Chemistry of inorganic compounds – Carbon or compound thereof – Oxygen containing
Reexamination Certificate
2000-05-19
2001-11-27
Bos, Steven (Department: 1754)
Chemistry of inorganic compounds
Carbon or compound thereof
Oxygen containing
C423S422000, C423S206200
Reexamination Certificate
active
06322767
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a process for recovering sodium carbonate or other sodium based chemicals from solutions containing sodium bicarbonate and especially sodium bicarbonate solutions obtained from solution mining trona deposits.
2. State of the Art
Sodium carbonate (soda ash) is approximately the eleventh highest volume chemical produced in the United States. It is used in the manufacture of glass, chemicals, soaps and detergents, and aluminum. It is also used in textile processing, petroleum refining, and water treatment.
For many years, sodium carbonate was produced by the Solvay process in which carbon dioxide was dissolved in water containing ammonia (NH
3
) and salt (NaCl) to precipitate sodium bicarbonate which was then separated by filtration and heated to form sodium carbonate. Because of high energy costs and problems with disposing of chloride-containing waste streams generated by the Solvay process, it has been abandoned in the United States in favor of obtaining sodium carbonate from naturally occurring trona deposits. Trona deposits are located in Utah, California, and Wyoming. Green River, Wyo. contains the largest known trona deposits in the United States and is actively mined by five companies.
Crude trona (“trona ore”) consists primarily (80-95 percent) of sodium sesquicarbonate (Na
2
CO
3
.NaHCO
3
.2H
2
O) and in lesser amounts, sodium chloride (NaCl), sodium sulfate (Na
2
SO
4
), organic matter, and insolubles such as clay and shales. In Wyoming, these deposits are located in 25 separate identified beds or zones ranging from 800 to 2800 feet below the earth's surface and are typically extracted by conventional mining techniques such as the room and pillar and longwall methods. The cost of these conventional mining methods is high, representing as much as 35 percent of the production costs for soda ash. Furthermore, recovering trona by these methods becomes more difficult as the best, most thickly bedded trona deposits are depleted. As a result, recovery of carbonate values from trona has fallen in some cases by as much as 5 to 7 percent. Development of new reserves is expensive, requiring a capital investment of as much as $100 to 150 million in 1995 dollars to sink new mining shafts and to install related equipment.
As its chemical composition indicates, trona ore requires processing in order to recover the sodium carbonate. Most of the sodium carbonate from the Green River deposits is produced from the conventionally mined trona ore via the “monohydrate” process. The “monohydrate” process involves crushing and screening the bulk trona ore which, as noted above, contains both sodium carbonate (Na
2
CO
3
) and sodium bicarbonate (NaHCO
3
) as well as impurities such as silicates and organic matter. After the trona ore is screened, it is calcined (i.e., heated) at temperatures greater than 150° C. to convert sodium bicarbonate to sodium carbonate. The crude soda ash is dissolved in a recycled liquor which is then clarified and filtered to remove the insoluble solids. The liquor is sometimes carbon treated to remove dissolved organic matter which may cause foaming and color problems in the final product, and is again filtered to remove entrained carbon before (going, to a monohydrate crystallizer unit, a high temperature evaporator system generally having one or more effects (evaporators), where sodium carbonate monohydrate is crystallized. The resulting slurry is centrifuged, and the separated monohydrate crystals are sent to dryers to produce soda ash. The soluble impurities are recycled with the centrate to the crystallizer where they are further concentrated. To maintain final product quality, it eventually becomes necessary to remove the impurities with a crystallizer purge stream.
The production of sodium carbonate using the combination of conventional mining techniques followed by the monohydrate process is becoming more expensive as tile higher quality trona deposits become depleted and labor and energy costs increase. As stated above, recovery of sodium carbonate (usually expressed as tons of sodium carbonate produced per ton of trona ore) has fallen as the higher quality, more readily accessible reserves have been mined. Furthermore, the costs of developing new reserves requires substantial capital investment, as much as $100-150 million in 1995 dollars.
Recognizing the economic and physical limitations of conventional underground mining techniques, various solution mining techniques have been proposed. Solution mining allows the recovery of sodium carbonate from trona deposits without the need for sinking costly mining shafts and employing workers in underground mines. In its simplest form, solution mining comprises injecting water (or an aqueous solution) into a deposit of soluble ore, allowing the solution to dissolve as much ore as possible, pumping the solution to the surface, and recovering the dissolved ore from the solution.
For example, a solution mining technique was proposed in U.S. Pat. No. 2,388,009 to Pike on Oct. 30, 1945. Pike discloses a method of producing soda ash from underground trona deposits in Wyoming by injecting a heated brine containing substantially more carbonate than bicarbonate which is unsaturated with respect to the trona, withdrawing the solution from the formation, removing organic matter from the solution with an adsorbent, separating the solution from the adsorbent, crystallizing and recovering sodium sesquicarbonate from the solution, calcining the sesquicarbonate to produce soda ash, and re-injecting the mother liquor from the crystallizing step into the formation.
A second patent to Pike, U.S. Pat. No. 2,625,384, discloses another solution mining method which uses water as a solvent under ambient temperatures to extract trona from existing mined sections of the trona deposits. The subsequent solution is recovered from the mine and heated before dissolving additional dry mined trona in it to form a carbonate liquor having more concentrated values of sodium salts which can subsequently be processed into sodium carbonate.
An additional complicating factor in dissolving trona deposits underground is that sodium carbonate and sodium bicarbonate have different solubilities and dissolving rates in water. These incongruent solubilities of sodium carbonate and sodium bicarbonate can cause bicarbonate “blinding” when employing solution mining techniques. Blinding can slow dissolution and may result in leaving behind significant amounts of reserves in the mine. Blinding occurs as the bicarbonate, which has dissolved in the mining solution tends to redeposit out of the solution onto the exposed surface of the ore as the carbonate saturation in the solution increases, thus “blinding” this surface—and its carbonate values—from further dissolution and recovery. Therefore it is anticipated that long term solution mining of a particular deposit may produce brines with lower sodium carbonate values and higher sodium bicarbonate values than those seen initially. This requires that a process be capable of handling the changing brine grade or that incongruent dissolution must be avoided by some means “Blinding” is an occurrence which has long, been recognized as a problem pertaining to solution mining and is described, for example, in numerous U.S. patents.
U.S. Pat. No. 3,184,287 to Gancy discloses a method for preventing incongruent dissolution and bicarbonate blinding in the mine by using an aqueous solution of an alkali, such as sodium hydroxide having a pH greater than sodium carbonate, as a solvent for solution mining. U.S. Pat. Nos. 3,953,073 to Kube and 4,401,635 to Frint also disclose solution mining methods using a solvent containing sodium hydroxide. Unfortunately, alkalis such as sodium hydroxide or lime are expensive and adversely affect the economics of these processes.
The concept of avoiding incongruent dissolution using a brine containing sodium carbonate was discussed by Gancy, supra, and also in U.S. Pat. No, 5,043,149 to Frint, which disclo
Chastain Richard W.
Neuman Thomas H.
Bos Steven
FMC Corporation
TraskBritt
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