F. López-Mateos

Mechanism and kinetic control of the oxyprecipitation of sulphuric liquors from steel pickling

Abstract Steel pickling liquors are one of the main environmental problems of steel making. Currently, there are several processes for the treatment of sulphuric liquors, although most recover only acid and haematite (α-Fe 2 O 3 ). We propose an oxyprecipitation process for this recovery, allowing several kinds of iron oxide or oxyhydroxide to be obtained. The aim of this paper is to determine the kinetic control and reaction mechanism. The influence of different variables is evaluated and two experimental ranges for the synthesis of goethite (α-FeOOH) and magnetite (Fe 3 O 4 ) are defined. A systematic study is carried out in these experimental intervals, and the results are analysed in order to determine the reaction order and the type of kinetic control. Finally, from these data, morphological and crystallinity studies and new experiments, the reaction mechanism is proposed.

Mathematical model of magnetite synthesis by oxidation of sulfuric pickling liquors from steelmaking

The aim of this paper is to propose a model obtained through experimental design which represents the synthesis of magnetite by oxidation-precipitation of sulfuric liquors from steel pickling. The studied variables are pH (5.5 - 6.5), stirring speed (250 - 1,000 rev * min-1) and flow of air used as oxidizer (5 -10 l * min-1). The model is valid within the experimental ranges and for a temperature of 70 degrees C. The proposed model is validated through statistical methods. The influence of each variable is studied related to the reaction mechanism and the optimal value of each one is determined.

Protocol for the synthesis of Ba-hexaferrites with prefixed coercivities

Abstract The ferrite industry is one of the largest consumers of iron oxides, usually in the form of hematite from the roasting of steel pickling liquors or from mineral sources. In previous papers we have tested an oxidation process for obtaining magnetite and goethite from steel pickling liquors and two mathematical models have been proposed for synthesizing these two iron oxides. In this paper, the characteristics of magnetite and goethite are related with the coercivity of hexaferrites synthesized when the former are used as raw materials. Constant conditions were used for the ceramic method. Using goethite, Ba-hexaferrite is obtained only when the goethite is precipitated at the lowest oxidizer flow. When magnetite is used, a protocol with a statistically validated correlation is proposed for synthesizing hexaferrites with coercive fields between 230 and 450 kA/m. This correlation is valid for an oxidation process carried out at stirring speed of 1000 rev/min and with an air flow between 7.5 and 101/min. A second empirical correlation is proposed for obtaining final coercivities between 450 and 525 kA/m, for which it is necessary to increase the calcination time.

Synthesis of M-type hexaferrites from steel pickling liquors (ID 109)

Abstract The recovery of steel pickling liquors is one of the main environmental aspects that the steelmaking industry must resolve. We propose the synthesis of barium M-type hexaferrite from these liquors as a recovery treatment. Two methods of synthesis have been studied: a variation of the ceramic method, and an oxicoprecipitation process. Products with excellent magnetic properties have been obtained.

Improvement of ceramic method for synthesizing M-type hexaferrites

Abstract The aim of this paper is to improve the standard ceramic method for the synthesis of hexaferrites, studying several alternatives to the usual raw materials of iron and barium oxides and new ways of developing the wet-milling. The research has been focused in the two first stages (the so-called red-milling and the presintering or calcination step) of this process, optimizing their variables. They are type of milling-fluid, temperature and time of calcination, heating rate and BaO/Fe2O3 molar ratio. The optimization variable was the coercive field. The use of iron oxides different than α-Fe2O3 yields hexaferrites with magnetic properties higher than commercial ones. Thus, these hexaferrites are optimum as permanent magnets and can replace other more expensive magnets.

KINETICS AND MECHANISM OF THE OXYPRECIPITATION OF WASTE HYDROCHLORIC PICKLING LIQUORS

Abstract The aim of the research project presented in this paper is to devise a process to treat waste liquors from the pickling of iron or steel with hydrochloric acid in rolling mills, recovering the high iron content in the form of oxides or oxyhydroxides which can later be used in other industrial processes. In addition to reducing water pollution, such a process attaches a new value to this ferrous waste. Chlorinated liquors are oxyprecipitated, leading to the formation of different combinations of iron oxides and oxyhydroxides, with the total removal of Fe+2 and the production of ammonium chloride solutions easily broken down into hydrochloric acid and ammonia. The precipitates have different industrial outlets, making the oxyprecipitation process an attractive solution for the treatment of waste pickling liquors.