Kaoli­n­ite. Gen­e­sis, prop­er­ties and appli­ca­tions

Kaolins are a group of clay min­er­als that are of great impor­tance in both the nat­ur­al sci­ences and var­i­ous indus­tri­al sec­tors. Their unique chem­i­cal and phys­i­cal prop­er­ties make them indis­pens­able for many appli­ca­tions, includ­ing ceram­ics, paper, cos­met­ics, phar­ma­ceu­ti­cals and elec­tron­ics. In this arti­cle, we will look at the ori­gin, prop­er­ties and main uses of kaolin.

Kaolins are includ­ed in the list of min­er­als of nation­al impor­tance approved by the Res­o­lu­tion of the Cab­i­net of Min­is­ters of Ukraine No. 827 dat­ed 12 Decem­ber 1994 as refrac­to­ry raw mate­ri­als and raw mate­ri­als for glass and porce­lain and faience.

Kaolin is named after the Kau-ling Hill in Chi­na, from which it has been mined since the third cen­tu­ry. Sam­ples of kaolin were first sent to Europe by a French Jesuit mis­sion­ary around 1700 as sam­ples of mate­ri­als used by the Chi­nese in porce­lain pro­duc­tion.

In its nat­ur­al state, kaolin is a white, soft pow­der con­sist­ing main­ly of the min­er­al kaoli­n­ite, or oth­er min­er­als of sim­i­lar com­po­si­tion (dikite, nakryt), which were formed as a result of exoge­nous or hydrother­mal kaolin­i­sa­tion of alu­mi­nosil­i­cates. In addi­tion, nat­u­ral­ly occur­ring kaolin usu­al­ly con­tains vary­ing amounts of oth­er min­er­als such as mus­covite, quartz, feldspar and anatase. In addi­tion, raw kaolin is often coloured yel­low by iron hydrox­ide pig­ments. It is often nec­es­sary to chem­i­cal­ly bleach the clay to remove the iron pig­ment and wash it with water to remove the oth­er min­er­als to pre­pare kaolin for com­mer­cial use.

Mineral composition

The rock-form­ing min­er­al is kaoli­n­ite. Its con­tent in pri­ma­ry kaolins in Ukrain­ian deposits varies con­sid­er­ably, usu­al­ly reach­ing 55–60% of the rock mass, and 30–40% in alka­line kaolins. In kaolins formed from albitites, labradorites, amphi­bo­lites and pyrox­enes, the kaoli­n­ite con­tent increas­es dra­mat­i­cal­ly due to the absence of quartz and is 90% or more.

The qual­i­ty of the min­er­al is reg­u­lat­ed by DSTU B V.2.7–60-97 Clay raw mate­ri­als for the pro­duc­tion of ceram­ic build­ing mate­ri­als.

Chemical properties

For­mu­la Al2Si2O5(OH)4
Com­mon impu­ri­ties Fe, Mg, Na, K, Ti, Ca, H2O

Physical properties

• Colour: vari­able: from white to cream and pale yel­low and even brown or brown.
• The colour of the dash: white;
• Hard­ness: 2 — 2½;
• Lus­tre: waxy, pearly, dull, earthy;
• Adhe­sion: very per­fect;
• Trans­paren­cy: trans­par­ent, opaque;
• Den­si­ty: 2.63 g/sm3;
• Frac­ture: uneven/irregular, con­choidal, sub­con­choidal, mica

Also char­ac­ter­is­tic of kaolins:

• Small par­ti­cle size, typ­i­cal­ly less than 2 microns, which increas­es the sur­face area per unit weight, mak­ing the rocks effec­tive for adsorp­tion and ion exchange.
• Plas­tic­i­ty. When kaolin is mixed with water in the range of 20 to 35 per cent, it becomes duc­tile (i.e. it can be mould­ed under pres­sure) and the shape is retained after the pres­sure is removed.
• Absorp­tion and des­orp­tion. Kaolin min­er­als have the abil­i­ty to absorb and retain water mol­e­cules, as well as adsorb oth­er mol­e­cules such as organ­ic com­pounds, heavy met­als and pol­lu­tants.
• Swelling. Some types of clay kaolin have the abil­i­ty to swell when hydrat­ed, which can be use­ful in a vari­ety of appli­ca­tions such as drilling flu­ids.

Geological and industrial types of deposits

Depend­ing on their ori­gin, kaolin deposits can be pri­ma­ry or sec­ondary.

Pri­ma­ry kaolin is found at the place of for­ma­tion. It is the end prod­uct of weath­er­ing of pale­ofeldspar rocks, igneous and meta­mor­phic crys­talline rocks of acidic and medi­um com­po­si­tion, as well as basic rocks. Pri­ma­ry kaolin is often of high puri­ty, white­ness and low amount of impu­ri­ties. This type of kaolin typ­i­cal­ly has large crys­tals that can be eas­i­ly iden­ti­fied with a micro­scope.

Sec­ondary kaolin is also called ‘clay kaolin’. It is formed as a result of geo­log­i­cal process­es of ero­sion and scour­ing of rocks such as gran­ite by water and oth­er fac­tors. Sec­ondary kaolin can be found in riverbeds, sed­i­men­ta­ry basins and oth­er places with large water flows. Sec­ondary kaolin is often of low­er puri­ty and white­ness than pri­ma­ry kaolin. Its crys­tals may be small­er and less even­ly dis­trib­uted, and it may con­tain more impu­ri­ties.

The largest deposits of kaolin in Ukraine

The world’s kaolin reserves exceed 10 bil­lion tonnes. Ukraine is one of the world’s lead­ing coun­tries in terms of explored reserves. Our coun­try is home to one of the world’s largest kaolin provinces, the Ukrain­ian Shield, where most pri­ma­ry and sec­ondary kaolin deposits are locat­ed. There are also known deposits in Zakarpat­tia and on the south­west­ern slope of the Voronezh crys­tal mas­sif (Sumy region).

Pri­ma­ry kaolin deposits include Hlukhovetske (devel­oped since 1905), Prosyanivske (oper­at­ed since 1894), and Velyko­hadomynetske (dis­cov­ered in 1969).

Sec­ondary kaolin deposits are Volodymyrske (in oper­a­tion since the late 19th cen­tu­ry), Novose­lytske (opened in the ear­ly 20th cen­tu­ry) and Polozske.

Application of kaolin

Due to its unique phys­i­cal and chem­i­cal prop­er­ties, kaolin has a wide range of appli­ca­tions:
1. in the pro­duc­tion of ceram­ic mate­ri­als such as porce­lain, earth­en­ware, ceram­ic tiles, san­i­tary ware, etc.
2. as coat­ing mate­ri­als and pig­ments to improve the qual­i­ty of paper.
3. in the pro­duc­tion of cos­met­ics for the man­u­fac­ture of face masks or soap (known as ‘white clay’).
4. in the man­u­fac­ture of rub­ber mate­ri­als to improve their mechan­i­cal prop­er­ties.
5. in micro­elec­tron­ic devices due to their dielec­tric prop­er­ties.
6. in the drilling of oil and gas wells to reduce drilling time and reduce min­er­al loss­es.
7. in water purifi­ca­tion and fil­tra­tion

Kaolins are also used in the chem­i­cal and food indus­tries, for the pro­duc­tion of linoleum, pen­cils, and glass­ware. Almost 50% of the world’s kaolin is used by the paper indus­try, 20%  is used for rub­ber pro­duc­tion. 5–10% is used to make ceram­ic prod­ucts.

Extraction and processing

Kaolin is extract­ed by open pit min­ing. Depend­ing on the geo­log­i­cal con­di­tions, the extrac­tion process may vary, but the gen­er­al process includes:

• Strip­ping — removal of top­soil and rocks;
• Extrac­tion of the min­er­al itself — kaolin is extract­ed using exca­va­tors, bull­doz­ers or oth­er spe­cialised equip­ment;
• Trans­porta­tion and stor­age;
• Pro­cess­ing — crush­ing, sep­a­ra­tion of kaolin from oth­er rocks and min­er­als, clean­ing from impu­ri­ties and con­t­a­m­i­nants.

Thus, the study of kaolin has revealed the ver­sa­til­i­ty of this nat­ur­al min­er­al and its impor­tance in var­i­ous indus­tri­al sec­tors. An in-depth analy­sis of the ori­gin, geo­chem­istry, min­er­al­o­gy and phys­i­cal prop­er­ties of kaolin has allowed us to bet­ter under­stand its for­ma­tion, struc­ture and appli­ca­tion pos­si­bil­i­ties.

The ori­gin of kaolin reflects the rich­ness of geo­log­i­cal process­es and the evo­lu­tion of the Earth­’s crust. Its prop­er­ties and chem­i­cal com­po­si­tion allow it to be used in var­i­ous indus­tries. From the ceram­ic indus­try to cos­met­ics and elec­tron­ics, kaolins make a sig­nif­i­cant con­tri­bu­tion to the vari­ety of prod­ucts we use in our dai­ly lives.

Final­ly, the study of kaolin high­lights the impor­tance of study­ing nat­ur­al resources and their opti­mal use.

The grow­ing need for sus­tain­able and envi­ron­men­tal­ly friend­ly mate­ri­als makes kaolins rel­e­vant for research and devel­op­ment of new tech­nolo­gies. Tech­no­log­i­cal advances make it pos­si­ble to study the min­er­al at a deep­er lev­el, which opens up addi­tion­al prospects for their appli­ca­tion.