Kinetics of Reaction of Roasting of Soda Ash and Ilmenite Ore Concentrate for Formation of Sodium Titanates

Alkaline metals and hydrogen titanates are of great interest for possible applications. The soda ash and Rosetta ilmenite ore concentrate briquette were investigated. The kinetic of formation of sodium titanate was studied in the temperature range 700°C to 900°C.


Introduction
Sodium titanates and peroxotitanates are inorganic ion exchangers that exhibit strong affinities towards a wide range of metals [1].Elvington et al. [2] indicated that both sodium titanates and peroxotitanates have shown to be effective for the removal of a wide range of metal ions from neutral and acidic solutions.Hobbs [1] indicated that sodium titanates exhibit good affinity for adsorption of noble metal, mercury and cadmium compound from water.Sodium titanates and peroxotitanates may find use in treating patients with toxic levels of metals, as therapeutic agents for treating cancer and bactericides for world treatment and in dentalmaterials.Walker and Schmitz [3] used monosodium titanates for the removal of strontium and plutonium from high-level waste solution.Alkaline metals and hydrogen titanates are also of great interest for possible applications such as photocatalysts as fuel cell electrolytes or cation exchangers in the treatment of radioactive liquid waste [4].Sodium titanate glycolate complex [4] was prepared by heating at temperature 198°C for 6 hours and the sodium titanate glycolate precursor decomposed on heating at 900°Cto form nanorods sodium titanates (Na 2 Ti 6 O 13 ) which has good photocatalytic activity for the decomposition of 4-chlorophenol in an aqueous slurry under UV radiation.Nanostructured alkali titanates are versatile materials that possess a variety of physical and chemical properties that can be tailored allowing their use in many application including catalysis and lithium ion batteries [5].
It was reported [6] that the alkali roasting of titaniferrous materials offered several advantages including the zero process wastes compared to the conventional process for beneficiation of TiO 2 ores and that in soda ash roasting the formation of liquid phase is common problem that results in product granulation, formation of rings on the kiln walls and lump formation.
Morsi et al [7] indicated that sintering of ilmenite ore in the presence of certain mole ratio of sodium oxide (2.5) with respect to titanium dioxide followed by roasting at 1000°C, 1100 °C or 1200°C for 120, 60 or 30 minutes respectively is feasible for complete titanium dioxide recovery.The formation of the higher titanates Na 8 Ti 5 O 14 and Na 6 Ti 2 O 7 during the sintering process favors the formation of lower titanates such as Na 2 TiO 4 during subsequent roasting process, which is easily soluble in dilute sulfuric acid.The contaminated soluble iron compound in the leaching can easily separated by a simple technique.
Safiulin and Belyaev [8] prepared TiO 2 with 92.6% purity from ilmenite using the method of alkali decomposition where the ore was heated with Na 2 CO 3 .Other authors [9] indicated that soda ash converts titanium, iron and all the impurities to their respective sodium compounds.The sodium compounds of iron and impurities are water-soluble and can separate during the water leaching stage.In addition, they indicated that the soda ash roasting of ilmenite takes place in two steps, the first step is the conversion of ilmenite to sodium titanate and sodium iron titanate, while in the second step the sodium iron titanate rearranges to form sodium ferrite and titanium dioxide.The rate of reaction described by Ginstling-Brounshtein model, which proves that the reaction is diffusion controlled, and the activation energy for the overall reaction found to be 130 kJ/mol.
Lasheen [10] studied the process of soda ash roasting of titana slag product from Rosetta ilmenite, and reported that the optimum conditions consisted of using a Na 2 CO 3 /slag ratio of 0.55:1 and a roasting temperature of 850°C for 0.5 hour duration period.In the present investigation, the kinetic reactions associated with roasting of soda ash and ilmenite ore concentrate are evaluated.

2.Experimental work 2.1. Materials 2.1.1. Ilmenite ore
A representative sample of Rosetta ilmenite concentrate was provided from the Nuclear Materials Authority (NMA) titanium project.The composition of Rosetta ilmenite ore concentrate contains about 43.6% TiO 2 , 27.5% FeO and 20.9% Fe 2 O 3 .X-ray diffraction analysis of Rosetta ilmenite ore concentrate is shown in Fig. 1, which indicated that the phases composing Rosetta ore concentrate mainly consist of ilmenite Fe 2 TiO 3 , Pseudo-rutile Fe 2 Ti 3 O 9 , hematite Fe 2 O 3 and a minor quantity of rutile according to the work of El Hossaini et al [11] and Hala et al [12].

Soda ash
Sodium carbonate anhydrous (Soda ash, analar grade) was provided from ADWIC CO.

Experimental procedures 2.2.1 Preparation and physical properties of the briquettes
The ilmenite concentrate was ground in a vibrating mill to a size less than 75 micrometers.The ground ilmenite concentrate powder was mixed with different percentages of soda ash and then pressed under certain loads.The briquettes were then subjected to drop number and crushing strength tests.The drop number indicates how often briquette can be dropped from a height of 46 cm before they show perceptible cracks or crumble.Ten briquettes were individually dropped onto 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.Ten briquettes were compressed between parallel steel plates up to failure to determine the average crushing strength [13].

Roasting process
Roasting of the produced briquettes was performed in a thermo balance apparatus shown in Fig. 2 [14].It consists of a vertical furnace, an electronic balance for monitoring the weight change of reacting sample and a temperature controller.The sample is 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 700°C to 900 °C and maintained constant to ± 5°C.Then the samples were placed in hot zone.
Samples were continuously withdrawn from the furnace at specific times and put in a desiccator to determine the percentage of titanium oxide converted to sodium titanates.The percentage of titanium oxide converted to sodium titanates was calculated according to the following equation:

Effect of the amount of soda ash mixed under constant pressure load on the quality of the briquettes
Fig.s (5 & 6) illustrate the effect of changing the amount of soda ash under constant pressure load 216.7 MPa on the quality of the produced briquette.It is evident that the increase of soda ash improves the physical properties of the briquettes.This may be attributed to the fineness of soda ash, which subsequently decreases porosity.

Effect of amount of soda ash added to ilmenite on the degree of titanium oxide conversion to sodium titanates
Fig. 7 illustrates the effect of changing the amount of soda ash added to the ilmenite on the degree of conversion of titanium oxide to sodium titanates at 900 °C (Pressure load on the briquette 216.7 MPa.).From this Fig., it is clear that the degree of conversion increased as the amount of soda ash increased.This may be attributed that the increase of soda ash leads to an increase the amount of sodium oxide than the amount of sodium oxide, which consumed in the side reaction such as the reaction with Fe 2 O 3 , Cr 2 O 3 , ….. etc. Fig. 7. Effect of change the amount of soda ash on the degree of titanium oxide converted to sodium titanates (pressure load on the briquette 216.7 MPa., time of roasting 80 min, and temperature 900 °C).

Effect of pressing load on the degree of titanium oxide conversion to sodium titanates
Experiments were performed on briquettes which contain 35% soda ash roasted at 900°C for 80 min, and the results are illustrated in Fig. 8, from which it is evident that as the load increased the conversion of titanium oxide to sodium titanates increased within the investigation range.This may due to that an increase of load leads to an increase in the contact between soda ash and titanium oxide.Fig. 8. Effect of the change of pressure load on the degree of titanium oxide converted to sodium titanates (amount of soda ash 35% of the ilmenite and temperature of roasting 900°C).

Effect of the roasting temperature on the degree of conversion of titanium oxide to sodium titanates
Experiments were performed on briquettes which contain 35% soda ash pressed at load 260 MPa., in the temperature range of 700°C to 900 °C in air.Plots of the conversion percentage as a function of time are shown in Fig. 9.It is clear that the conversion rates increased with increasing temperature.The analysis of the curves relating the roasting percentage and time of roasting shows that each curve has three different slopes indicating three different conversion rates.The first rate is high, while the second is somewhat slower and the third is slowest one.The increase of conversion percentage with rise of temperature may be due to the increase of number of reacting moles having excess energy, which leads to the increase of conversion rate [18].In addition, the raise of temperature leads to an increase of the rate of mass transfer of the diffusion and rat of desorption [18 -21].

Conclusions
‫-‬ The pressing pressure load in the briquetting process of ilmenite and soda ash powder increased both the drop damage resistance and crushing strength.‫-‬ The degree of titanium oxide converted to sodium titanate at constant temperature increased as the soda ash increased.‫-‬ The rate of titanium oxide converted to sodium titanate when a constant amount of soda ash (35%) of ilmenite used increased with increasing temperature.‫-‬ The kinetic roasting ilmenite with (35%) soda ash briquettes show that the reaction process is controlled by diffusion process and the energy of activation is (135 kJ/mol).

Fig.s 3
Fig.s 3 & 4 show the relation between the change of pressure load (MPa) at constant amount of soda ash (35%) on the drop number (drop damage resistance) and cold crushing strength of the ilmenite briquettes.From these Fig.s, it is clear that as the pressing pressure load increased both the drop damage resistance and crushing strength increased.This may be because increase pressure load increases the compaction of briquette and subsequently the Vander Waals forces increased [15&16], also the increase of briquetting pressure leads to progressive crushing of the macro pores [17].

Fig. 3 .
Fig. 3. Relationship between the change of pressure load and drop number of mixture of ilmenite with fixed amount of soda ash (35% of the ilmenite ore concentrate briquette).

Fig. 4 .
Fig. 4. Relationship between the change of pressure load and cold crushing strength (C.S) of mixture of ilmenite with fixed amount of soda ash (35% of the ilmenite ore concentrate briquette).

Fig. 5 .
Fig. 5. Relationship between the change of the amount of soda ash and drop number of the briquette pressed under constant pressure load 216.7 MPa.

Fig. 6 .
Fig. 6.Relationship between the change of the amount of soda ash and cold crushing strength (C.S) of the briquette pressed under constant pressure load 216.7 MPa.

Fig. 9 .
Fig. 9. Effect of change temperature on the conversion degree of titanium oxide.
Two models were used to determine the controlling mechanism[22] 1. Solid state reaction at surface of particles for cylindrical symmetry 1-(1-R) 1/2 = k t 2. Solid diffusion model (Ginsling-Brounshtein) for cylindrical symmetry R + (1 -R).ln(1 -R) = kt Where: (R) is fractional roasting, (t) is time of roasting, and (k) is the rate constant.Plots of the RHS against time for reacted fraction (R) from titanium recovery curves showed no concordance with the first model, where as the diffusion model showed good agreement with experimental data as seen from Fig.10 and11.The values of slopes of f(R) = R + (1 -R).ln(1 -R) against time represent the reaction rate constants k at each temperature.Their logarithms were plotted in an Arrhenius type plot in Fig.…as ln k against 1/T.A straight line could be fairly fitted through the points and the obtained slope E/R = 16.178K, corresponding to an activation energy = 135 kJ/mol.

Fig. 13
Fig. 13 illustrates X-ray analysis of the roasting ilmenite mixed with 35% soda ash at temperature 900 o C From which it is clear that the main phases NaTi 2 O 4 , NaTiO 2 , Na 2 Ti 3 O 7 , Na 8 Ti 5 O 14 and Na 2 Fe 2 Ti 8 O 16 .