Quartz sand for high-end transparent quartz glass

The main quality indexes of transparent quartz glass are purity and bubbles, which require quartz sand to have very low impurity and gas content. The practical experience of quartz raw material purification and melting shows that the main raw material characteristics affecting the quality of quartz glass are the types and contents of impurity elements, gas content and particle size distribution of quartz sand.
1. Types and contents of impurity elements in quartz sand
There are some residual impurities in quartz sand, including Al, K, Na, Li, CA, Mg, Fe, Mn, Cu, Cr, Ni, B, Ge, etc.
Table 1. Impurity element requirements of quartz sand for quartz glass

Application	Origin	Main impurity elements and content/ppm
		Al	Fe	Ti	Ca	B	K	Na	Li
Electric light source	China	Total 80-100ppm
Chemical industry	Czech	42	3.0	3.0	8.0	-	18.0	5.0	-
semiconductor	China	20	0.18	0.5	0.1	0.1	0.2	4.0	1.8
semiconductor	USA	15	0.3	-	0.4	0.1	0.7	0.9	0.7
semiconductor	Russia	14	0.9	0.4	0.1	-	0.3	0.6	-
Fiber optic tube	Brazil	11	1.8	0.4	0.1	-	0.3	0.4	-
Optical mirror	USA	8	0.05	-	0.7	0.04	0.05	0.05	0.2

Impurity of aluminum element
Aluminum is the main impurity element of quartz sand, and it is difficult to replace Al3+ of Si4+ in the crystal lattice. A small amount of aluminum will not affect the appearance quality of quartz glass, Aluminum is the main impurity element of quartz sand, and it is difficult to replace Al3 + of si4 + in the crystal lattice. A small amount of aluminum will not affect the appearance quality of quartz glass, but after Al3+ replaces si4+, equilibrium charge ions(Na+、Li+  etc.) will be introduced. In addition, under irradiation of X or γ,  Al3+and Ge3+ will produce color centers which will harm the irradiation resistance of quartz glass.
Alkali metal impurities
At high temperature (> 1000 ℃), alkali metal impurities can induce silica crystal precipitation from quartz glass, which has a great influence on the thermal stability of quartz products.
At the same time, alkali metal impurities will reduce the melting point and softening point of quartz glass, and seriously deteriorate its performance. Quartz glass devices used in electronic industry, such as quartz crucible, diffusion furnace tube, quartz galvanic tube, bracket, etc., require high purity and high temperature resistance, and non-metallic impurities will diffuse into electronic components, so the alkali metal impurity content in quartz glass should be reduced as far as possible.
 
Table2.Raw material requirements for continuous melting transparent quartz glass tube
 

Index	Grade of quartz sand
	Super	Class A	Class B	Class C
Total impurities(ppm)	<30	<50	<100	<150
Impurity particles(Grains / Kg)	Nothing	<5	<10	<20
Moisture (%)	0.3	0.3	0.5	0.5
Glass grade	A&B grade	Above C grade >40%	Above D grade >70%	Above D grade >40%

Transition metal impurities
The content of transition metals (Cr, Cu, Fe, Mn, Ni, Zn, etc.) also has a serious influence on the high purity quartz glass. If trace metals dissolve or dissolve into the silicon melt, its conductivity will be destroyed; And most transition metals will absorb ultraviolet, visible and far-infrared light, which will damage the optical properties of quartz glass. For optical fiber, the micro inhomogeneity caused by impurities will increase the loss of optical fiber, and even lead to signal distortion.
Boron impurity
For some applications of quartz glass, such as CZ crucible, low boron content with good consistency is required. The content of boron is directly related to the geology of the deposit. This is because boron ions enter into the silicate framework structure and produce strong chemical bonds, rather than being contained in boron bearing minerals in isolation.
2.Gas content of quartz sand
The fluid inclusion in quartz ore will become the gas in glass melt during melting. Due to the high viscosity of SiO2 melt, it is not easy to remove the wrapped gas. Instead, it grows into bubbles and produces gas lines when drawing pipes or rods, which affects the quality of quartz glass. With the increase of melting temperature, the melt viscosity can be reduced and the gas can be eliminated, but with the increase of SiO2 volatilization, the material yield will be reduced and the energy consumption will be increased.
The process of quartz sand and the melting process of quartz glass, such as resistance furnace material, and even furnace drawing, have exhaust process. Some gas-liquid inclusions are cracked before the quartz material melts and the gas is discharged; Some of the other gases (such as small molecular gases he and H2) will not be discharged immediately, but the diffusion coefficient in the high temperature solid or melt at 800-1400 ℃ is large and can be permeable. In addition, the gas with small diffusion coefficient such as water, oxygen, nitrogen, HCl, SO2, H2S can be removed by prolonging the exhaust time. Co, CO2, CxHy and other gases are very difficult to discharge from solid or melt, and their content affects the transparency of quartz glass.
3. Grain size distribution of quartz sand
The size distribution of quartz sand affects the bubble number and melting process parameters of quartz glass. Because of the difference of surface activation energy, small particles melt first than large ones, and the fused quartz will cover the surface of the fused quartz particles, which makes the bubbles impossible to be eliminated. The gas content of small particle gap is much, which increases the difficulty of exhaust. Small particles are also easy to volatilize and waste raw materials; Coarse quartz is difficult to melt completely, forming the particle structure in glass. The actual size of raw materials depends on the melting process.
Table3. Grain size of quartz sand used in quartz glass melting process

Process	Resistance furnace	Continuous melting furnace	Hydrogen oxygen gas refining	Plasma
Size/ mesh	40-80	60-120/80-140	80-200	120-200

The best appearance of quartz sand is spherical. The needle shaped raw materials cause poor feeding, forming a shed phenomenon, leading to the formation of bubbles in the melting process. Due to the diversity of Genesis and mineral characteristics, when selecting quartz glass raw materials, quartz production enterprises should investigate the inclusion characteristics of quartz ore, select quartz raw materials with small amount of inclusions or easy to explode, and determine the processing technology of quartz powder according to the occurrence state of harmful impurity elements in quartz ore. So as to build a suitable sand washing plant.