Rotterdam 57/ Gravel processing on the water

Rotterdam 57 for gravel processing on the water

A floating screening and washing installation

With the production of the sand classification plant ‘Rotterdam 55′, depending on the mining area, an amount of gravel, grain size >4 mm, is released. In the past, we decided not to process the gravel simultaneously with the sand on board but to store the unwashed and unsorted gravel in an underwater depot within the mining location so that it could be processed separately on a gravel processing plant. That’s why sand and gravel ships can be loaded independently from each other. By storing the unprocessed gravel during the sand production and loading it into the underwater depot the gravel processing installation ‘Rotterdam 57′ can work independently from the sand production at all times.

Mining process
The floating installation ‘Rotterdam 57’ is located above the underwater depot, which uses a large depot crane to move the unwashed and unsorted gravel from the depot into a storage silo onboard of the installation. The grapple of the crane has a content of 6 ton and is positioned through a location determination system through which the gravel can be located underwater and where the gravel has been mined before. The gravel is then transported from the storage silo through the sorting and washing part of the installation.

Screening process

The produced gravel is specifically intended for the concrete and asphalt industry. The gradations which are used can be separated into 2 main groups, fine gravel with a grain separation between 4-16mm and coarse gravel with a build up between 4-32mm. The screening with the right gradation happens on board with a 3-tier tumbler screen. By using the adjustable valve there is a choice between the 2 upper screening decks so that the maximum demanded grain size of 32 or 16mm can be chosen. During the screening process there is always a residue product, during the production of the 4-32mm, a gradation 32-Xmm (X stands for the largest size which can be dredged by the suction dredger, usually a maximum of 100mm) and with the production of 4-16mm,gradation 16-Xmm. The 16-Xmm is then added to the rough gravel in the underwater depot so that it can be used again with the production of 4-32mm. The gradation 32-Xmm is not suitable for further production and is put in a separate depot through which it can be used as shore protection or gravel for the use in gabions.

Washing process

Besides the sorting, the cleaning of gravel is one of the most important processes on board of the plant. For this process, two gravel washers are placed, the first one is a pre-washer in which the clay residue is removed, next to that for the next screening process the second washer is placed and adjusted for the smaller clay particles and pre-historic wood which is rinsed and taken out. The second to last step is to get the washed and screened end product right before the end of the process through a so called after wash screen and then the clean end product is loaded onto the ships.


Depending on the grain composition of the rough gravel, the Rotterdam 57 has a production capacity of a 100 ton/hour gradation 4-16mm and 140 ton/hour 4-32mm. With a work schedule of 60 hours, an average production is possible of 6.000 ton gravel which is transported in a sustainable way through the Dutch waterways to our clients.