Insitu Rare Earth Recovery System

The solution for maximizing flyash value, improving the environment, and achieving national security relative to rare earth supply could be a system which labeled the Insitu Rare Earth Recovery System.

Overview

A number of power plants and many waste to energy plants use two stage scrubbing systems.   Waste to energy plants in Europe sell by product hydrochloric acid, gypsum, and valuable metals while producing a contaminant free ash.

A number of 250 MW two stage scrubber systems operated by Philadelphia Electric produced magnesium sulfite for further processing to sulfur products. These scrubber systems operated from 1980 to 2012.  High efficiency particulate first stage rod deck scrubbers captured the particulate and HCl.  The second stage rod deck scrubbers captured the SO₂ using MgO.

The first stage operated at close to pH 6 but could have operated in the manner of the European waste to energy plants. They recirculate water which quickly captures the HCl and operates at pH 1.  The 30 percent hydrochloric acid slip stream maintains the equilibrium.  Ion exchange resins are used following the initial solids separation to capture the metals. The acid and metals are then sold.

This process could turn out to be a very important step forward for the world wide coal industry.  But let’s just focus on the implications for CCR compliance for coal fired boilers in the USA. A  specific example would be A.B Brown 1 & 2.

A permit for a gas plant was denied so Vectren is considering replacing its double alkali scrubbers with one oxidation scrubber. At the same time it is talking to suppliers about beneficiating flyash as part of its CCR compliance.

Instead of the oxidation scrubbers and upgrade of the precipitators which are far beyond their useful life the system could be replaced by the two stage scrubbing process.

The plant could buy lower cost higher chlorine coals and produce the rare earth’s feed stock, hydrochloric acid and gypsum or ammonium sulfate. It would produce flyash which is free from metal and other toxic contaminants.

The tried and true scheme used in many Chinese and some U.S. power plants is a final wet precipitator. This can be installed in the top of the second absorber or can be stand alone.  An alternative instead of the second absorber would be a dry scrubber system.

Beneficiation of ponded sludge could be accomplished by adding it as a reburn product (similar to the WE Energies Prairie Station) to the boilers.

The concentrated rare earths and other metals could then be shipped elsewhere for further processing.

Costs:  One of the biggest costs encountered by those who are mining flyash landfills is the size reduction of the flyash. In the proposed system the flyash is at its original size from sub-micron to plus 100 microns. So a major cost is eliminated. There is no cost to acquire hydrochloric acid  or to build a mixing system. It can therefore be speculated that if the University of Kentucky and Chinese government determinations that extracting rare earths from coal is economical then this scheme is much more so.

What about the cost of the two stage scrubber? One of the reasons that some power plants have opted for two stage scrubbing in the past was to capture the HCl before it gets mixed with the gypsum. One small pre scrubber is all that is needed for the HCl and particulate capture.  This means the second scrubber can be 316L instead of a more expensive alloy. There are no chlorides to be washed from the gypsum and no worries about FGD wastewater contamination.

Bob McIlvaine was President of Environeering in 1969 when the company was contracted by United Engineers to furnish the two rod deck scrubbers for multiple units at Philadelphia Electric. Environeering was the partner to Combustion Engineering and furnished the marble bed scrubbers for the world’s first commercial limestone FGD scrubber at Union Electric Merrimac. The rod deck scrubber was an improvement which provided efficient absorption but also varying particulate removal efficiency depending  on the velocity and turbulence created  thru the rod decks.

Most of the first FGD scrubbers replaced precipitators and removed particulate down to 0.1 lbs/ mmbtu. This was better than needed or that was being achieved in most precipitators. When the limits were reduced to 0.05 lbs it required  as much as 20” w.g.  scrubber pressure drop to meet the limits without a  precipitator.

Environeering furnished rod deck scrubbers to Northern States Power for pre scrubbing on ten units.  When the limits were reduced to 0.03 lbs it became advisable to keep the precipitators in operation.  Environeering prior to acquisition by what is now Babcock Power installed  hundreds of rod deck scrubbers in many different industries.  The concept was so successful that other scrubber suppliers developed similar products.

The ability of the rod deck scrubber to remove a varying amount of particulate depending on the pressure drop combined with ease of utilizing multiple decks, each with a different pressure drop opens the door for classification of flyash.  If the rare earth processing would benefit from such as classification it can be done insitu at low cost.

McIlvaine Mini Scrubber

Particle separation in a scrubbing stage is a function of the pressure drop which in turn is a function of velocity, turbulence and to a lesser extent the liquid flow. Many organizations have had venturi scrubber pilot plants.  These plants can be placed at a plant and a slip stream can be processed. This is the only way to predict particle separation. Testing devices such as cascade impactors are almost useless due to the variations in shape and specific gravity of particles.

McIlvaine took the concept one step further and reduced the orifice scrubber to just 1 CFM so it could be used in standard sampling trains. Dupont, AERE Harwell, AirPol, Nalco, Martin Marietta and other organizations have successfully used this device to predict scrubber performance. It was given the commercial name of the McIlvaine Mini Scrubber. McIlvaine offered the drawings and concept free of charge to the Institute of Clean Air Companies. Several members began using the device but the organization never banded together to make it a standard bid tool.

This device can be used by researchers to determine the benefits of a multi stage scrubber to separate flyash particles into appropriate fractions for leaching. The multi stage scrubber could capture the particles emitted from the boiler in several fractions.

System Details

The flyash and HCl is captured in the rod deck scrubber which operates at pH 1. A bleed stream of 30 percent acid with the particulate is then sent for further processing.

.

This schematic provides the mixing of HCl and flyash in a system which eliminates the first stage precipitator.  It is therefore attractive for old coal-fired power plants in the U.S. as well as for new coal-fired power plants in China. It does incorporate a wet precipitator.  Coincidently, this is common  in China for other reasons (to meet tough new particulate standards).

The HCl content does not have to be high. The first stage scrubber starts with water and then reaches equilibrium with 30 percent dirty acid. A portion is bled to maintain this percentage.

The advantages of using high chlorine coals would be that these coals are less expensive and the byproduct sales volume of acid will be higher.

The Chinese believe coal flyash is already a very attractive source for REEs and are pursuing it aggressively. One reason is that the CO₂ emissions are 75 percent less than from extraction through mining.  With the insitu option the energy is much less than the processing of flyash from ponds and therefore compares even more favorably to energy required for mining ores.

European Waste to Energy Plants

Horsholm Denmark has a waste to energy system with metals recovery and acid sale. At the end of the incineration process, the extracted acids, heavy metals and gypsum are sold for use in manufacturing or construction. Small amounts of highly concentrated toxic substances, forming a paste, are shipped to one of two warehouses for highly hazardous materials, in the Norwegian fjords and in a used salt mine in Germany.

Other Multiple Scrubber Variations

TCR Process

 Four Step Scrubbing Process