Schematic design of a scalable technological process

This scheme represents the final summary of the project, putting all the pieces together and presenting a working conceptual desigh of the production line for the novel adsorbent material obtained from crustacean shells.
The goal of this scheme is to provide concrete technical solutions for production of 100 g of final adsorbent material per batch. Currently the production method is under evaluation for patenting, hence some key details were hidden for protection of the intelectual properties.
The process design achieved here is based on the results obtained on the species Atlantic blue crab (Callinectes sapidus), however, by deduction, this process is aplicable to other similar species, althugh exact values and numbers may be different.

Process stage
Step summary Description

Stage 1: Native shells
The crustacean shells may be sourced from seafood industry entities, such as fish markets, restaurants or processing plants. The shells should be rinsed with water and air-dried. Thorough cleaning is not necessary. Shells of hard-shelled species are preferred, such as the Atlantic blue crab (Callinectes sapidus).
The total mineral (shell) mass fraction of the blue crab amounts 52.13 % ± 1790.015 (mean ± standard deviation). The image shows the shell mass breakdown by the crab`s anatomic parts. Notably, the most useful calcium carbonate mass is contained within the carapace and the legs, which also contain the lowest amount of edible meat. Contractss may be established with seafood industry entities, so to collect the shells. The logistical aspect of shell collection is outside of the scope of this scheme, and has to be negociated on case-by-case basis.
Basically there are two main ways to acquire the shells: either by contracting the food chain entities to provide shells after they have extracted the meat they need, or acquiring the fresh crustaceans and separating the shells manually. The latter method is more labour- intensive, however, since Atlantic blue crabs are invasive species in many regions, management plans may call for removal of the specimens from nature, and adsorbent material production may be a way to extract some added value from this biomass.
Certain companies offer devices for mechanical separation of crab meat from the shells. One such company is Akiyama Machinery based in Japan. The link to the company`s website is provided below. Other designs more suitable for household use exist, such as the "Crab Master", however, these work on the basis of shells crushing, and are not recomended for use in this context.
Check Akiyama Machines site

Treatment 1: Carotenoids extraction
This step has a two-fold aim: firstly, the carotenoids are valuable compounds in nutritional and biomedical fields, and their extraction presents an additional valorisation option; secondly, while in contact with an organic solvent, lipids and other organic dirt are dissolved away, freeing additional porous surfaces of the shell.

This treatment is optional

Carotenoids extraction in an organic solvent is not mandatory, and it may be advantageous not to spend time and waste solvents on extraction if there are no further valorisation plans in place for carotenoids. The alternative is to rinse the shells several times with running warm (> 50 C) water and dry them well.

Solvent immersion in brief:

  1. The shells will firstly have to be mechanically crushed to fit the opening of the extraction container.
  2. Exposure to an organic solvent such as acetone does indeed act to increase shell porosity up to 5 times the original value. For a 1 kg batch of shells, a glass container of 2 litre capacity and a fairly wide neck is appropriate.
    NOTE: the container should not be plastic, as organic solvents may dissolve certain types of plastic.
  3. The shells should be kept in the solvent for at least 9 hours (time optimized within another study), however, overnight immersion is also acceptable is this is more convenient for the operator. The level of the solvent should be adjusted to cover all the shells, which is somewhat less than 2 litres. The container should be capped to prevent solvent evaporation.
  4. After the alocated time is elapsed, the solvent should be drained from the container, and the shells spread on a clean tray and air- dried for 2 hours.
    NOTE: Valuable carotenoids can be purified from the solvent, and in this way the bulk of the same solvent may be re-used for another shell batch. Re-use of the solvent without distillation was not tested, however, due to its saturation with dissolved matter, its efficiency in increasing the shell porosity is expected to dwindle with repeated use.

Solvent choice

We recommend using acetone in this step. This solvent dissolves carotenoids better than ethanol for instance, and will extract to some extent the remaining water within shell structure. Ethanol, methanol and ethyl acetate may also be used, however, we have quantitative data on porosity only for acetone, and we observed that hexane, on the other hand did not seem to interact with the shells at all.

Re-use of acetone

After distillation were conducted, indicating that as much as 80% of the solvent may be recovered and used again for another batch of shells. The extraction efficiency was tested by comparing the concentration of dissolved carotenoid astaxanthin after 5 hours of immersion of shells in paralel in fresh and dissolved acetone. This data is available from the Principal Investigator upon request.

Stage 2: Carotenoid-depleted shells
After extraction, the shells remain dry and clean of loose organic matter. The major shell constituents remaining are chitin and Mg-rich CaCO3. Also, the porosity of the shells increased significantly (see details).
X-ray diffraction (left) shows that the bulk mineral constituent of the shell is now Mg-rich CaCO3 (reflection peaks at 23.21, 29.57, 36.27, 39.70, 43.57, 47.9 and 48.92° 2?). Fourier-transform infrared absorption spectroscopy (FTIR) shows that the organic phase mostly rerers to chitin (bands around 866, 1068, 1545, 1670, and 2880 - 3100 cm-1. These figures were reproduced from our previous publication in the journal Water (Open access), where more scientific details are given.
At this stage the shell material is stable and does not require refrigerated storage. It can be stored in plastic containers until further processing. It is not odorous and is dry enough for subsequent milling step.

Treatment 2: Milling into micropowder
Milling is done by mechanical methods, and a wide variety of milling machines designs may be used. In our trials, we used a planetary ball mill and a vibratory disc mill. The former yields slightly better reproducibility regarding the powder size, while the latter device design has a significantly larger throughput.

General remark

In addition to calcite minerals, shells contain an organic scaffold (network) consisting of chitin fibers. Chitin is a tough natural polymer which gives the shell certain elasticity and resistance to tearing, so crushing by simple rolling will not work for production of fine powder. Shearing and impact forces are thus needed. Two main device designs are appropriate: ball mills and vibratory disc mills.
Example of a milling jar for ball mills Example of a ball mill device

Ball mills

Ball mills work on the principle of a horizontal force acting to move the crushing medium (balls) inside grinding jars (containers) which contain both the sample to be milled and the balls. This principle is well described here. The most common European producer of ball mills of small to medium capacity is Retsch but here are also other companies that produce intermediate-capacity mills. Certain mining companies produce big ball mills with drums of several cubic meters, however, these are oversized considering the shell volume available would probably under-utilize the device.
Milling cycles in planetary ball mills will usually have a duration of 3 to 5 minutes.
Example of a milling jar for disc mills Example of a vibratory disc mill device

Vibratory disc mills

Vibratory disc mills pulverise hard and brittle materials by horizontal circular motions. The sample is loaded between the discs of the jar (shown to the right) and the jar is subsequently secured in the device grinding holder. There is a video at the Retsch one of common producers), demonstrating the mill operation. Although the jar capacity here is lower (250 ml), the milling cycles are short (under 1 minute) and the small number of jar components makes it very easy and rapid to empty and refill the jar for the next cycle.

Important specifications

Crab shells are not a particularly difficult material to handle with ball or vibratory disc mills, however, few specifications should be observed:
  • Milling throughput:The mill itself (the machine) and the milling jars (containers) of various volumes are usually sold separately as the device and an accessory, respectively. When choosing a mill, the operator should heep in mind that the device should be able to house the biggst available jars (500 ml for ball mills, 250 ml for disc mills).
  • Jar material: there is a wide variety of materials the jars can be made of, such as (hardened) stainless steel, tungsten, agate, sintered aluminium etc. Since the shells are mostly calcite minerals, steel jars are sufficient, and are the most cost-efficient, as the prices for jars made from other materials may be greater.
  • Versatility: The presented mill designs can be used to grind a variety of other materials for other purposes when not powdering shells. It should be kept in mind, however, that the material to be milled has lower hardness than the jar material.

Stage 3: Shell powder
The powder is the shell form on which further treatments and analyses can be done. Details show the composition of the shell: the chitin and CaCO3 will be the main shell components that we will follow throughout the subsequent treatment, as well as changes in shell porosity. We are not particularly concerned with particle size, as this detail is not expected to significantly affect the properties of the final adsorbent material.
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Powder properties
Particle size
Principle of dynamic light scattering

Particle size can in principle be determined by laser diffraction or by mechanical sieve shakers. The first method requires suspending a small aample aliquot in water, relies on the principle of different particles scattering the laser light in a size-dependent manner. The devices are electronically controlled, and a fine resolution can be achieved. This website summarizes well the available granulometer types and their mode of operation.
A less expensive alternative are the vibratory sieve shakers, however, these are not as precise.
Laser diffraction granulometers are characterized by the cost of scientific equipment, their acquisition only for determining the size of the milled powder may not be financially feasible.

Our DLS results on shell particles obtained by 3 to 5 minute milling runs in a planetary ball mill indicate that the aprticles had a diameter distribution between less than 1 to about 250 microns, with the 1 - 50 microns claass containing most of the particles. Full reports are discussed in our publications in journals ACS Sustaiinable Chemistry & Engineering and ACS Omega

Particle porosity

The porosity measurements results by the BET method for the BC1 adsorbent: (a) nitrogen adsorption and desorption isotherms, (b) pore diameter distribution.

Treatment 3: calcination - removal of organics
In this step, the bulk organic components of the shell (chitin, residual protein) are removed by volatilization. This is done by calcination of shell powder at a temperature above the degradation threshold of chitin.
Thermogravimetry Prior to thermal treatments, we conducted thermogravimetry analyses (TGA) in air and argon atmosphere. It is not necessary to conduct TGA for every shell batch, as it is expected that the thermal steps will be very similar, and oru curves presented below may be used. A significant difference may occur in powder mass loss, as the shells containing greater organic fraction will lose more mass in this step.
Figure shows the TGA curves for the powder obtained by milling of native and carotenoids-depleted (post-extraction shells) up to 1000 C. Boxes indicate the temperature ranges where the current and the second (see "Treatment 5") calcination should be conducted. The small variation in mass loss is due to different content of organic material between the two shell types. 		Calcination

A tube oven is often used for calcination in controlled atmosphere; the sample is placed in a quartz tube, and the tube can be connected to a vacuum pump or to an argon, nitrogen, or other gas container. The tube is mobile, and when the sample is ready for calcination, the tube stage is rolled into the heating tunnel.

Treatment 4: NaOH treatment
After the thermal treatment, it is necessary to wash the powder with sodium hydroxide solution to remove any remaining organic residue.

Preparation of the NaOH solution


Sodium hydroxide (NaOH) goes by the commercial name of caustic soda. For a medium-scale process, such as described here, technical grade NaOH is appropriate. This substance can be purchased by kilograms from local producers. Producers can also be searched over common online shops like Ebay or similar. The operator has to keep in mind to purchase solid NaOH (not solution), and we recommend purity of at least 50%. If surplus NaOH is purchased, the operator has to ensure dry storage conditions.
The recipe for NaOH stock solution calls for mixing of 311 g NaOH per 1 L of water. Depending on the exact mass of the powder after the first thermal treatment, the appropriate amount of stock NaOH solution should be used. Every 10 g of sample will require 103 ml of stock solution.
NOTE: The dissolution of NaOH in water is an exothermic reaction! Hence, it shuld be done gradually, in glass container, and preferably on a magnetic stirrer.
NOTE: Use of glass containers and tools is preferable since this material is much easier to clean than plastic.
It should be kept in mind that the stock solution above reffers to the analytical purity NaOH. When technical grade NaOH is used, the actual quantity of substance has to be increased proportionally, to reach the appropriate NaOH concentration.

The treatment


After the correct amount of powder and the stock solution is prepared, they may be combined. In this step, it is necessary to stirr it on a magnetic stirrer for 15 minutes.
When the reaction time has elapsed, residual NaOH should be washed away. In our lab-scale tests we used the centrifuge, however, for the medium scale process it is more feasible to spread the powder over a sieve, posibly the same one used for smaller size exclusion after milling. Then, clean water may be trickled over the filter for several minutes, until the pH returned to above 6. The entire sieve with powder is then placed into a drying oven and dried at 130 C for 4 hours. The powder can then be recovered into a new recipient and should be kept at a temperature for additional 48 hours, covered with parafilm.

The equipment

Magnetic stirrer is a basic laboratory device, and can be acquired from almost any lab equipment vendor. Considering the sizing of this proces, the stirrer capacity of at least 5 L is necessary. Heating plate is not mandatory. Stirring magnets exist in different sizes, and are not expensive.
The drying oven , being a standard laboratory device, comes in a wide variety of sizes. For drying of powder here, a smaller model with 60 - 80 L capacity is sufficient, however, it must have active ventilation system.

Treatment 5: final calcination
In this step, CaCO3 will transition to CaO and CaOH by reacting with residual oxygen. This will further improve prospects for porosity enhancement
The equipment is the same as used in a prior, first calcination, only this time the temperature is raised to 700 C, to the range where CaCO3 starts to be decarboxylated. Most calcination ovens that can reach 500 C can also reach 700 C. Since on the medium-scale proces the energy cost will be an issue to consider, and the initial powder mass is reduced by this point (due to all the treatments), the operator may decide to hold the treatment until another batch of shell powder reaches this stage in order to conduct a joint treatment.

Treatment 6: Hydrochloric acid wash
In this final step, mild hydrochloric acid (HCl) is applied to corrode the remaining calcitic structure and roughen the pore walls, ultimately resulting in growth of the pore surface area.

Preparation of the acid solution