R&D project done for US government, DOE (Department of Energy
ASA Equipment Solutions played a key role in supporting a rare element recovery R&D project by providing a comprehensive range of high-quality equipment, including glass-lined reactors, stainless steel tanks, a 50 HP containerized boiler, a 1.3 megawatt generator, precision laboratory tools, and a Halar-coated basket centrifuge. By offering a strategic mix of new, rebuilt, and used equipment, ASA Equipment Solutions delivered significant cost and time savings, helping the project maintain efficiency without compromising on performance or reliability.
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Centrifuges
Prior to considering the various system options, it is important to clearly define the process. An accurate definition
of the process is essential at initial briefings with the centrifuge supplier because subsequent
screening tests and feasibility studies must be based on relevant data, rather than on
vague and sometimes misleading assumptions.
A potential user should clearly define exactly what they require the centrifugal process to achieve. For example, does the material require clarification, classification, degritting, thickening,
dewatering, washing or separating and repulping, and is the process a solid/liquid, liquid/liquid or a three phase
liquid/liquid/solid application?
Other considerations of equal
importance are: g force; cake dryness; solids’ level in discharge liquors; product
temperature; viscosity; specific gravity; pH; and batch or continuous process. The decision to use either a batch or a continuous machine depends on several factors.
In the situation where it is essential that no
cross-contamination occurs between batches, centrifuges can be installed with pre-programmable, validated, clean-inplace (CIP) washing systems, which also eliminate the need to open the casing
between cycles for cleaning and maintenance.These machines are particularly suitable for use in applications
in the pharmaceutical and fine chemical industries
Recommendations from pharmaceutical authorities such as the Food and Drug
Administration (FDA), USA, have led to the introduction of stringent codes of practice that are rigidly enforced right
across the international process industry. These recommendations in turn have
created a need for centrifuge design to be validated, and have driven the development of machines that combine the
very best of good manufacturing practice (GMP) and state-of-the-art programmable logic controls (PLCs).
Having clearly defined the process, i.e. objectives and required variables, it is now
possible to proceed with selecting the type of centrifuge to meet the specified criteria. Broadly speaking centrifuges can be divided into filtration centrifuges and sedimentation centrifuges. Figure 1 highlights the different types of sedimenting and filtering centrifuges. Preliminary screening tests will quickly
indicate the best option.
Disc Bowl The disc bowl type centrifuge operates at speeds of 3000 to 20 000 times gravity,
providing a continuous clarification system that is suitable for materials with a solids content of 1-2 % or less. It is designed to separate either a solid/liquid or two liquid.
phases on a continuous basis. Solids settle on the wall of the bowl and are discharged either manually or automatically by
intermittent opening of the bowl.The disc stack greatly increases the effective
settling/clarification area, and the liquid and solid phases travel up or down the disc
surfaces.The liquid discharges through one or more paring discs.
R&D project done for US government, DOE (Department of Energy).
ASA Equipment Solutions supplied several pieces of equipment for a rare element recover R&D project. Items supplied were glass liner reactors, stainless steel tanks, 50 HP containerized boiler, 1.3 megawatt generator, laboratory equipment and Halar coated basket centrifuge. Equipment supplied was new, rebuilt or used. By purchasing used and rebuilt equipment project saved both time and money.
Clarifying Centrifuge Operation and Selection
CENTRIFUGE
These two categories can be further split into subcate- gories, depending on whether the centrifuge runs on a continuous or batch basis. Decanting centrifuges effect a separation by having a “heavier” solid (i.e., a solid with a higher specific gravity than the liquid in which it is suspended) settle in or from a lighter liquid (i.e., a liquid with a lower specific gravity than the solid).
Continuous centrifuges
These two categories can be further split into subcate- gories, depending on whether the centrifuge runs on a continuous or batch basis. Decanting centrifuges effect a separation by having a “heavier” solid (i.e., a solid with a higher specific gravity than the liquid in which it is suspended) settle in or from a lighter liquid (i.e., a liquid with a lower specific gravity than the solid).
Batch-filtering centrifuges
All batch- filtering centrifuges are equipped with a feed device that introduces the slurry into the cylinder, as well as a device that allows for the addition of various wash fluids after the initial solid/liquid separation is achieved. In some cen- trifuges, the feed and wash mechanisms employ the same components, while in others, separate pipe and nozzle con- figurations are used.
Guidelines for selecting centrifuges
If the application is not new, there is good chance that you can find documentation of a process that defines the type of centrifuge currently operat- ing on this application. This does not imply that it is the best centrifuge for the process, but it may suggest a comfort factor in this technology. There may also be a personal or plant pref- erence for a particular type of centrifuge for the application.
The characteristics of the solids and liquids handled in a process will influ- ence centrifuge selection.
Specific gravities of the solids and liquids — If the solids are lighter than the liquid, a decanting centrifuge is not an option. If the specific gravities are very close, but the solids are slightly heavier, a decanting centrifuge may be considered, but only if either the particle size or the centrifugal force improves the settling of the solids.
Particle size — Coarse solids with particle sizes greater than 100 µm are generally best suited for filtering- type centrifuges. Finer solids that measure less than 10 µm are best handled in decanter centrifuges. Given that most solids have a broad particle-size distribution, the per- centage of solids finer than 100 µm will influence the overall expected recovery of solids. It may also dictate the selection of either batch or continuous equipment. Be- cause batch centrifuges typically operate with filter media of varying porosity and/or a heel of solids, they can be used on finer particles and will give a higher recovery of solids than continuous, filtering-type centrifuges.
The need to achieve a certain outcome with respect to the recovery of solids, nature of the recovered solids or liquid, and other product character- istics will dictate the choice of equipment.
Centrate clarity — Decanting centrifuges provide the best clarity of all centrifuge types. Filtering centrifuges typically are not used where centrate clarity is the princi- pal process requirement because they use either a filter medium or a screen.
Solids recovery — To maximize the percentage of solids captured, decanting centrifuges or batch-basket cen- trifuges are favored. Continuous, filtering-type centrifuges will typically lose 1–5 wt.% of solids finer than 100 µm through the screen, depending on particle size and feed rate.
Solids dryness — Filtering centrifuges provide the driest solids, while decanting centrifuges provide the wettest solids. Since the final dryness of the solids will be tied to the particle size of the solids, the difference in dry- ness between a filtering centrifuge and a decanting cen- trifuge may be as little as only a few percentage points. The batch-basket centrifuge, because of its ability to ex-
tend the drying time, may produce the driest cake of any centrifuge (e.g., large particles, 50–100 µm could be less than 5% solids in the final cake). But, one must determine whether such “dryness” is cost effective.
FAQS
Rare element R&D project done for US government, DOE (Department of Energy)
ASA Equipment Solutions provided a range of equipment to support a research and development project focused on rare element recovery. The items delivered included glass-lined reactors, stainless steel tanks, a 50 HP containerized boiler, a 1.3 megawatt generator, various laboratory tools, and a Halar-coated basket centrifuge. These pieces played a vital role in the project’s operational success.
The equipment supplied came in new, rebuilt, and used conditions, allowing the project team to meet their needs efficiently without compromising on quality. By opting for a mix of rebuilt and used equipment, the project was able to significantly reduce costs and expedite the setup process, saving both time and financial resources.
Decanting batch-basket centrifuge
The imperforate (i.e., without holes or perforations) basket centrifuge is another type of vertical batch cen- trifuge used in dewatering certain unfilterable solids, such as electrochemical waste and other slimy waste materials (Figure 7). It is very similar to the filtering basket cen- trifuge, except that it has a solid cylinder that requires no filter medium. A typical cycle of a decanting batch-basket centrifuge is described below:
Accelerate speed.
After the cake from a pre- vious cycle is discharged, the basket must be accelerated back to full speed, since the best separation takes place at the maximum centrifugal force.
Feed
The feed valve is opened and the basket is filled with the slurry. Once the basket is filled, the feed valve is left open and the clarified liquid overflows the basket lip, while the solids settle against the basket wall.
Breakover
For any given set of process parameters, there will come a time when the centrate will begin to carry over the solids that are fed to the centrifuge.
Skim
Once the solids settle, a skimmer tube is moved into the clear pool under the basket lip; the movement of the basket forces this liquid out through the tube.
Discharge
The basket is then slowed to a low speed and a scraping device is used to deflect the solids out of the centrifuge. The cycle is then repeated. The Table compares the different types of centrifuges and their process variables.