Although the hydrothermal reaction can be achieved at atmospheric pressure, it is preferred to employ pressure greater than atmospheric, i.e.,<a href="http://www.sellercrusher.com/solutions/crushed-stone-machine-supplier.html">crushed stone machine supplier</a> up to pressures approaching the capacity of the conventional pressure vessel, in order to decrease the curing time and to improve the strength of the resultant hardened agglomerates. Generally, economic considerations dictate that the upper limit of pressure used should be about 500 psig. The pressure preferably is in the range of about 150 to about 350 psig.
It can be appreciated that the retention time of the agglomerates in the reaction chamber depend upon several process variables, such as pressure, temperature and atmosphere of the chamber, composition of the agglomerates, etc. In any case, this time must be sufficient to obtain hardening and bonding of the individual particles of the agglomerates into a hardened,<a href="http://www.sellercrusher.com/solutions/crushed-rock-equipment-for-sale.html">crushed rock equipment for sale</a> high strength condition. When higher temperatures and pressures are used, the time for the hydrothermal curing step is generally in the range of about 5 minutes to about 8 hours, preferably about 30 to 60 minutes.
The hardened agglomerates are removed from the reaction chamber, and upon cooling are ready for use. When removed from the reaction chamber, the hot, hardened agglomerates usually contain approximately 0.5 to 1.5 weight percent free moisture. Although the agglomerates have superior strength characteristics upon cooling, it has been found that the compressive strength of the hardened agglomerates can be increased significantly by rapidly drying them, immediately after removal from the reaction chamber and before appreciable cooling has occurred, to remove at least a substantial portion of the moisture remaining in the agglomerates. This drying can be accomplished in any convenient manner, such as in an oven or by blowing warmed air over the agglomerates. Generally, a temperature of about 100 to about 350°C, preferably about 150° to about 250°C, can be used for this drying step.