Alginate production methods
1. Sodium alginate
"Alginate" is the term usually used for the salts of alginic acid, but it can also refer to all the derivatives of alginic acid and alginic acid itself; in some publications the term "algin" is used instead of alginate. Alginate is present in the cell walls of brown algae as the calcium, magnesium and sodium salts of alginic acid. The goal of the extraction process is to obtain dry, powdered, sodium alginate. The calcium and magnesium salts do not dissolve in water; the sodium salt does. The rationale behind the extraction of alginate from the seaweed is to convert all the alginate salts to the sodium salt, dissolve this in water, and remove the seaweed residue by filtration. The alginate must then be recovered from the aqueous solution. The solution is very dilute and evaporation of the water is not economic. There are two different ways of recovering the alginate.
The first is to add acid, which causes alginic acid to form; this does not dissolve in water and the solid alginic acid is separated from the water. The alginic acid separates as a soft gel and some of the water must be removed from this. After this has been done, alcohol is added to the alginic acid, followed by sodium carbonate which converts the alginic acid into sodium alginate. The sodium alginate does not dissolve in the mixture of alcohol and water, so it can be separated from the mixture, dried and milled to an appropriate particle size that depends on its particular application.
The second way of recovering the sodium alginate from the initial extraction solution is to add a calcium salt. This causes calcium alginate to form with a fibrous texture; it does not dissolve in water and can be separated from it. The separated calcium alginate is suspended in water and acid is added to convert it into alginic acid. This fibrous alginic acid is easily separated, placed in a planetary type mixer with alcohol, and sodium carbonate is gradually added to the paste until all the alginic acid is converted to sodium alginate. The paste of sodium alginate is sometimes extruded into pellets that are then dried and milled.
These essentials of the process are illustrated in the flow diagram in Figure 32.
The process appears to be straightforward, certainly the chemistry is simple: convert the insoluble alginate salts in the seaweed into soluble sodium alginate; precipitate either alginic acid or calcium alginate from the extract solution of sodium alginate; convert either of these back to sodium alginate, this time in a mixture of alcohol and water, in which the sodium salt does not dissolve.
The difficulties lie in handling the materials encountered in the process, and to understand these problems a little more detail of the process is required.
To extract the alginate, the seaweed is broken into pieces and stirred with a hot solution of an alkali, usually sodium carbonate. Over a period of about two hours, the alginate dissolves as sodium alginate to give a very thick slurry. This slurry also contains the part of the seaweed that does not dissolve, mainly cellulose. This insoluble residue must be removed from the solution. The solution is too thick (viscous) to be filtered and must be diluted with a very large quantity of water. After dilution, the solution is forced through a filter cloth in a filter press. However, the pieces of undissolved residue are very fine and can quickly clog the filter cloth. Therefore, before filtration is started, a filter aid, such as diatomaceous earth, must be added; this holds most of the fine particles away from the surface of the filter cloth and facilitates filtration. However, filter aid is expensive and can make a significant contribution to costs. To reduce the quantity of filter aid needed, some processors force air into the extract as it is being diluted with water (the extract and diluting water are mixed in an in-line mixer into which air is forced). Fine air bubbles attach themselves to the particles of residue. The diluted extract is left standing for several hours while the air rises to the top, taking the residue particles with it. This frothy mix of air and residue is removed from the top and the solution is withdrawn from the bottom and pumped to the filter.
The next step is precipitation of the alginate from the filtered solution, either as alginic acid or calcium alginate.
Flow chart for the production of sodium alginate
2. Alginic acid method
When acid is added to the filtered extract, alginic acid forms in soft, gelatinous pieces that must be separated from the water. Again flotation is often used; filtration is not possible because of the soft jelly-like nature of the solid. If an excess of sodium carbonate is used in the original extraction, this will still be present in the filtered extract so that when acid is added, carbon dioxide will form. Fine bubbles of this gas attach themselves to the pieces of alginic acid and lift them to the surface where they can be continuously scrapped away. The processor now has a jelly-like mass of alginic acid that actually contains only 1-2 percent alginic acid, with 98-99 percent water. Somehow, this water content must be reduced. It is too soft to allow the use of a screw press. Some processors place the gel in basket-type centrifuges lined with filter cloth. Centrifuging can increase the solids to 7-8 percent and this is sufficient if alcohol is to be used in the next step of converting it to sodium alginate. It is also now sufficiently firm to be squeezed in a screw press. The 7-8 percent alginic acid is place in a mixer and, allowing for the water contained in the alginic acid, enough alcohol (usually ethanol or isopropanol) is added to give a 50:50 mixture of alcohol and water. Then solid sodium carbonate is added gradually until the resulting paste reaches the desired pH. The paste of sodium alginate can be extruded as pellets, oven dried and milled.
3. Calcium alginate method
When a soluble calcium salt, such as calcium chloride, is added to the filtered extract, solid calcium alginate is formed. If the calcium solution and filtered extract are mixed carefully, the calcium alginate can be formed as fibres - bad mixing gives a gelatinous solid. This fibrous material can be readily separated on a metal screen (sieve) and washed with water to remove excess calcium. It is then stirred in dilute acid and converted to alginic acid, which retains the fibrous characteristics of the calcium alginate. This form of alginic acid can be easily squeezed in a screw press. A screw press with a graduated-pitch screw is usually used; the squeezing action must be applied very gradually, otherwise the material will just move backwards and out of the press. The product from the screw press looks relatively solid but still contains only 20-25 percent alginic acid. However, it is dry enough to form a paste when sodium carbonate is mixed with it to convert it to sodium alginate. Sodium carbonate is added to the alginic acid in a suitable type of mixer until the required pH is reached, then the paste is extruded as pellets, dried and milled.
The disadvantage of this second method, compared to the alginic acid method, is that an extra step is added to the process. The advantage is that the handling of the fibrous calcium alginate and alginic acid is much simpler and alcohol is not needed. Alcohol is expensive and while it is usually recovered and recycled, recovery is never 100 percent, so its use adds to the costs.
Other important factors in alginate production are colour control of the product, water supply and waste disposal.
If the original seaweed is highly coloured, e.g. Ascophyllum, the alkaline extract will also be highly coloured and the process will eventually yield a dark product that commands only a low price as it is limited to use in technical applications. Lighter coloured seaweeds, such as Macrocystis, yield a lighter coloured alginate suitable for food and other applications. Colour can be controlled by the use of bleach - sodium hypochlorite - that is added to the filtered alkaline extract or even to the paste at the final conversion stage. Care must be taken, since excessive bleach can lower the viscosity of the alginate, reducing its value. Sometimes the seaweed is soaked in a formalin solution before it is extracted with alkali. The formalin helps to bind the coloured compounds to the cellulose in the cell walls, so much of the colour is left behind in the seaweed residue when the alkaline extract is filtered.
Large quantities of water are used in the process, especially when diluting the thick (viscous) initial alkaline extract to a viscosity suitable for filtration. A plentiful and reliable water supply is a necessity for an alginate factory to survive.
Waste waters from filtration are alkaline, they contain calcium from the calcium precipitation (excess calcium gives a more fibrous calcium alginate) and acid from the acid conversion step. In some countries the waste is pumped out to sea. Where environmental concerns are greater, or when water supplies are limited, recycling is not too difficult and its costs may be partly offset by the lowering of the quantity and cost of water used by the factory. A means of disposing of solid wastes - the seaweed residue and used filter aid - must be found. There have been several positive studies reported on the use of this waste to adsorb heavy metals, such as cadmium, zinc and copper, from industrial liquid wastes (e.g. Romero-Gonzalez, Williams and Gardiner, 2001). Attempts to ferment this waste to produce ethanol from the cellulose content appear to be less promising, in economic terms (Horn, Aasen and Oestgaard, 2000).
1.2 Other alginate products
Sodium alginate, produced as described above, is the main form of alginate in use. Smaller quantities of alginic acid and the calcium, ammonium and potassium salts, and an ester, propylene glycol alginate, are also produced. Calcium alginate and alginic acid are made during the calcium alginate process for making sodium alginate; each can be removed at the appropriate stage and, after thorough washing, can be dried and milled to a required particle size. The other salts are made by neutralization of moist alginic acid with the appropriate alkali, usually ammonium hydroxide or potassium carbonate; sufficient water or alcohol can be added to keep the material at a workable consistency; they are processed as described for the paste-conversion method in the calcium alginate process for sodium alginate production.
Propylene glycol alginate, an ester of alginic acid, has different properties and uses from the salts such as sodium alginate. It was first patented in 1947 and has been the subject of further patents as methods for its production were improved. It is made by taking moist alginic acid (20 percent or greater solids) that has been partially reacted with sodium carbonate, and treating it with liquid propylene oxide in a pressure vessel for 2 hours at about 80°C. The product is dried and milled.