Study On The Hydration Of Coal-Measured Kaolinite Surfaces In Aqueous Solutions

One of the vital reasons why it is difficult to achieve sedimentation and clarification of the high marlaceous coal slurry is that there are a lot of fine clay mineral particles in the slurry. Thick hydration layers will be formed on the surfaces of clay mineral particles in the aqueous solutions to increase the repulsive hydration force between two particles because of the large number of hydrophilic groups of surface. The conventional technology of aggregation based on the DLVO theory cannot make these fine particles agglomerate and settle effectively. Thus, to study the hydration mechanism and look for ways to crack the hydration layers on clay mineral particle surfaces are the important parts of high efficiency method of high marlaceous coal slurry treatment. In consideration of kaolinite being a kind of main mineral in high marlaceous coal slurry, in this work the surface physicochemical characteristics, and effects of solution chemical properties and surface charging characteristics on hydration of kaolinite particles have been researched based on the test analysis, theoretical calculations and computer simulations methods. In addition the hydration mechanism of kaolinite particles has been analyzed and the hydration model has been established according to molecular dynamic simulation. This work will provide a theoretical support for cracking hydration layer on the surface of kaolinite particles and high efficiency clarification treatment of high marlaceous coal slurry.The research results of the physicochemical characteristics of kaolinite surface and the interaction between particle surface and coal slurry show that kaolinite particle surface contains a lot of hydroxyl groups such as> SiOH and> A1OH. These hydroxy groups will produce protonation at low pH solutions resulting in the increasing of solution pH and the numbers of surface hydroxyl groups, on the contrary produce deprotonation at high pH solutions resulting in the reducing of solution pH and the increasing of the numbers of> SiO and> A1O groups. In alkaline slurry, kaolinite can reduce the solution pH value, and the other elements in the structure of kaolinite particles and adsorbed on surface can dissolve into the solutions, thereby increasing the concentration of the solution metal cations. The presence of metal cations in the solutions can restrain the protonation of the kaolinite particle surfaces, and promote the deprotonation of the kaolinite particle surfaces inversely. The research results of kaolinite surface charging characteristics shows that the IEP and PZNPC of kaolinite are around pH3.00and pH5.65respectively. All the surfaces of kaolinite are negatively charged and theζpotential of entire kaolinite particle is around-60mV. The K+and Na+ionsaffect the kaolinite electrokinetic properties by compressing the electrical double layer of the particles and weakening the protonation or strengthening the deprotonation of the kaolinite particle surfaces. The Mg2+and Ca2+ionsreduce the absoluteζpotential of kaolinite particles by compressing the electrical double layer of the particles and having specific adsorption on the kaolinite particle surfaces. Once the ion concentration of Mg2+and Ca2+increases to0.1mol/L, theζpotential of entire kaolinite particle becomes around zero and keeps steady in a lager range of solution pH. The aluminiumcompound inorganic salts can decrease the deprotonation of the kaolinite particles by reducing the solution pH caused by the hydrolysis of Al3+ions, resulting in theζpotential of kaolinite particles decreasing. Additionally, on account of the low IEP of clay mineral particlesand high IEP ofAl(OH)3(S)particles, once the A1(OH)3(S)precipitate onto the particle surface, the IEP of particles with A1(OH)3(S) on surfacewill increase to approach the pH of coal surry and reduce theζpotential of the entire particles. The smaller the size of kaolinite particle is, the larger the acid-base response coefficientα value is, and the more hydroxyl groups there are on the particle surface, and the stronger the protonation/deprotonation capability of particles is.The research results of the hydration mechanistic of kaolinite particle surfaces prove that the hydration capability of kaolinite particle surfaces is stronger than that of ions in solutions. The K+and Na+ions can make the shear plane in the electric double layer moves by changing the free water content in the diffuse electric double layer and affecting the hydration capacity between water molecules and kaolinite particle surfaces, resulting in the changing of the thickness of hydration layers on the particle surfaces. The Mg2+and Ca2+ionscan make the shear plane move to the particle surfaces and reduce the thickness of hydration layer by having specific adsorption on the kaolinite particle surfaces which can affect the surfaces hydration capacity. In the Al3+ions solution the A1(OH)3(S) will deposit on the kaolinite particle surfaces and increase the size of kaolinite particles greatly. The molecular dynamics simulation shows that the water molecules adsorb on the surface of kaolinite particles to form a dense, firmhydration layer in two types. The water molecules are similar to the "hole water" in type A, and the water molecules are similar to the "connect water" in type B. Type A is the major adsorption form of water molecules on the surface of kaolinite surfaces.