Caustic soda is widely used in scrubber applications, but broader distribution of magnesium hydroxide may underpin its uptake in hybrid and closed-loop applications
With washwater discharge curbs and requirements in freshwater shipping promising to expand the market for hybrid and even closed-loop scrubbers, magnesium hydroxide suppliers are trying to fix supply chain bottlenecks to compete with caustic soda.
Caustic soda is a byproduct of the chlorine industry and comes as a clear solution. It has a wide range of applications. “It is among the widest traded chemicals in the world,” says Mareneco business development and marine operations manager Pierre-Alexis Mosnier. For these reasons, most scrubber manufacturers recommend caustic soda, he says.
Besides broad availability, caustic soda has useful properties such as the ability to quickly neutralise the sulphur in the exhaust gas. Negatives include its corrosive nature – it is classified as Class 8 corrosive – and its property of crystallising at 12 C, which means it can freeze in pipes, says Mr Mosnier; hence, heating would be required in cold waters. Additionally, Mr Mosnier notes its pricing is volatile and fluctuates greatly.
Magnesium oxide and sodium bicarbonate through dry scrubbers could be alternatives. But disposal of the solid byproduct in sodium bicarbonate would be a tremendous challenge, says Mr Mosnier.
While magnesium oxide is available as powder and loaded in bags, requiring dilution before use, a promising alternative is magnesium hydroxide, which is available as a suspension and can be directly loaded for use, he says.
Magnesium hydroxide has a pH of 10 so it doesn’t need special handling and freezes at 0 C, but as a suspension it can settle requiring recirculation.
The key issue, however, is availability. Europiren, the supplier of naturally occurring magnesium hydroxide product MagTreat, has sought to address availability of the chemical in marine markets by supplying it through barges. Europiren managing director Henk Don says barges help in providing greater access, since many ports, such as Rotterdam, discourage trucks as they hinder the functioning of cranes. “We now supply to vessels in Rotterdam and Antwerp through barge. We can ship the powder to somewhere close to the port and then make the slurry there. Ever-changing schedules of the vessels can be accommodated,” he says.
The barge can cover all of Europe, he notes. “In three to six months, we will have a more extended supply network, starting into Singapore, China, Korea and also North America,” he adds.
Regarding the stability of magnesium hydroxide, since it is supplied as suspension, Mr Don says the chemical is stable for at least for three months. “We still recommend circulation once per week in the tank,” he says.
There is a possibility that the product can get struck in the pump as a solid. “If the water evaporates, you end up with the solids in the pump. It is standard practice in land industry to flush the system before use. This applies in the marine environment, too,” he adds.
Mr Don says the chemical is more expensive upfront but is cheaper on a per tonne of fuel consumed basis, since 30% less chemical is used, requiring 75% less volume than caustic soda.
Mr Don says magnesium hydroxide is naturally occurring as deposits in the US, Russia, China and other regions. Europiren mines and mills the powder in Russia and manufactures the slurry in six plants.
In synthetic magnesium hydroxide, dolemite or limestone is calcined and reacted with brine to obtain a precipitate. While the solids content in synthetic magnesium hydroxide product is around 40% to 60%, the standard MagTreat slurry has 65% to 75% solids, which means less water, less storage and lower cost.