Precipitation/Crystallization Capabilities

Historically, the method used to remove soluble metals and select other constituents from wastewater was through addition of an alkali and precipitation. While the solids produced contained substantial waters of hydration and were difficult to dewater, this approach did remove the contaminants of concern from the wastewater stream. Over the past decades, proprietary technologies have been developed to improve on this approach. These methods typically involved a means or method to create more crystalline solids from the wastewater constituents. EnCOSS personnel have the unique benefit of skills in crystallization and precipitation that result from technology transfer from the inventors of the most advanced and successful technologies in this area. John Schubert was with the original licensee of the HDS (High Density Sludge) process developed by Bethlehem Steel, when Bethlehem transferred the technology for outside use in the late 1980s. John was also with the company that licensed the HARDTAC technology from DuPont, and was one of a few individuals who received the technology transfer for that process. John has applied both of these technologies in a variety of applications.

To understand the difference in performance of the various precipitation/ crystallization technologies available, one indicator of the benefit of these approaches is the particle size distribution of the solids that result from the process. Consider the three particle size distributions shown below:

The first figure shows a particle size distribution from a conventional precipitation process. This distribution reflects a broad range of particle sizes with poor uniformity. Due to the large proportion of particles below 1 micron in size, and the wide distribution of solids, this material would not be expected to dewater particularly well. In comparison, the next figure illustrates the product of a High Density Sludge or HDS type of process.

In the HDS process, the precipitation reaction occurs on the surface of recycle particles, which produces an “onion-skin” effect in which waters of hydration are minimized and some crystallization can take place. The end result is the particle size distribution shown. The median particle size in this distribution is about 5 microns, but the important factors are the uniformity of the solids and the lack of a significant portion of the solids present below 1 micron in particle size. (In comparison, the median particle size for the conventional precipitation process shown above was about 3 microns.) Note that the particle size distributions shown herein are not for the same feed wastewater, so direct comparison of results from one process to the next is not valid, although the principals illustrated by the distribution plots are valid.

A final particle size distribution is shown to the right from a crystallization process, known as HARDTAC. This process produces true crystalline solids through use of a crystallization reactor and design methods appropriate to that. The size distribution shown in this curve reflects a median particle size of about 40 microns. Note that other wastewater applications of this technology have produced median particle sizes closer to those from the HDS process, but with improved particle size distribution characteristics for dewatering.

Using principals that are applicable in both of these technologies, EnCOSS is able to offer modular and custom designed processes that produce the optimal result in terms of dense precipitates or crystalline solids, using only the level of technology necessary to provide the optimal return on investment. Areas of expertise include precipitation of dense precipitates/crystalline solids for a variety of metals and acid-solublizied components of wastewater streams, byproduct recovery from spent acid streams and regeneration of spent potassium hydroxide/potassium fluoride streams for reuse. EnCOSS personnel have extensive experience in providing these technologies to solve complex problems in the metals, inorganic chemicals and mining industries.

EnCOSS is capable of providing precipitation/crystallization systems as part of facilities designed, built, owned and operated by EnCOSS, or as facilities engineered by EnCOSS, with critical equipment provided by EnCOSS, for a customer to own and operate. EnCOSS is capable of providing operational support at a variety of levels depending on customer needs. EnCOSS has extensive experience in retrofitting these processes into existing facilities, and improving the performance of existing facilities through process optimization. For more information, contact the EnCOSS office closest to your location.