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Process Selection Criteria for Refining Trona to Commercial Products

Presented at The First International Soda Ash Conference (ISAC) - June 11, 1997

By Roger Aitala and Mark Aitala, Isonex, Inc.



Introduction

Trona deposits vary in geological and chemical characteristics. Geographical location and site characteristics such as environmental matters, specific energy resources, distribution methods, and trade barriers are key elements in a trona-refining project. A commercially successful trona-refining project, capable of surviving the inevitable competition in this global business, rests on the rational integration of the major project parameters.

There should be no inconsistencies or discontinuities between the ore from the mine, the feed to the refinery, and the market requirements. They must match or economic disaster is possible after having invested the several hundred million dollars required to build a world-class trona refinery. The major parameters in process selection are: the characteristics of the trona deposit; whether the project involves an expansion or a "dry desert"; available forms of energy and its costs; and the geographical market under consideration for soda ash and suitable derivatives.

This paper outlines a logical methodology for process and product slate selection. Existing commercial technologies, mining and refining technologies under development are briefly reviewed.


The Project

There are three main possible types of trona-refining ventures:
  1. Expand an existing trona-refining complex
  2. Develop a new complex in a known deposit
  3. Develop a new complex in a new deposit

The mission in all cases is: structure a trona-refining operation which will maximize economic returns for the long term, consistent with safety and environmental safeguards.


The Trona Deposit

Location, especially as affecting distribution
  • Distance from major consumers
  • Transportation/distribution systems in place or required to reach target markets
  • Logistics for people and materials during construction
Surface or Deep
  • Owens Lake, California; Lake Natron, Tanzania; and Magadi, Kenya are examples of surface deposits.
  • Wyoming and Beypazari, Turkey are examples of massive, deep rock trona deposits.
  • Hydrology. Presence of acquifers and relationship to trona beds, artesian flows, water infiltration rates. Ground waters may be a welcome source of water in desert areas with scarce surface water.
Geological Information and Delineation and Sizing of Reserves
  • Seismic data
  • Geological description, including extent of faulting
  • Trona-refining complexes are capital-intensive; therefore, reserves need to be adequate to support a project for many years, preferably over 20.
Complete Ore Analysis of Extensive Samples

Certainly, a high trona assay is desirable, but other considerations like low salt, low kerogens, and a high degree of homogeneity in the ore in all directions also have economic impacts. Dry and wet analyses are necessary to determine among others:

  • Water-insolubles at what temperature? And break down.
  • Calcium Carbonate (CaCO3)
  • Shortite (Na2CO3· 2CaCO3)
  • Pirssonite (Na2CO3·CaCO3· 2H2O) (Can scale equipment)
  • Gaylussite (Na2CO3·CaCO3· 5H2O) (Can scale equipment)
  • Burkeite (Na2CO3· 2Na2SO4)
  • Nahcolite (NaHCO3)
  • Silicates form sodium silicates and can show up in some products
  • Wegscheiderite (Na2CO3· 3NaHCO3, extra carbon dioxide as bicarbonate)
  • Sodium chloride
  • Thenardite (Na2SO4, especially in California's soda lakes)
  • Organics (Kerogens cause color, foaming, and increase costs)
  • Others (Borates, Potassium, Magnesium salts, double salts, complete analysis)

Product Slate

The probable main products to be considered are:
  • various forms of sodium carbonate, especially dense soda ash
  • caustic soda
  • sodium bicarbonate.
  • downstream derivatives may also be of interest in certain geographical markets.

Market Requirements

Product specifications, especially for the physical characteristics, such as granulation, flowability, particle and bulk density.
  • Soda ash
  • Sodium Bicarbonate (Technical or USP Grade)
  • Caustic soda (50% or solid)

Product samples from pilot operations are highly desirable. Potential customers will consider your project more seriously and can confirm its suitability for their applications.

Distribution Considerations

Trona-refining complexes usually require only fuel from other locations. Therefore, the distribution plan mostly concerns the product(s).

Wyoming trona-refiners distribute most soda ash via fleets of owned or leased, closed hopper cars. Trucking has declined dramatically. Most of the product destined overseas is handled by the American Natural Soda Ash Corporation (ANSAC). Most of the railroad cars, shipping and receiving terminals are dedicated to soda ash to prevent cross-contamination. ANSAC ships most of the soda ash via unit trains and eight ocean-going vessels.


Mining Alternatives and Plans

These are interdependent with process selection. The mining plan must establish early in the project whether the deposit will be mined exclusively mechanically or will be followed by solution mining. The long-term mining plan may in some cases include mining beds at different depths. In Wyoming, mining technologies have evolved and improved considerably. For example, Figure 4, shows that the trona miners' productivity has almost doubled over the past twelve years.

Mechanical vs. Solution

FMC has been practicing post-mechanical solution mining commercially since November 1995.

Tg Soda Ash has been operating a caustic soda plant based on "mine waters" since 1994 and may have been recycling some liquids to the mine. Tg Soda Ash has also obtained recently a patent for carbon dioxide stripping of mine waters containing sodium carbonate and sodium bicarbonate. Tg has also applied for a process patent, presumably involving solution mining of trona, refining to soda ash and caustic soda, and recycling to the mine.

These new solution-mining technologies hold promise for further improvements in miners' productivity and reduction in mining costs.

Advantages of Solution Mining for a New Complex
  • Leaves most of the insolubles in the mine
  • Lower capital cost underground and in refining system
  • Lower labor cost, especially in mine
  • Lower energy consumption
  • Post-mechanical solution mining increases recovery and project life.
Projected Mine Effluent
  • Solid trona, size of ore rocks
  • Trona solution, degree of saturation, and how affected by return solution
  • Temperatures
  • Complete Analysis
  • Expected Consistency

Process Selection

As already mentioned, these must be considered in conjunction with the mining alternatives. Starting with the probable main product, dense soda ash:

Dense Soda Ash from Trona (See simplified Flowdiagrams 1A and 1B)

Stoichiometry for main reaction:

2Na2CO3 · NaHCO3 · 2H2O Ð-90o-150oC-ð 3Na2CO3 + CO2 + 5H20

The overall stoichiometry yields water, which is an important observation for projects located in desert regions.

The above reaction can be carried out on solid trona at about 150o C or by steam-stripping the carbon dioxide from a water-solution at lower temperatures. Solvay operators may remember similar equipment was called decarbonators or DCB's. Wet calcination has many advantages over dry-calcination, except that the degree of decomposition is not as complete. This can be remedied by adding small amounts of lime, which will convert the bicarbonate to sodium carbonate.

The separation of the sodium carbonate from all other ore components can be carried out via many processes. Figure 1A is a simplified flowdiagram for the dry-calcination process used by all Wyoming refiners. Figure 1B is a simplified flowdiagram taken from a recent Tg Soda Ash patent showing one of the several possible process configurations.

In both cases the final product is obtained by precipitating sodium carbonate monohydrate crystals in evaporators, centrifuging the crystals, and dehydrating to the anhydrous dense soda ash in steam tube dryers or fluid bed dryers.

Na2CO3·H2O Ð-------ð Na2CO3 + H20

As is the case in all reactions in this field, they are reversible and are driven in one direction or the other by concentrations, temperature, and by removing components from the reaction medium.

The monohydrate evaporator-crystallizers systems are complex. Among other considerations, they must be designed and operated to control the size distribution of the crystals to match the granulation specifications for the final product. Even crystal shape and single crystal diameter to length ratio may be important and require additives and/or shape modifiers. Formation of complex crystal clusters "stars" must be avoided as they tend to break and generate fines. Some dry classification of the final product can also be done, but at extra cost.

In Wyoming, about 1.8 tons of trona yields 1 ton of dense soda ash. The specific yields at each producer depend on the assay of the mined trona, which can vary from 79% to 96%, and on the details of the operation.

Phase diagrams, such as shown in Figure 5, are the main road maps guiding the process design. As for all road maps and ventures, it is crucial that they be for the right system with the relevant components. They show what solid sodium carbonate hydrates are in equilibrium with the liquid phase as a function of temperature and concentration of sodium carbonate. They can be difficult to interpret, especially when sodium chloride, calcium carbonate, sodium silicates and other compounds, such as gaylussite, pirssonite are included. These can be important in order to avoid regions conducive to scaling of internal equipment surfaces. Equipment can be descaled by washing, but frequent washing reduces effective plant capacity and increases energy costs to remove the excess water.

Many other issues must be addressed in the process design:
  • How is salt removed? (Purge and Decahydrate crystallizer)
  • How are insolubles removed? (Leave in Mine, Filtration, Tailings pond)
  • How are kerogens removed? (Carbon Treat, High-Temperature Calcination, Oxidizing Agents)
  • Overall water balance (Decahydrate system, condensate recovery)
  • Sulfur content of fuels
  • Overall energy consumption vs. capital cost (Multi-effect evaporation, Mechanical vapor recompression, Heat recovery)
  • Formation and control of return solution for steady operation above and below ground. Temperature, composition, and flowrate.
  • Disposal of insolubles to tailing ponds or mine
  • Environmental issues

A decahydrate system is an effective way to accomplish several objectives: recover sodium carbonate values from dilute solutions also recovering relatively pure water, concentrating purge streams, and providing an economic feed for downstream products.

The challenge is to design a process which optimizes the overall economics for an integrated multi-product complex. That requires that all major process streams and economic elements be analyzed as an integrated system. Optimization of processes for each product may not lead to the same solutions.

Innovative Refining Processes Under Development

Papers are being presented at this meeting on: separation of insolubles via electrostatic precipitation of calcined trona; wet calcination of trona in microwave ovens. Other technologies have been attempted or can be visualized which would separate the insolubles or other components via physical means such as classification or flotation.

Innovation has always been a characteristic of the soda ash industry both in synthetic and natural-based processes. Many of these novel processes have attractive features, but the definitive main test is whether they can yield the product purity and physical quality obtainable via processes based on crystallizations.

Sodium carbonate not meeting the specifications of the generally accepted commercial product may target specific market niches. One disadvantage is that a different grade product may require an independent distribution system. Another concern should be that all American producers can degrade their present processes to compete with a lower grade. The net affect could be an increase in distribution costs and a decrease in profit margins.

Caustic Soda from Trona (See Simplified Flowdiagram 2)

Overall stoichiometry for main reaction:

Na2CO3 + H2O + CaO Ð------ð 2NaOH + CaCO3

The sodium carbonate feed can be obtained by: dissolving trona in a dilute caustic soda recycle stream (Solvay Minerals); by taking a slip stream from a weak soda ash solution, possibly from a decahydrate system, (FMC); or by using mine waters (Tg Soda Ash-Elf Atochem).

The lime can be purchased or obtained by calcining the precipitated calcium carbonate. Theoretically, makeup calcium carbonate could be obtained from shortite, which is often present in trona deposits. Economic considerations may lean toward the purchasing alternative.

Important process details must deal with side reactions involving, silicates, burkeite, and saponification of kerogens, which may cause foaming in the evaporators.

Sodium Bicarbonate from Trona (See Simplified Flowdiagram 3)

Most sodium bicarbonate is used in baking soda, dentifrice, deodorants, and pharmaceuticals. All these end uses must meet stringent, high-purity specifications. Usually, they can be met most economically by starting with high purity raw materials.

Overall Stoichiometry for the main reaction is:

Na2CO3 + H2O + CO2 Ð------ð 2NaHCO3

Sodium bicarbonate crystals are formed in an absorption tower; they are recovered by centrifugation, dried, and classified to meet granulation specifications. The process selection is mostly governed by product specifications, but the relative amounts of soda ash and coproducts or derivatives in the product slate also play a role. Most likely, the demand for sodium bicarbonate is much lower than for soda ash and a liquid feed containing sodium carbonate in solution is adequate.


Developing and Selecting the Best Project Alternative

The project's viability is evaluated mainly by comparing the project's cost structure vs. existing and potential firms in the market under consideration for comparable product quality. The project must also meet the firm's financial objectives and risk limits. The major elements effecting process selection during the project development phase of the venture are:

  • Product(s) Slate and Specifications
  • New or Old Mine
  • New or Existing Operation
  • Product Slate
  • Capital Cost
  • Operating Cost, especially energy and labor components
  • Must be competitive with existing and planned projects

They must be considered almost simultaneously, which is possible with computers. Compromises are not possible in environmental and safety matters, but many compromises are required among different disciplines and to balance technical and economic factors, and risk/reward ratios. It is the essence of "design".

Do it early in the project and do it thoroughly, before the project generates a life of its own and revisions to incorporate the best overall mining/refining technologies are discouraged by project executives responsible for meeting budgets and schedules. You do not want to build a brand new, but technically obsolete at startup complex. It might require considerable, expensive retrofitting later to remain competitive.

Each trona-refining project is unique. Most projects are expansions of existing operations. Some benchmarks for an economically viable new project in Wyoming are:

  • Minimum economically-minable trona reserves: 75 million tons
  • Minimum trona assay: 80%
  • Major product: dense soda ash meeting ANSAC's specifications
  • Planned capacity: 2 million short tons per year, probably built in two phases
  • Mining plan: start with mechanical and follow with solution
  • Process: final product should be derived from dehydration of sodium monohydrate. Front end should be able to process solid trona and/or solutions.
  • Project life: 20 years minimum
  • Preliminary cost targets for a dry-desert project: order-of-magnitude capital investment $360 million in Phase 1 for 1 million tons per year; totally loaded operating costs before corporate charges and royalties at about $61 per ton in Phase 1.
Have an exciting trona-refining project, safe, profitable, and long-lived!