Kinetic Reduction of Mill Scale via Hydrogen

Mill scale is very attractive industrial waste since it is rich in iron (about = 72 % Fe) and it is suiTab. for direct recycling to the blast furnace via sintering plant. In this paper the characterizations of raw materials were studied by different methods of analyses. The produced briquettes were reduced with different amounts of hydrogen at varying temperatures, and the reduction kinetics was determined. Two models were applied and the energy of activation was calculated.


Introduction
In several iron and steel making processes, about 500 kg/ton of solid wastes of different nature are generated. One of this wastes is the mill scale which represents about 2% of steel produced [1]). Mill scale is very attractive industrial waste due to very rich in iron (about = 72 % Fe) [2]).
In the whole world 13.5 million tons of mill scales are generated annually [3]. Mill scale is suiTab. for direct recycling to the blast furnace via sintering plant [4]. Approximately 90% of mill scale is directly recycled within steelmaking industry and small amounts are used for ferroalloys, in cement plants and in the petrochemicals industry [5][6][7][8]. Some authors [9] studied the replacement of some amount of Baharia high barite iron ore concentrate by mill scale waste and they indicated that replacement of some iron ore by mill scale increased the amount of ready made sinter, sinter strength and productivity of the sintering machine and productivity of blast furnace yard. .
The reduction of iron oxides via gaseous and solid reductants has been already extensively studied [1].
Some investigator [1] studied the kinetics of scale reduction by carbon monoxide in the temperature range 800-1200 ˚C and the results indicated that un-reacted shrinking core model with one interface under chemical reaction control fits well with the experimental data at the initial stage of reduction and the activation energy about 80 kJ/mol. Some authors [2] found that the best reduction of mill scale was obtained at 1050 ˚C at 180 min in pure carbon monoxide gas. …………………………………………………………….
It was indicated that with increase the temperature, the percentage reduction increases with increase in time and the activation energy of reduction depend upon the type of binder (10).
Some author [11] found that varying the total pressure of the reduction system by hydrogen and carbon monoxide had no effect on the rate of reduction. Commercial iron powders are classified in three types, reduced iron powder, atomized iron powder, and electrolytic iron powder, depending on the production method, and are used in various applications, taking advantage of their respective properties. Iron powders are used in many different industries for many different applications. This study aims at investigating the reduction kinetic of Egyptian mill scale briquette via hydrogen to produce iron powder.

Experimental work 2.1. Material
The rolling mill scale used in this work was provided by mill of Egyptian iron and steel Co. The sample was submitted to chemical and x-ray analysis.
The chemical analyses of mill scale are illustrated in Tab.I.  X-Ray analysis is illustrated in Fig.1. From which it is clear that mill scale mainly consists of magnetite, wustite, iron, quartz and hematite.

Preparation of the briquetting and its physical properties
The mill scale was grinding in vibrating mill to powder with size less than 75 μm. The mill scale powder (10 g) are mixed with 2% molasses and then pressed under different pressure. The briquette subjected to drop number test and crushing strength tests. The drop number indicates how often green briquette can be dropped from a height 46 cm before they show perceptible cracks or crumble. Ten green briquettes are individually dropped on to a steel plate. The number of drops is determined for each briquette. The arithmetical average values of the crumbing behavior of the ten briquettes yield the drop number.
The average crushing strength is done by compressed 10 briquettes between parallel steel plates up to their breaking [12].

Reduction process
The reduction of mill scale by hydrogen was done in a thermo balance apparatus. (A schematic diagram of thermo balance apparatus is shown in Fig. 2 [13]. It consisted of a vertical furnace, electronic balance for monitoring the weight change of reacting sample and temperature controller. The sample was placed in a nickel chrome crucible which was suspended under the electronic balance by Ni-Cr wire. The furnace temperature was raised to the required temperature (650-950 ˚C) and maintained constant to ± 5 ˚C. Then samples were placed in hot zone.
The nitrogen flow rate was 0.5 l/min in all the experiments (at initial time in order to remove air before each experiment and also after the end of reduction). The weight of the sample was continuously recorded at the end of the run; the samples were withdrawn from the furnace and put in the desiccators.    Fig. 3 and 4 shows the relation between the change of pressure load at constant amount of molasses (2%) on the drop number (drop damage resistance) and cold crushing strength of the briquette It is clear that as the pressing pressure load increased both the drop damage resistance and crushing strength increased.This may be due to the fact that increase pressure load increases the compaction of briquette and subsequently the Vander Waals forces increased [14][15], also the increase of briquetting pressure leads to progressive crushing of the macro pores [16].    Figure it is clear that the degree of reduction increased as the hydrogen flow rate increased. This may be attributed that the increase of flow rate leads to an increase of number of hydrogen mole in the bulk phase, which in turn leads to the raise of hydrogen adsorption and subsequently the rate of reaction increased [17] or the increase of flow rate increased the gas diffusion across the boundary layer subsequently the reduction ion increased [18]. Also may be the higher flow rate prevailing in the reaction zone which enhances the rate of hydrogen absorption and subsequently the rate of chemical reaction steps increased [19]. .

Effect of temperature on the reduction degree of mill scale briquettes
Experiments were performed for briquette which pressed at load 216. 85 M.Pa. , in the temperature range of 650 to 950ºC in hydrogen atmosphere (2 L/min. Plots of the reduction percentage as a function of time are shown in Fig. 6. From this Fig., it is clear that the reduction rates increased with increasing temperature. At high reduction temperatures (more than 900 and up to 950˚C), with increasing temperature, the oxygen removal favorable increased. The analyses of the curves relating the reduction percentage and time of reduction, shows that each curve has 3 different slopes indicating 3 different values of reduction rates. The first value is high, while the second is somewhat slower and the third is slowest one. The increase of reduction percentage with rise of temperature may be due to the increase of number of reacting moles having excess of energy which leads to the increase of reduction rate [17,20]. Also the raise of temperature leads to an increase of the rate of mass transfer of the diffusion and rat of desorption [18][19][20][21].
Where R is fractional reduction, t is time of reduction, k is the rate constant. Fig.7. illustrate the relation between 1-(2/3) R-(1-R) 2/3 against time of reduction for different reduction temperature. From which it is clear that the straight line was observed.

X-ray analysis of some reduced samples
Figs. 11 and 12 illustrate x-ray analyses of a reduced mill scale at different temperature (650-950˚C) in hydrogen atmosphere. From which it is clear that the reduction at high temperature more iron is produced and reached to about 100%.

Conclusions
Mill scale is very attractive industrial waste due to very rich in iron (about = 72 % Fe).The pressing pressure load in the briquetting process of mill scale powder increased both the drop damage resistance and crushing strength increased.
The degree of reduction of mill scale briquettes at constant temperature increased as the hydrogen flow rate increased.
The reduction rate of mill scale briquettes under a constant flow rate of hydrogen (as reducing agent) increased with increasing temperature.
The kinetic reduction of mill scale briquettes show that the reduction process is controlled by diffusion process and the energy of activation is ranged between (61.5-72.25KJ/mol).