To folks unfamiliar with the petroleum refining industry, or, even just generally, the chemical processing industry, our titling of this dispatch, "USDOE Polishes Coal Liquids", might be confusing.
However, one of the final steps undertaken at a petroleum refinery is the clarification of, and the removal of very fine solids from, the liquid products.
All non-gaseous fossil fuels, from sweet crude petroleum to tar sands to Coal, bear some load of very fine, microscopic inorganic mineral matter that makes it's way through whatever refining process the fossil fuel is put through, and, which should be removed before any liquid fuels derived from those materials are passed on to market.
In large bodies of heavier liquid fuels held in place for longer periods of time, as in the fuel tanks of big ships for example, microscopic mineral particles in the fuel which are small enough that, in and of themselves,  wouldn't cause problems, can cause organic molecules in the fuel to coagulate around them, thus making the minute particles somewhat larger and sometimes troublesome.
In both the above cases, the liquids are passed through what is known in the refining and, more generally, the chemical engineering trades, as "polishing", before they are sent to market or, on board some large ships, which can and do use cruder types of liquid fuels, to the engine.
Some references might help to clarify the concept further, as in:
What is fuel polishing; "Fuel polishing is the process of removing contamination such as water and particles from fuel to ensure that it remains in line with fuel specifications. Ideally, fuel should have an ISO particle code of 18/16/13 and a dissolved water content no greater than 200 parts per million. Fuel polishing will ensure that the fuel is 'clean and dry' meaning free from water and dirt thereby reducing the possibility of engine or fuel system damage"; and:
Polishing: Definition from Answers.com; "In petroleum refining, removal of final traces of impurities, as for a lubricant, by clay adsorption or mild hydrogen treating".
The above "clay adsorption" simply means mixing fine clay, or other reactive chemical molecules or particles, into the product liquid, which causes finer contaminant particles in the liquid to coagulate on and accumulate around the clay, thus making larger particles which are easier to filter out mechanically.
There are even specialists servicing those kinds of needs in the petroleum refining industry, such as the Pall Corporation. See:
And, they've published materials which can tell you more about fuel polishing processes, as in:
http://www.pall.com/pdfs/About-Pall/PR900a.pdf; "Separations Technology in Petroleum Refining".
In one sense, "polishing" liquid fuels makes them more ready for market and use, just as a car dealership might apply a final "polish" on their units before putting them on the showroom floor.
And, that's exactly how close and ready Coal liquids, according to one of the agencies that formed our own USDOE, are, or were, for market; since, although they don't actually use the word or terminology, they developed, more than three decades ago, at the Oak Ridge National Laboratory, a process to "polish" liquid hydrocarbon fuels made from Coal.
As seen in excerpts from the initial link in this dispatch to:
"United States Patent 4,132,639 - Improving the Sedimentation and Filterability of Coal-derived Liquids
Date: January, 1979
Inventors: Sidney Katz and Billy Rodgers, TN
Assignee: The United States of America
Background and Government Interests: This invention was made in the course of, or under, a contract with the U.S. Energy Research and Development Administration. It is related in general to the art of coal liquefaction and particularly to the separation of finely divided solid particles from coal liquefaction products.
Abstract: An improvement in the separation of suspended solids from coal-derived liquids by a separations process in which solids size is a separations parameter is achieved by contacting the coal-derived liquid containing suspended solids with an effective amount of an additive selected from the group of sulfuric acid, phosphoric acid, phosphoric anhydride and salts of sulfuric and phosphoric acid, and maintaining the contacted liquid at a temperature within the range of about 150 to 400 C and for a time sufficient to achieve the desired separation rate.
In a process for separating suspended solids from the carbonaceous liquid product resulting from the liquefaction of coals ... in which solids size is a separation parameter, said suspended solids including ash and insoluble organic matter, the improvement comprising maintaining said carbonaceous liquid product containing solids at 150 - 400C to increase the effective particle size of said solids as a function of time while contacting said carbonaceous liquid product containing solids with an additive consisting essentially of material selected from the group of H2SO4 (and) H3PO4, and salts of sulfuric and phosphoric acid, for a time sufficient to enhance the rate of increase of said effective particle size said additive present in a concentration of about 0.01-3.0 weight % with respect to said carbonaceous liquid containing solids, and separating said suspended solids from said carbonaceous liquid product at said temperature. 

The process ... in which said additive is selected from the group of P2O5, H3PO4 and salts of sulfuric and phosphoric acid. 

The process ... in which said additive is present in a concentration of about 0.1-1.0% with respect to carbonaceous liquid product containing solids. 

The process ... in which said additive is selected from the group of P2O5, H2SO4, H3PO4, Al2(SO4)3,
NH4HSO4, AlPO4, (NH4)2HPO4, K3PO4, Na3PO4, and (NH4)3PO4.
(Don't be distracted by the chemical formulae. Some of the above are just stuff that can be made from ground up "phosphate" rock, which is, literally, as common as dirt in places like Florida; and, some of which, like the ammonium, "NH4", sulfates and phosphates are used in the compounding of fertilizer. - JtM) 

The process ... in which said additive is present in a concentration of about 0.1-1.0% with respect to carbonaceous liquid product containing solids.
(And, as above, not only is this stuff as common as dirt, we don't really need very much of it.) 

The process ... in which said contacted liquid is maintained within said temperature range for sufficient time to allow suspended solids to settle to provide a clarified carbonaceous liquid product substantially free of solids. 

The process ... in which said temperature is within the range of 275.-375 C (and) in which said additive is selected from the group of salts of sulfuric and phosphoric acid. 

The process ...  in which said process for separating suspended solids is a filtration process.
(The above is as in our introductory references and comments concerning "fuel polishing" and the Pall Corporation's  "Separations Technology in Petroleum Refining".)
Description and Background: A variety of processes have been proposed for converting solid carbonaceous material such as coal ... into a liquid form reduced in impurities such as sulfur and ash. Some liquefaction processes convert the carbonaceous material into a liquid fuel suitable for combustion or a synthetic crude suitable for use as a feed for a gasoline refining plant.
Typically, coal liquefaction processes involve reacting finely crushed coal with a source of hydrogen, either a hydrogen donor solvent such as tetrahydronaphthalene or hydrogen gas or both.
(Concerning the above "tetrahydronaphthalene", perhaps better known to our readers as "Tetralin", see:
WVU Hydrogenates Coal Tar | Research & Development; concerning: "Hydrogenation of Naphthalene and Coal Tar Distillate; Abhijit Bhagavatula; West Virginia University; Morgantown, West Virginia. 2009. Abstract: The hydrogenation of naphthalene and coal-tar distillates has been carried out in a Trickle Bed Reactor (to obtain) the hydrogenated product, tetralin (1,2,3,4 Tetrahydronaphthalene)"; and:
Exxon 1982 CoalTL Uses WVU CoalTL Hydrogen Donor Solvent | Research & Development; concerning: "United States Patent 4,345,989 - Catalytic Hydrogen-donor Liquefaction Process; 1982; Assignee: Exxon Research and Engineering Company; Abstract: Coal ... is converted into lower molecular weight liquid hydrocarbons by contacting the feed material with a hydrogen-donor solvent. The hydrogen-donor solvent used ... will normally be a coal-derived solvent ... generally employed in coal liquefaction reactors. Representative compounds of this type include ... tetrahydronaphthalenes (i.e., "Tetralin"s.)")
The reaction products are of very complex chemical composition and contain insoluble materials such as ash and heavy organics which must be separated from the product prior to use or downstream catalytic reactions. This invention is directed to a separation method for separating suspended solids from coal-derived liquids. For purposes of our process, "coal-derived liquids" is intended to include the carbonaceous liquid product resulting from the liquefaction of coals ...regardless of the particular liquefaction process and the term is intended to include solutions of liquid coal products in miscible solvents such as hydrogenation solvents. 

The separation of suspended solids from coal-derived liquids is one of the more formidable problems facing the establishment of a large-scale coal liquefaction industry. The solids content of most coal-derived liquids is largely present as sub-micron particles, including inorganic ash particles. Some coal-derived liquids contain as much as about 10 weight % solids consisting of particles, 90% of which are smaller than one micron. Several methods of separating coal-derived liquids have been proposed including filtration, magnetic separation, flash distillation, hydroclones, centrifuges, and settling. With the exception of flash distillation each of these methods depend upon the size of the particles as a separation parameter and would be enhanced if the particles were made significantly larger than their original size.
Summary: It is an object of this invention to provide a method for separating solids from coal-derived liquids including solutions of liquid coal products which does not require the separation and recovery of an aqueous phase (and) to provide a method for increasing the size of particulate solids in coal-derived liquids including solutions of liquid coal products which does not require the addition of expensive materials or the recycle of large quantities of precipitation solvents or liquefied coal product fractions. 

It is a further object to provide a method for enhancing the separation of solids from coal-derived liquids which requires the addition of only small quantities of inexpensive material which need not be recovered."
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And, that's the gist of it.
And, it really is only an adaptation of standard petroleum refining techniques so that they can be applied to hydrocarbon liquids made from Coal.
Small amounts of cheap and common minerals are ground up and added to the Coal liquids. The mineral particles, due to the electrochemical nature of their surfaces, cause minute particles of solid contaminants in the liquid hydrocarbon to stick to them, like dust on an electrostatically-charged piece of glass; and, the thus larger, agglomerated particles are then easily filtered out using standard, and currently-employed, refinery techniques.
Better than three decades ago, Uncle Sam was putting the final spit and "polish" on his Coal liquid cruiser.
How come he ain't rolled it out onto the showroom floor, yet, so we all can take a big, public gander at it?
It's far, far past time he, or someone, did.

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