed of very fine particles of metallic oxide (Fe,Cr, Ni).

Fig9: SEM-EDX images for the three studied dusts, in the first raw the calcium is put in white, in the second raw the chromium is put in white (image width: 1.6mm)

The thickness of this protecting layer is in line with the time of reaction, consequently this is considered as the most plausible explanation for the difference in reaction time. Nevertheless additional work should be performed to confirm this conclusion. The origin of the thickness difference should be studied from the EAF process perspective.

Dust BDust A Dust C

Impact of the product temperature

The material temperature has a strong influence on the reaction kinetics. Tests have been carried out using preheated reactants before the mixing. The following hydration trials have been performed on dust B because the reaction time is compatible with the observer time. The results are presented at Figure 10.

Fig10: impact of the reactants temperature on the reaction time

Those results confirm the accelerating effect of a higher temperature on the hydration speed. The temperature of the environment seems to have an important impact on the disintegration of the protecting layer.

Impact of the amount of added water

The amount of water added impact the reaction time. Below the amount of water chemically absorbed by the dust (table 1) the reaction time increases. This can be explained by the fact that when almost all the water has reacted, the likelihood of contact between water and free lime become quite small and consequently the hydration reaction speed decreases.

Above the amount of water chemically absorbed by the dust (table 1), the impact is not fully understood. An increase of the reaction time is observed for some dust and a decrease for others, this could be explained by the relative importance of the following mechanisms:

Increasing the water addition increases the heat capacity of the material (Cpwater = 4.18kJ/kg K and Cpdust = 0.8kJ/kg K). A heat capacity increase slows down the rise of temperature and consequently the reaction speed.

Increasing the water addition increases the likelihood of contact between water and free lime witch increase the reaction speed.

4. Conclusions

The work carried out in this study resulted in a better comprehension of the hydration reaction of the free lime contained in EAF dust. It is worth noting that with the sensors (temperature and current) used in this study, the end of the reaction can be detected but not its reason (total consumption of free lime or a lack of water). For an optimal use of those sensing results, the amount of free lime in the considered dust must be known or measured. This can be obtained by utilisation of the straightforward methodology that has been developed in this study.

Additionally, the work carried out also put in light the main parameters influencing the kinetics of the hydration reaction. Those parameters are:

the dust nature, which influences the thickness of the protective layer made of small metallic oxide particles;

the material temperature; the amount of added water.

5. References

[1] European Commission: “Best Available Techniques (BAT) Reference Document for Iron and Steel Production”, 2010, 443. [2] Strohmeier et al., Steelworks Residues and the Waelz Kiln Treatment of Electric ArcFurnace Dust, 1996, pp. 87-90.[3] L. Avernant, A. Roques, M. El. Ouafi: “Recyco-Isbergues : Rapport d’activité – Année de référence 2013”, 2014. [4] W. Pietsch: “Roll pressing”, 1978, 94-97.[5] E. E. Pressler, Stephen. Brunauer, D. L. Kantro: “Investigation of Franke Method of Determining Free Calcium Hydroxide and Free Calcium Oxide”, Analytical Chemistry, 1956, 896–902.[6] R.F. Gebhardt: “Rapid Methods for Chemical Analysis of Hydraulic Cement”, ASTM STP985, 1988.[7] H.G. Midgley: “The determination of calcium hydroxide in set Portland cements”, Building Research Establishment Department of the Environment, Garston, Watford, 1978.[8] V.S. Ramachandran: “Differential thermal method of estimating calcium hydroxide in calcium silicate and cement pastes”, Division of Building Research National Research Council of Canada Ottawa, 1979.[9] T. Sofili, V. Novosel-Radovi, S. Cerjan-Stefanovi, A. Rastovcan-Mioc: “The mineralogical composition of dust from an electric arc furnace”, Material Technology 39, 149-154, 2005.[10] C.Z. Rizescui, Z. Bacinschi, E.V. Stoian, A. Poinescu: “Characterization of Steel Mill Electric Arc Furnace Dust”, Advances in Waste Management, 139-143, 2010.