Brief introduction to Olivine:
Olivine sand is a naturally occurring potential industrial mineral consisting of mainly magnesium iron orthrosilicate (Mg1-x Fex)2SiO4, which is a continuous solid solution between Forsterite (Mg-end member: Mg2SiO4) and Fayalite (Fe-end member: Fe2SiO4). Potentiality of olivine sand strictly depends on the Mg to Fe ratio. Olivine is a major component in earth forming rocks and is found with varying compositions due to alteration by weathering. It is therefore important to get a thorough understanding of the various structural modifications prior to its applications in different fields.
Occurrence: It occurs as a primary mineral in certain metamorphic rocks in Norway, Greenland, Turkey 
etc. In India this mineral is available in Salem district of Karnataka and Keonjhar district of Orissa. Metamorphism of impure dolomite or other sedimentary rocks with high magnesium and low silica content also produces Mg-rich olivine. Olivine of Norway origin has the highest MgO and least iron content, which is utmost desirable for various refractory applications. Since long this mineral is denominated as a refractory material, but its catalytic activity is still unraveled.
Insight view of Olivine crystal:
Olivine belongs to orthorhombic system with space group Pbnm. It has a spinel-like structure similar to magnetite (Fe3O4) but uses one tetravalent and two divalent cations (M22+ M4+O4) instead of two trivalent and one divalent cations. Here, the atomic structure can be described as a hexagonal, close-packed array of oxygen ions with half of the octahedral sites occupied with magnesium or iron ions and one-eighth of the tetrahedral sites occupied by silicon ions.
Physicochemical properties and advantages:
- No (or very less) free silica content.
- Slightly basic in nature, used to sequester CO2 for enhanced weathering by cost effective technique.
- No tendency to hydration, hence no special attention is required for storage.
- Low heat conductivity (0.031 cal/cm/sec/oC) and good insulating characteristics.
- High refractoriness (fusion point 1700 -1760oC) due to high content of forsterite (m.p 1890oC), which also imparts resistance towards reaction with molten metals and slag resulting in longer life.
- Inert and has high chemical and mineralogical stability (due to strong forsterite mineralogic bonding), easily applicable for high temperature processes.
- Specific gravity 3.26-3.4 and bulk density 1.5-2.0 g./cc .
- Does not react with alkaline binders
- Good compatibility with refractory materials and can work well in conjunction with other refractory materials.
- Low thermal expansion of 0.8% at 1000oC resulting in resistance to thermal shock.
- High volume heat capacity (specific heat between 20 and 1000oC is 0.23 to 0.3 cal/gm/cc), useful as a heat storage material.
- High abrasion resistance (Moh’s scale hardness 6.5-7) and thermally stable, hence can be used in fluidized bed reactor for steam gasification process.
- Favorable price due to wide availability.
Conventional applications:
- Since long olivine has been used as a refractory material. It is used to make refractory brick, block and used as a casting sand, as a ballast for concrete platforms.
- As a Tap Hole Filler in steel melting furnaces.
- Olivine sand is sometimes used as an alternative to silica, primarily by foundries that cast high-alloy steels. It possesses several advantages compared with silica, the most important being a lower and more uniform thermal expansion and inertness toward certain difficult alloys, manganese steels.
- Aluminium foundry industry uses olivine sand to cast objects in aluminium.
- As a mineral plastering of building
- As a fertilizer in enriching MgO & Iron in Soil.
- As a mould coat in steel foundries and manganese steel foundries.
- Most olivine is used in metallurgical processes as a slag conditioner. High-magnesium olivine (forsterite) is added to blast furnaces to facilitate slag formation and to reduce emission of CO2.
- It is also used as a substitute for dolomite in steel works.
Disadvantages:
- Olivine particles must be crushed to a very small size (≤1 μm), or else particles rapidly sink out of the surface mixed layer before being dissolved.
- Olivine is used almost solely with alkaline binders and this leads to low sand reclamation rates with conventional equipment.
- The refractoriness of olivine is related to its iron content which for steel casting should not be greater than 8% and for certain goods like large carbon steel castings, maximum 6% is desired.
Current research trends:
Literature survey evidenced that, olivine has enough strength against attrition in a fluidized bed reactor, and iron content makes it a suitable in situ catalyst for oil gasification processes from biomass, which is an attractive renewable source of fuel and energy in this energy crisis period. Hence, olivine can also be used as an inert bed material in fluidized bed reactor for biomass gasification alternative to silica sand and bauxite.
Our objective:
- Thorough investigation of the sample composition, physical/chemical parameters, quantification of the constituent elements either by classical analysis and/or by instrumental analysis like XRF, ICP-OES, TG/DSC, FTIR, XRD etc.
- Since in the structural matrix of olivine, different phases of iron, metallic iron (Feo), FeO, Fe2O3, Fe3O4 and MgFe2O4, can be present /arise /deplete under different reaction conditions; hence it is an urgent need to understand that how iron behaves as a catalyst in fluidized bed reactor. Therefore, proper pre-treatment of olivine is required to provide the catalytically active phase of iron, mimicking those in a gasifier.
- Depending on the analytical results project the samples for proper applications.
Thermogravimetric and Differential Scanning Calorimetric analysis:
Key Findings:
- Mass loss of < 0.2% in the first region (from ambient to 110oC) corresponds to the loss of adsorbed water. Indicates least tendency towards hydration.
- Mass loss of ~ 3.6% in the second region (from 110oC to 750oC) corresponds to the serpentine (Mg3Si2O5(OH)4) dehydroxylation. An endothermic peak at 650oC also confirms this phenomenon. Serpentine forms from the metamorphism of olivine due to water incorporation into its crystal structure.
- The exothermic peak at ~825oC attributed to recrystalization of olivine and the formation of hematite phase of iron oxide from the olivine matrix.
Analysis of X-Ray Diffraction Pattern:
Key Findings:
- Here XRD patterns mainly indicate the peak appears or disappears on calcination of olivine at 950o
- It can be observed that normal olivine contains no prudent iron oxide phase but minor amount of serpentine phase which is eliminated upon heat treatment.
- Calcination at 950oC pulls off the hematite phase of iron oxide from the sample bulk along with the prominent appearance of major crystalline phases of olivine (shown in figure below).
Plausible reactions on calcination of Olivine:
Catalytic activity:
Olivine is able to exert its catalytic property when it is calcined at ≥ 800oC (at which maximum amount of syngas is thermodynamically achievable) in the gasification reactor. At that condition, gases such as H2 and CO, reduce these iron oxide to metallic iron (Feo) and thereby able to transport oxygen to the gasification zone. This oxygen-transport lowers the necessary amount of steam required for the gasification and increases tar degradation as well as fuel conversion.
Plausible reaction at gasification reactor: Fe2 O 3 + CO+ 2H2 →2Fe(0) + CO 2 + 2H2 O
Therefore a thorough understanding of olivine’s behavior in oxidizing-reducing condition is crucial for control of its catalytic properties in fluidized bed reactor.
X-ray Fluorescence analysis of different Olivine samples:
Observation: Well resolved calibration curves (R2 0.999) as obtained from XRF analysis using pressed bead and fused bead both, indicate the concomitance of result that were obtained by classical analysis .
Conclusions:
- In conclusion, it can be inferred that the refractoriness of olivine is strictly related to its iron content (6-8%) which makes it most desirable candidate in refractory and steel industries.
- On the other hand, catalysis activity of olivine is also dependent on its iron content since particular phases of iron influence its catalytic property in fluidized bed reactor for biomass to oil gasification process. Therefore, precise characterization with proper pre-treatment is highly required prior to its application in different fields.
- Phase transition, thermal decomposition, adsorption/desorption phenomenon can be well enquired using TG-DSC analysis.
- Identification of different phases appears/disappears under heat treatment or ambient condition can be well resolved by X-ray diffraction pattern and temperature programmed reduction technique.
- The composition (%) of different olivine samples (treated or untreated) can be investigated thoroughly by wet chemical method and instrumental techniques like XRF and ICP analysis.
- Finally, after thorough characterization olivine samples can be projected towards catalytic applications for biomass gasification process as an alternative energy source beyond its application in refractory and steel foundries.
CONTRIBUTED BY DR. SOUMITA MUKHOPADHYAY AND DR. ARIJIT GOSWAMI UNDER THE GUIDANCE OF PROF. BARUN KUMAR GUPTA