Haematite, reduction

The reaction mechanism for the solid state reduction is the same as that described above for the hydrogen reduction of haematite, namely the formation of a porous iron product which results from the penetration of pores in the reacting pellets by reducing gases, and the migration of the reaction products, C02 and H20 through these pores back into the gaseous phase. 

In order to analyse the packed bed process, it is necessary to consider both heat transfer from the solid to the gas and reaction heat which may be transmitted to the gas. The composition of the gas, and hence its physical properties, are determined by the rate of reduction, which in turn depends on each layer of the packed bed, and on the degree of reduction which has already occurred. In the reduction of haematite, there are three stages in the reduction, corresponding to the formation of Fe304 and FeO before the metal is formed. The thermal data for the reduction processes can be approximated by the respective heats of reduction by H2 and CO gases. Taking 1000°C as a typical mean temperature, the mean value for the heats of reaction per 2 gram-atom of iron are... 

The temperature at which the reduction by the hydrogen takes place varies with the different oxides and also with the same oxide, depending on its physical condition. " Crystalline haematite, as the naturaJ ferric oxide is called, requires to be at a red heat (about 500° C.) before reduction begins to tadce place, while if iron is precipitated from one of its salts (as ferric hydrate by... 

Diffraction patterns can be used to identify the various phases in a catalyst. An example is given in Fig. 10.3b, where XRD is used to follow the reduction of alumina-supported iron oxide at 675 K as a function of time. The initially present oc-Fe2C>3 (haematite) is partially reduced to metallic iron, with Fe3C>4 (magnetite) as the intermediate. The diffraction lines from platinum are due to the sample holder [10]. 

It is common practice to add the etheramines, which are partly neutralised (5-30%) with acetic acid, direct to the mineral slurry. As in all flotation operations, the collector is sensitive to slime in iron ore applications, and in particular for haematite, a lot of fine material is produced during the grinding process. The presence of slime will increase the collector consumption drastically, and give poor silica reduction and decrease the iron recovery. Therefore, a de-sliming stage is often used prior to flotation. In this, the iron oxide particles are depressed with starch or dextrin alkalized com starch is also commonly used. The depressant is added in a conditioner formulation before the collector is added. The pH varies between 8 to 10.5 in haematite flotation, with 10.5 being the common pH used, while magnetite flotation often takes place at pH levels of 8 to 10. 

For stoichiometric reasons, the hydrogen consumption for the reduction of haematite into wiistite is only about half of the hydrogen used for reduction of the wiistite into metallic iron. As a consequence of these thermod5mamic and stoichiometric restraints, only roughly 50% of the hydrogen (1/3+1/2- l/3=l/2) introduced at the bottom of the shaft furnace is used for the reduction process, whereas the rest is found -diluted with steam - in the top gas [495]. 

Let s see how this interpretation applies to the other reactions 1 have mentioned. You can probably see that the reduction of iron oxide fits in well the Fe ions of haematite (recall Figure 4.1) accept electrons and are turned into Fe atoms they are reduced. The oxide ions of the ore surrender their electrons (two of them from each 0" ion) and so, according to the new definition, are reduced to O atoms. That these O atoms attach to CO molecules is interesting, and shortly 1 shall invite you to think about it further, but for the moment there is a reduction (of Fe to Fe) accompanied by an oxidation (of 0" "to O). 

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