The World of Mechanochemistry

 

 

Mechanochemistry studies the impact of mechanical treatment on the physico-chemical properties and on chemical reactions of substances.

Some facts on mechanochemistry

The founder of the modern physical chemistry V.F. Ostwald suggested to introduce the term “Mechanochemistry” in the way analogous to the “Photochemistry” and “Electrochemistry” terms. These terms reflect the causal dependence of the chemical reaction on the way it is initiated.

During the prehistoric period people used mechanochemical phenomenon when they struck fire from a flint. In the XVII-XVIIIth centuries, flintlock guns and pistols were used.

Mechanochemistry

Flintlock dueling pistols

Chemical transformations caused by friction are sometimes assigned to an independent branch of mechanocemistry called tribochemistry. Chemistry of ultrasound and chemistry of shock waves are also considered to be the branches of mechanochemistry.

Mechanochemistry

The model of particle
collision process. Heinecke, 1984.

During the treatment of powders in mills, disintegrators and other analogous milling equipment mechanical energy is transferred to the treated particles. As a result, comminution (particle size reduction) occurs, and new particle surface is formed. Nevertheless it is not the only result of the mechanical treatment. One can also observe:

Crystal deformation
Formation of many defects;
Shear stresses;
Reduction of crystallite sizes;
Crystallites aggregation;
Heat release;
Local rise of temperature and pressure;
Emission of light and electrons;
Phase transitions;
Amorphisation;
Chemical bonds breaking;
Acceleration of diffusion processes;
Generation of centres of high activity
on the newly formed surfaces.

All these phenomena can be qualified as mechanical activation effects.

The activity of solid bodies during deformation, friction, or destruction
is caused by the occurence of the vibrationally and electronically excited states
of interatomic bonds, mechanically stressed and disrupted bonds,
including free radicals, coordinately unsaturated atoms, various structural defects, as well as by the ionization of the substance particles, and the stabilization of electrically charged centres.

Mechanical energy consumption can initiate decomposition of substances (including destruction of polymers), polymorphic transformations, heterogeneous reactions of solids with gases and liquids, solid-phase synthesis in powder mixtures, and other reactions. Mechanical energy intake also takes part in mechanical wear of frictional and working tools surfaces during mechanical treatment processes, in destruction of constructional materials, working at static or dynamic loads in active media, for example, in the corrosion of strained metal.

Mechanically activated substances possess high reactivity, which is important for many practical applications:
sintering of mechanically activated powders occurs at tens and hundreds degrees lower temperatures compared to the non-activated substances;
it is possible to carry out mechanochemical reactions directly in a milling apparatus (for instance, in a planetary mill), and the solid-phase mechanochemical synthesis is possible;
the solubility and the speed of dissolution of solids in liquids grow;
the increase of catalysts’ activity is also possible.

One of the important issues in the investigation of physico-chemical properties of mechanically activated compounds is exploration of the factors leading to reactivity increase.

Application of mechanical activation

Activation by milling as a new way to speed up physico-chemical processes is used more widely nowadays. It has already overgrown the scopes of laboratory investigations and is used to speed up technological processes or as a method to change the technological parameters of the treatment regimes of mineral raw materials.

The activation of mineral substances by milling is successfully used in coal technology, hydrometallurgical processes intensification, in the production of fertilizers, constructing materials, composite mixtures, etc. It creates a new perspective for the recycling of mineral raw materials stored in rock waste disposal areas, for the improvement of complex and far-sighted use of mineral resources, and also for lowering the environmental damage caused by industry.

Activation by milling has promising perspectives in the processes of leaching, extraction, selective and overall dissolution of various substances. The elimination of the process limiting stages allows the manifold acceleration of the transfer of solid components into the dissolved state. The energy consumption by activation processes is compensated by the time saved and by the improved recovery of the dissolved components.

Another promising trend of using the activation by milling is the preparation of composite mixtures. Composite mixtures are widely used in various sectors of industry. They are made ready as a batch before pyroprocesses; used in the preparation of moulding powder; used for the preparation of solid solutions to serve as catalysts, or other agents; ceramics industry works on the basis of composite mixtures; they are also used during the preparation of moulding sands, fluxing agents; used for surface-covering of electrodes; used for press-forming of cermet parts and adhesive compositions.

Activation by milling should find various applications in multipurpose use of mineral resources and lowering the damage made by the industrial by-products to the environment. In particular, the applications can include the following: industrial waste utilization and rock waste disposal area elimination; waste water purification accompanied by valuable (and harmful) components trapping on the activated surface; upgrading of peat, coal, and oil shale before combustion with simultaneous recovery of metals, sulfur, and other valuable components; replacement of the sulfide and arsenic-containing concentrates roasting for the roasting-free process based on the mechanical activation.

Most probably, activation by milling will be further developed as a new way in chemical synthesis of inorganic compounds.

Besides, activation by milling can form the basis of principally new technological processes, where a secondary operation can become the primary one.

Nowadays, high-energy density milling equipment of industrial scale (planetary mills) is commercially available. Therefore there is every reason for wide implementation of the achievements of mechanochemistry.

Conferences


There are the following international conferences on mechanochemistry:

INCOME

Conferences
ON MECHANOCHEMISTRY AND MECHANICAL ALLOYING
2003 Braunshweig, Germany
2006 Novosibirsk
2008 India
The next conference is planned to be held on August 31 – September 3, 2011 in Herceg Novi, Montenegro.
ISMANAM
International Symposium
on Metastable and Nano Materials
2006 Warsaw, Poland
2008 Buenos-Aires, Argentina
The next, 18th International Symposium on Metastable, Amorphous, and Nanostructured Materials (ISMANAM 2011) is planned to be held on June 26 - July 1, 2011 in Gijon, Spain

Mechanochemists

Senior and adult generation of mechanochemists: Doctor of Chemistry, Professor, Honored Scientist of Russian Federation Evgeniy G. Avvakumov and Doctor of Chemistry Alexander M. Kalinkin. Braunschweig, Germany, 2003.

The present text is an introduction into the world of mechanochemistry. The following literature was used:


On-line chemical encyclopaedia http://www.xumuk.ru/encyklopedia/2606.html


The new handbook of chemist and production engineer. Processes and apparatuses in chemical technologies. L.F. Bilenko. (In Russian)

http://chemanalytica.com/book/novyy_spravochnik_khimika_i_tekhnologa/09_protsessy_i_apparaty_khimicheskikh_tekhnologiy _chast_I/5252#8.5.

Avvakumov E.G. “Mechanical methods for activation of chemical processes”. Novosibirsk, “Nauka” publishing house. 1986. P. 305. (In Russian)

Boldyrev V.V. “Experimental methods in mechanochemistry of solid inorganic substances”. Novosibirsk, “Nauka” publishing house. 1983. P. 64. (In Russian)

Butyagin P.Yu. in “Progress in Chemistry”, 1984, vol. 53, issue 11, pp. 1769-89. (In Russian)

Chajkina M.V. “Mechanochemistry of natural and synthetic apatites”. Novosibirsk, the publishing house of the Russian Academy of Science Siberian Branch. 2002. P. 203. (In Russian)

G. Heinecke, Tribochemistry. Akademie-Verlag, Berlin, 1984.

One can find some other references on the page «Publications».

 

 
Active-nano (Andrey V. Petrov)