How to improve the whiteness of titanium dioxide?

In the various indicators of titanium dioxide, whiteness is one of the more important indicators, because of the high whiteness of titanium dioxide, so called the "King of White", but the whiteness of titanium dioxide is not fixed, but according to the production method and process parameters change, of course, the higher the whiteness is the better. So, what is the whiteness of titanium dioxide? What are the factors? How to improve the whiteness of titanium dioxide? Powder net small editor after combing the existing research data to make the following finishing.
01
definition of whiteness
White is a special color attribute with a high light reflection ratio and low color saturation, which is used to determine the degree to which a reflected object can appear white based on

. White is located in a fairly narrow range in the color space, they are the same as other colors, can be expressed in three-dimensional quantities (i.e., light reflection ratio Y, purity C, main wavelength), but people are used to different white objects in a one-dimensional whiteness (W) order to quantitatively evaluate its whiteness, and the ideal white (such as high purity sulfuric acid, its whiteness value of 100%) as a parameter standard. Whether it is visual assessment of whiteness, or the use of instruments to assess whiteness, must be based on the recognized "standard" basis. Therefore,
so-called whiteness refers to the degree of whiteness from the ideal.

whiteness measurement
whiteness is calculated and measured in a variety of ways, so it is necessary to explain what calculation method is used when comparing whiteness, because the whiteness values obtained by different calculation methods are completely different. There are many instruments on the market for measuring whiteness, and the measurement principles of different instruments may vary, resulting in different results when measuring with different instruments for the same sample. At present, the instruments used to measure whiteness have a great influence on whiteness due to their light source model and angle of view, so the choice of test method has a considerable influence on whiteness data.
a new generation of WSD-3C fully automatic whiteness meter
03
whiteness factors
(1) impurities.
in the titanium dioxide process, especially the titanium dioxide process of sulfate method, most of the work is to remove impurities in the product, because impurities seriously affect the application performance of titanium dioxide, especially whiteness. Different production plants, the use of different sources of raw materials, titanium dioxide contains impurities in the type and quantity of great differences. In addition to a very small part of these impurities from the corrosion and wear of equipment, mainly from the current domestic sulfuric acid method titanium dioxide production plant widely used titanium-containing raw materials - titanium iron ore.
these impurities themselves are colored and are easily colored in white titanium dioxide
. Compared with titanium dioxide by chloride, titanium dioxide by sulfate is of poor purity and high impurity content.
(2) the crystal form of titanium dioxide.
Goldstone crystal type of titanium dioxide is more likely to absorb short-wave light than titanium type, if in the sharp titanium type of titanium white powder mixed with too many gold-redstone crystals, titanium dioxide to light absorption coefficient K value will increase, the whiteness and gloss of the finished product will be worse, and the goldstone crystal type is more sensitive to impurities, when harmful impurities reach a certain content, mixed with gold-redstone crystals of titanium-type titanium dioxide will also produce a sharp titanium-color phenomenon. Therefore,
for sharp titanium-type titanium white powder, the purer the crystalline whiteness, the better.
(3) particle size and particle size distribution.
particle size and particle size distribution are the main factors affecting the whiteness of titanium dioxide, which is mainly influenced by the reflection and scattering of light by titanium dioxide particles. According to the existing measurement data,
in a certain particle size range, the smaller the particle size of titanium dioxide, the higher the whiteness value of
, which is mainly due to the smaller the particle size of titanium dioxide, the larger the surface area, light reflection, diffuse reflection enhancement. However, when the particle size is less than 0.1 microns, light diffraction occurs, and titanium dioxide crystals become transparent, which also affects whiteness. Therefore
in the finished titanium white powder to minimize particle size greater than 0.35 microns and particle size less than 0.1 micron particles.
the relationship between average particle size and whiteness
Source: classification of whiteness of titanium dioxide and its influence factors (Guo Huiliang, etc.)
(4) particle shape.
particle shape refers to the presence of titanium dioxide particles, which can be divided into spline, rod and layer. According to the available measurement data, the whiteness of layered titanium white powder is slightly lower, the spter and rod whiteness is very close, therefore, although the shape of titanium white powder particles can slightly affect whiteness, but not the main factor.
photo of titanium dioxide particles
Source: classification of titanium whiteness and its factors (Guo Huiliang, etc.)
(5) titanium content.
Titanium content refers to the total content of titanium dioxide in titanium dioxide white powder, for pigment grade titanium dioxide white powder in the content of titanium dioxide is generally more than 90%, specifically 92% to 98%, other substances are mainly coating agents and a small number of impurities. According to the available measurement data,
with the increase of titanium dioxide content, the whiteness value also increased.
the relationship between titanium dioxide content and whiteness
Source: classification of whiteness of titanium dioxide and its factors (Guo Huiliang, etc.)
(6) various additives.
In production, many additives, such as filters, lubricants, etc., may contain long carbon chains of organic compounds, carbonized at high temperatures, so that titanium white pink phase becomes dull, or even with a distinct red-brown.
(7) surface treatment.
to improve the performance of titanium dioxide, many manufacturers of titanium white initial products surface treatment, that is, coating. The choice of coating agent and the order and time of coating have some influence on the whiteness of titanium dioxide. In the wet coating, the viscosity of the system has a greater effect on the coating effect, the adhesion is not conducive to the dispersion of the coating agent, and the whiteness of titanium white powder is reduced.
04
Method to improve whiteness
According to the production experience analysis, when the whiteness of titanium dioxide increases, its cover, coloration index will be correspondingly increased, therefore, to achieve the same effect of the use of less titanium dioxide, the cost will be reduced. There are some main points about the method of increasing whiteness in production.
(1) preferably the source of the mine.
raw materials are the first step in production, only qualified raw materials to produce qualified products, titanium dioxide for impurities content is extremely sensitive, sometimes to 1×10
-6
, therefore,
to choose a good source is the top priority
. Iron, manganese and molybons are important impurities whose contents need to be rigorously tested and precisely controlled.
(2) matching equipment and operations.
there are some titanium dioxide factory, in order to save investment because of the simple, save some processes, the result of a large number of impurities into titanium white powder, seriously affecting the whiteness of the product.
(3) controls the quality of the titanium solution.
if the indicators of titanium liquid are not qualified, it will have adverse effects on the hydrolysalization of the next link, so that the size, shape and particle size distribution of titanium dioxide particles is not suitable, the particle lattice has certain defects, resulting in a reduction in product whiteness. The preparation of titanium liquid and the purification of titanium liquid should be strictly controlled, and the excess titanium liquid should not be supplied to the next process.
(4) hydrolysing process optimization.
is an important process in the production process of titanium dioxide by sulfuric acid, and also an important factor affecting the whiteness of titanium dioxide. Usually the uniformity of pressurized hydrolysal products is poor, the washing speed is slow, the finished product whiteness is poor, and
normal pressure hydrolysaling can overcome this disadvantage, so that the particle shape is uniform, whiteness rises.
(5) washing process is optimized.
washing is the key link to remove Fe
2 plus
from the solution, if not removed in time, it will be oxidized to Fe
3 plus
, whether it is iron or tri-iron, will make titanium white powder whiteness reduced. Monitoring should be strengthened to ensure that the ≤ pH is 1.5 and the ≥ is 0.06Mpa.
(6) calcination condition.
calcination is the process of dehydration, desulfurization, crystal transformation, crystalline growth, and the whiteness of the finished product is directly related, do not under-burn or over-burn. Generally in front of the rotary kiln set up a combustion chamber, to prevent high temperature flame direct contact with the material, and control the temperature, speed and calcination time, according to experience to control the parameters.
(7) sanding conditions.
is the first job in the post-processing phase, the task is to break the material for later coating treatment. When the material is too thick, the coating is uneven, too fine will lead to a decrease in the finished product cover force, and waste grinding resources, therefore, the average particle size of the general control in 0.3 to 0.6 m.
(8) coating process.
coating mode, coating amount, coating uniformity, etc. can affect the whiteness of the product, usually, phosphorus coating, aluminum, tantalum will make the whiteness have an upward trend, while the coating silicon will make the whiteness unchanged or even decreased, and when the coating is uneven will also seriously affect the whiteness of the finished product.
(9) gas powder operating conditions.
Airflow crushing uses the self-grinding effect of the material, cutting titanium dioxide particles with medium pressure steam, so that the material and materials, materials and the inner wall between the strong collision and achieve the purpose of fine shredding, so that titanium dioxide particles reach the range of the finished product particle size requirements. After air flow crushing, the particle size of titanium dioxide is reduced and the particle size distribution is improved. According to experience, the finished particle size is generally crushed to 0.2 to 0.35 m. In operation, the main control steam pressure and feed speed, if the finished particle size is large, can be increased steam pressure, improve the feed speed to adjust.
Tration
to improve the whiteness of the product titanium dioxide powder, theoretically mainly control the size, shape and particle size distribution of titanium dioxide particles, impurities of the type and content, combined with the actual production situation, the site mainly control the indicators of titanium liquid, Hydrolysis operation, calcination operation, etc., so that the finished titanium white powder particle size in about 0.2 m, smooth and uniform shape, narrow particle size distribution, product impurities type and content are minimized to improve the whiteness of finished titanium powder.