**3.2 Stability of MO-NFs**

*Advances in Microfluidics and Nanofluids*

**2.3 Alternative synthesis methods**

naturally occurring ores.

*2.3.1 Biosynthesis of MOs NPs*

*2.3.2 Plasma assisted synthesis*

attractive for MO-NPs synthesis.

ore, composite materials can be obtained [35, 36].

**3. Preparation and stability of metal oxide nanofluid**

*2.3.3 Extraction from ores*

particles.

the mixture.

**3.1 Preparation**

Most of the methods of synthesis of nanomaterials allow a great flexibility in the choice of the precursors and the morphology of the material through the control of key parameters such as temperature, concentration, or pH. However, they also required the use of toxic chemical reactants or additives. In this regard, alternative synthesis routes have been exploited, with mitigated results, including biosynthesis, plasma assisted. Extraction from ores as for it fits in the strategy of valuation of

This synthetic approach includes the use of vegetal substances (plant extracts or agricultural wastes) but can also involve living organisms such as bacteria and fungi. Natural extracts used for the preparation of the MO materials are available and renewable, in addition to contain organic groups that can react with the metal

The plasma state is obtained from the ionization of noble gases, which increases the reactivity of the medium rich in electrons and chemical species upon reaching a very high temperature, up to 10000K [29]. Non-thermal plasma is an advantageous alternative that can be operated at atmospheric pressure [30]. Among the different types of systems used for plasma production, gliding arc discharge is reported to

Under humid air as feeding gas, glidarc discharge produces acidic and very oxidizing environment in which many transitions metal ions precipitate as hydroxides or oxides [31–33]. However, one of the major disadvantages associated to this technique is the poor control on the size range and morphology of the

Mineral ores are naturally occurring rock, from which the metal content can be extracted via hydrometallurgy [34]. During MO extraction, the ore is successively crushed, roasted, leached, and precipitated. The leaching step consists in the solubilization of the metal species under its ionic forms by the action of a strong acid or a strong base. Usually, impurities remain insoluble and are removed from

The metal ions remaining in solution are then concentrated by solvent (water) evaporation before further treatment for the precipitation. When the mineral contains one major species, single MO particles are produced, but for more complex

Preparation of NFs is the most challenging step, as far as the experimental studies with NFs are concerned. This is so because nanofluids need special requirements

ions is solution in the absence of further additives [27, 28].

**134**
