Environmental issues impact every aspect of industrial development and production today. Mining, although removed from many of the harsher environmental standards and costs felt by other industries since the advent of RCRA legislation, has found itself under increasing scrutiny by state and federal regulators. This is particularly so when the effects of mining migrate beyond property boundaries and present a potentially adverse effect on the environment of its neighbors.
The final plant will incorporate a custom designed co-precipitation process in a modular design equipment format and consist of:
- Heavy Metal Precipitation
- Primary Coagulation
- Polishing Filtration
- Final pH Adjustment
- Sludge Dewatering
STC Process Technologies conducted a process engineering and laboratory evaluation of technologies available to process water to very low microgram per liter standards. Standards that are some of the lowest in the nation. The investigation evaluated:
Conventional Sodium Hydroxide Precipitation
The precipitation process included the study of base hydroxide precipitation combined with co-precipitation and sulfide precipitation options. The data demonstrates that a co-precipitation process using ferrous iron as the co-precipitant would be the most cost effective precipitation process for this waste water source.
As the analytical data base demonstrates, a simple and effective co-precipitation treatment system is fully capable of effectively achieving all permit standards for seep water discharge.
Precipitation pH was maintained between 10 and 11, as predicted by several series of titration curves developed on all major mine seep waters. The Titration curves would also be used for future pH control system design.
Chemical stabilization and solidification is a state-of-the-art hazardous waste treatment technology developed by Silicate Technology Corporation (STC). This solidification/stabilization technology uses proprietary silicate compounds and can be employed in combination or as three separate technologies:
- Fixation and solidification of inorganics (heavy metals) contained in contaminated soils and sludges.
- Fixation and solidification of organics contained in soils and sludges.
- Removal of organics and heavy metals from contaminated water.
Field Pilot Project
In the first stage, the pH is adjusted to approximately 10.0 by lime addition with the concurrent addition of iron salts, which act as a primary coagulant. Lime addition is by means of an air diaphragm pump controlled by a pH monitor. Iron salts are added as a 20% solution by an electronic metering pump which is keyed to the system feed pumps.
In the second stage a polyelectrolyte solution is added to promote floe growth. This solution consists of both polymer and sulfide additions and is introduced by electronic metering pumps keyed to the system feed pumps.
The primary pilot plant goal – which was to establish that the STC Process Technology bench-scale process design technique could be duplicated at full-scale continuous operations – was achieved. A secondary goal was to incorporate any design changes and/or improvements in the full-scale plant that would develop during the course of pilot plant management and evaluation.
The purpose of this extensive treatability test work is to develop a chemically competent, economically optimum and mechanically reliable precipitation process scheme.
The treatment facility will be fully automated for chemical dosage control and influent and effluent flow control. Moreover, the plant will also have an automated control loop that circulates water to equalization basin for a period of time in order to stabilize the process prior to discharge to the environment. The normal out-of-standard plant shut down controls are also incorporated in the control system.
Manpower requirements for the proposed treatment facility will generally require only one employee for less than an eight hours per day, except for periodic preventative maintenance operations.
The system consists of three modules that were factory-built, tested and disconnected for transport. Each module has a transport carriage designed for haulage by a standard semi-tractor. To prevent the need for special overheight load hauling permits, one of the modules utilizes a detachable gooseneck transport structure in addition to the standard fifth wheel connection.
The sand filter features an enhanced gravity flow concept to increase capacity over standard units. The open topped gravity flow feed concept improves filtration by utilization of small carry-over floc particles as a filter aid. This enhancement would have been lost due to impeller shear if the solution were pumped. Another advantage, over pressure filters, is better operator control. Visual observation of the filtration process enhances operator understanding of the functioning of the filters. The filter design also greatly reduces the large backwash water requirement normally associated with gravity sand filters. The filter is divided into four (4) sections. During normal filtration all four (4) sections are filtering. During backwash, clean filtrate from three (3) sections is used to backwash the fourth section. Backwash solution is drawn off above the filter bed in the “dirty water” section, instead of at tank top, to quickly remove solids from the system.