Ultra-fine calcined kaolin is a porous structural material of high whiteness, high dispersibility and have good coating properties,plasticity, fire resistance,chemical stability and electrical insulating properties,which is widely used in petrochemical, ceramics, paints, coatings , cosmetics , rubber , pharmaceutical and other industries , and in the electronics, aerospace and other high-tech fields also continue to be applied, the paper industry is the biggest user .
The paper-making industry take kaolin as white pigment to instead of the expensive titanium dioxide.Currently in the international arena,calcined kaolin has become a standardized paper-making raw material, is used as paper coatings and functional filler.It can be slightly reduced in the whiteness of the paper and optical properties,to instead the existing processes 50-60% Tio2.As their substitutes,are bound to have strict requirements.The coating grade kaolin , must have high brightness, high dispersion, pigments and other chemical additives have good mutual solubility , high resolution optical coefficient , low density, low wear , and other fine-grained and relatively inexpensive properties.Furthermore, in terms of abrasion resistance, product type , pore volume , and other rheological properties of the product also has a good performance requirements to ensure that the paper quality and printing results.The main parameter determining kaolin as a filler or coating pigments in papermaking comprising include whiteness, the particle size distribution , rheological , chemical inertness and wear resistance.Ultra-fine calcined kaolin can improve the whiteness of the paper , opacity, opacity and printability bulk and can improve the ink absorption , offset , reducing print mottle , improved fidelity offset.In the United States , from its use in the field of high-grade securities extended to ordinary newsprint paper , photographic paper and paper dictionary .
High whiteness is one of the essential feature in quality paper kaolin.The common whitening & remove impurities methods include:Dithionite method,acid hydrogen reduction method,flotation method,acid treatment,and calcination,etc.
Calcination:the kaolin after through 600-1000℃ calcined,the whiteness is improved.
The reason is generated a polymer melt,also increase the light scattering gap,and the kaolin product calcined under１０５０℃,could obtain good oil absorption and Large surface area. The fixed carbon content in kaolin is about ２％,exist in Kaolinite crystals gap,to make the kaolin appearing gray or white-gray.This kaolin is often used roasting method in addition to high-temperature oxidation of carbon to improve its whiteness.But iron kaolin minerals, such as pyrite (FeS), siderite (FeCO3) and limonite (Fe2O3 · 3H20), firing at high temperatures will be transformed into Fe2O3, resulting in raw yellow or brick-red , so the baking process need further removal of iron and titanium.When calcined at 550 ~ 700 ℃ kaolin , under the effect of adding NaCl, S02, SiO2, O2 and other components , the decomposition in C12, the participation of the fixed carbon kaolin , iron and titanium oxides are converted to low melting point, high volatility FeC13 ( boiling point 315 ℃) and TiCl, ( boiling point 136 ℃), while the carbon is converted to CO and C02, so that the carbon , iron , titanium, and kaolin separation.
Firing mechanism:about 110 ℃, various adsorbed discharged at about 500 ℃,kaolin OH group in a state of vapor escape;at 650 ℃,the complete decarboxylation into metakaolin .
The reaction equation is:Al2O3·2SiO2·2H2O→Al2O3·2SiO2+2H2O↑
When the temperature rose to about 980℃, metakaolin decomposed, and a large number of small Al-Si spinel crystalline mullite weak , amorphous SiO2. When the temperature rose to 1250 ℃, the mullite into crystalline .
From the above reaction formula,we can know that the kaolin removal of the water of crystallization and formed metakaolin at about 700℃，and began to produce alumina spinel when 95O℃,higher temperatures the formation of large mullite,also free the cristobalite which is impact the paper abrasion.So for the paper &coating industry applies , the medium temperature (800 ℃ ~ 1000 ℃) calcined products have good whiteness , good cover , light scattering is high, the appropriate void volume and bulk , the pressure can be reduced when the light loss of height and opacity to minimize mullite phase appeared to suit the requirements of high-speed paper coating .