Water in ores and minerals often create problems in transportation and mineral processing. As a consequence, determination of water in those materials is of significant importance. In this regard, classification and subsequent estimation of various forms of water present in iron ore warrants a special mention.
Water in iron ore can be classified in three parts. Some of the water is attached by physisorption based on various Van der Waals forces and essentially removed around 110℃. This water loss is attributed to the moisture content of the ore. Another part is chemisorbed to the ore by means of hydrogen bonding and driven off around 150-350℃. The third and the final part which is essentially the water of crystallization (Combined water/CW) present in goethitic phase require 350-500℃ temperature for complete removal. Current practice involves the measurement of moisture around 1100C and the presence of other types of water is often overlooked. This can cause serious problem in ore processing as water can be detrimental in high temperature metallurgical processes. Therefore, it is important to have a measure of total water content in iron ore and identify different types of waters present in the ore in a single analysis.
Presence of goethite in iron ore can be identified and quantified using X-ray powder diffraction. In this work presented herein, two iron ores are subjected to thermogravimetric (TG) analysis. Water loss in the aforementioned temperature regions are calculated and presented in table 1. To further verify the results, the same two samples are subjected to X-ray powder diffraction and the amount of goethite is estimated from the powder data. The water content of the goethite phase is then calculated and compared with the TG data. The results of both the experiments are tabulated in table 1.
The samples were characterized by TG analysis in a Perkin Elmer STA 6000 instrument. The experiments were carried out within the temperature range 30-600℃ with a heating rate of 10℃/min. Powder X ray diffraction patterns were obtained from Bruker D2 Phaser diffractometer. Phase quantification was performed by Rietveld refinement using TOPAZ software package. Iron ores used for the purpose are obtained from commercial sources.
TG analysis and Powder X ray diffraction:
As evidenced from the TG curves, sample A shows a loss around 0.177% upto 110℃, another 0.177% in the temperature range 150-350℃ and the third and the final weight loss of 0.137% in the temperature range 350-500℃. Sample B showed a similar pattern with weight losses of 0.213%, 0.022% and 0.058% respectively. To further corroborate these observations the samples were subjected to powder X ray diffraction and amount of goethite phase was estimated. Amount of water present in goethite (FeO(OH)) was calculated theoretically and it was found to be in excellent agreement with the weight loss observed in TG curves in the temperature range 350-500℃. As suggested by literature survey, quantification of low concentrations of phases is always an issue in PXRD which prompted us to explore the option of TG analysis to determine the water of crystallization in suitable ores and minerals. The results are tabulated in table 1.
Table 1: Results obtained from TG analyzer and PXRD
|Sample Name||% Of CW (TGA)||% Of Goethite (PXRD)||% Of CW (PXRD)||%Moisture||% chemisorbed water|
PXRD pattern of Sample A and B
In the present work, thermogravimetry has been successfully employed to analyze moisture, chemisorbed water and water of crystallization (combined water) in iron ore. The method is accurate, repeatable and does not require manual expertise. It is also observed that the method can overcome the deficiency of PXRD in determining phases present in low concentration. Albeit having limited repeatability in determining very low concentration of phases, PXRD still remains the principal tool to resolve the presence of different phases which share an overlapping water loss region in the thermogram. As a consequence, choice of the ore based on the mineralogical characteristics is of utmost importance to apply the technique of TG analysis in determination of water content and the method requires significant screening before analysis.
CONTRIBUTED BY ALAKTA SAHA, DR. SATIRTHA SENGUPTA & DR. ARIJIT GOSWAMI UNDER THE GUIDANCE OF PROF. BARUN KUMAR GUPTA