Manganese nodules have been lying on the deep sea soil for centuries before they became known. The first ones to discover the nodules were Murray and Renard, during their scientific cruise between 1872 and 1876 (Murray and Renard, 1876). The Clarion-Clipperton Zone, located between Mexico and Hawaii, appears to have a virtually inexhaustible amount of mineral resources. The manganese nodules consist mainly of man- ganese, but because they also contain metals such as copper and cobalt, the nodules become more and more interesting for our modern society. Another name for the manganese nodules is polymetallic nodules, be- cause of the content of many other metals.
By means of a vertical hydraulic transport system the manganese nodules will be transported upwards from the seabed. Given that the nodules are located at a depth of about 5000 meter, it is plausible that they come into contact with the pipe wall during transport. When a nodule interacts with the wall, abrasive wear can occur. Abrasive wear, or sliding wear, causes the nodule to degrade, or decrease in mass, through the sliding contact with the pipe wall.
The main research question of this Master Thesis is formed as follows:
What is the contribution of abrasive wear to the degradation of polymetallic nodules, in a vertical transport system?
The contribution of abrasive wear is tested on the basis of the equation of Archard (1953). In the MTI laboratory in Kinderdijk various manganese nodules are tested in a specially designed experimental setup. The polymetallic nodules available for this Master Thesis originate from the Clarion-Clipperton Zone (CCZ). The first set of experiments is focused on single nodule degradation. The nodules are weighted before and after a test run, hereafter the nodule mass after a test run is divided by the initial nodule mass, resulting in the normalized remaining mass. Each test run consists of a specific sliding length. With the sliding length and normalized remains mass, a prediction can be made of the material specific wear rate factor, noted as k [m2/N]. The experiments resulted into a value of k = 4.16·10−9 [m2/N]. With this k-factor, in the worst case, 97% of the original nodule mass is remaining after 1000 meters of sliding contact.
Subsequently, a batch of nodules is tested in the experimental setup, with the expectation that the con- tribution of abrasive wear increases to the total nodule degradation, due to the presence of particle-particle interaction. Recently, Blue Mining has conducted trials in Freiberg, Germany, in an abandoned mine shaft. A vertical pipe of almost 130 meters has been placed inside, and the effect of abrasion on a batch of nodules has been observed. During the batch experiments in Kinderdijk, where the batch of nodules was in a horizontal position, it is seen that the contribution of abrasion is greater, compared to the tests in Freiberg.
Using the open-source program OpenFOAM, a vertical turbulent pipe flow is modeled. The pipe has simi- lar dimensions to the pipe of the experimental setup. The simulation was done using a Large Eddy Simulation (LES). By means of a one-way coupling, the data of the flow field is linked to the movement of the simulated particle. Subsequently, point particles were released at various points at the entrance of the pipe. The first series of point particles consists of a diameter of one-third of the pipe diameter, corresponding to the com- mon size of a manganese nodule. The collisions between the point particle and the pipe wall are solved with a Discrete Element Method (DEM), which makes use of a (non-linear) spring-damper system.
To test the sensitivity of the model, point particles of one-sixth and one-tenth of the pipe diameter are also released. The number of collisions per total number of simulations are compared with each other. After the sensitivity analysis it turned out that in 15% of the simulations there was contact with the pipe wall, regardless of the diameter of the particle. When this 15% is compared to the experimental results using single nodules, there is a weight loss of around 0.5%. This holds a weight loss of only a single - to a few - gram(s) for the average polymetallic nodule.
It can be concluded, according to the experimental trials with polymetallic nodules from the Clarion- Clipperton Zone and by means of a one-way coupled CFD-DEM model, that the contribution of abrasive wear results into a (maximum) remaining mass of 97%. However, it is advised to extend the computer model with a two-way (or multi-way) coupling between the liquid and the solids. It is also recommended to conduct vertical abrasive wear tests with nodules, in order to validate the model and gain more insight in degradation behaviour.