The Particle Growth In Carbonation Of Sodium Aluminate Solution

The carbonation of sodium aluminate solution is the key process to control the chemical purities and the physical properties of the products in alumina production by sintering process.The quality of the alumina produced by Sintering process in our country at present does not reach the standard of sandy alumina,it is the bottleneck that how to obtain the product which can meet the sandy requirement in carbonation decomposition process.This paper profoundly investigated the agglomeration kenetics and the control of physical properties of products in carbonation process from the intrinsic factors of carbonation process. The conclusions were as follows.1) The agglomeration of fine grains(＜20μm) is playing a dominant role when the primary particle number is big,while the agglomeration of coarse particle(20～45μm) also occurs significantly with relatively small primary number.During the late period of carbonation,the agglomeration hardly occur with low amount of precipitation.2) The mass relative supersaturation was first put forward,the driving force of Al(OH)₃ precipitation is not only proportional to that of the absolute supersaturation but also in inverse proportion to the solid content,the decrease rate(-dN_i/dt) of the particles number of Al(OH)₃ in the system was used to characterize the agglomeration rate of carbonation decomposition.3) The model of particle agglomeration rate is deriveded -dn_i/dt=k_i dδ/dt, that is,the change rate of every particle size region of Al(OH)₃ is proportional to that of mass relative supersaturation.4) Because the three processes(nucleation,agglomeration and crystal growth) are difficult to match,under laboratory conditions,the carbonation product with good physical properties can not be obtained without seeded additions. 5) The particle size of carbonation product increases at first,then decreases by increasing the seed coefficient,and reaches the maximum when the seed coefficient is about 0.32.The particle size of carbonation product increases along with increasing the particle size of seeds.6) Under fine seeds addition,no matter how the amount of seeds and the decomposition rate change,the agglomeration took place inconspicuous between particles,and the product can not accord with the sand requirement.7) In the course seeds addition,the seed coefficient is about 0.3,temperature＞90℃,using the slow stirring speed,higher supersaturation should be maintained to provide the enough binder in the earlier stage of precipitation.By significantly decreasing the ventilation rate of CO₂ in late period,the crystal particle can continuously grow with low supersaturation. Thus well distributed product of Al(OH)₃ with coarse size can be gained(d(0.5) is 75.57μm,-45μm＜2.5%,+150μm＜0.02%).