1. Field of the Invention
This invention relates to a process for the preparation of micro inorganic foamed granules.
2. Description of Prior Arts
Inorganic foamed granules are lightweight, and have been accordingly used widely as lightweight aggregates for construction of buildings, extenders for fertilizer, detergent or paint, absorbents, soil improvers, etc. Particularly, the micro inorganic foamed granules having a mean particle size of not larger than 500 .mu.m are employed as fillers for imparting heat insulating properties to buildings and adjustors for adjusting specific gravity of cement used for an oil well or geothermal well, in addition to the above usage.
Particularly, the use as adjustor for adjusting specific gravity of cement used for an oil well or geothermal well is one of the quite important uses.
It is required that the micro inorganic foamed granules for employing as specific gravity adjustor should have apparent specific gravity in the range of 0.6 to 0.9 and particle size of not larger than 500 .mu.m (preferably not larger than 200 .mu.m). Further, high strength is required, and the micro inorganic foamed granules having hydrostatic pressure strength of not lower than 300 kgf/cm.sup.2 at 70% of indestructibility are generally employed for the above purpose.
The known micro inorganic foamed granules are classified into three groups, that is, granules manufactured from natural volcanic glassy materials, those manufactured from artificial glassy materials and those contained in coal ash.
The micro inorganic foamed granules of natural volcanic glassy materials are produced by the steps of pulverizing materials such as obsidian, pearlite and volcanic ash to make the particle size not larger than 100 .mu.m, removing a portion of fine powder, adding thereto additives such as SiO.sub.2 powder and Al.sub.2 O.sub.3 powder in an appropriate amount and firing the mixture at a temperature not lower than 1,000.degree. C.
The micro inorganic foamed granules of natural volcanic glassy materials prepared in the above method generally has a low apparent specific gravity and can be manufactured at relatively low cost. However, hydrostatic pressure strength thereof is extremely low. Therefore, the micro inorganic foamed granules of natural volcanic glassy materials cannot be employed as adjustor for adjusting the specific gravity of cement used for an oil well or geothermal well. The foamed granules are used exclusively as heat insulating materials or lightweight aggregates for construction of buildings, which do not require mechanical strength.
The micro inorganic foamed granules of artificaial glassy materials are produced by steps of adding a foaming compound such as carbon to artificially prepared glassy materials, fusing the mixture and spraying.
The above micro inorganic foamed granules are easily manufactured so as to have the particle size of not larger than 100 .mu.m. The apparent specific gravity and hydrostatic pressure strength of the obtained micro inorganic foamed granules are satisfactory enough to employ as adjustors for adjusting specific gravity of cement used for an oil well or geothermal well. However, manufacturing cost thereof is high.
Inorganic foamed granules contained in coal ash in a very small amount are known. The inorganic foamed granules show favorable properties such as 0.6 to 0.7 of apparent specific gravity and 400 kgf/cm.sup.2 of hydrostatic pressure strength (measured at 70% of indestructibility) for employment as adjustor for adjusting specific gravity of cement used for an oil well or geothermal well. However, the foamed granules are generally contained in coal ash in the amount of not more than 0.5 wt.%. For this reason, the foamed granules are not collected industrially in Japan, which makes the granules very expensive.