General guide to concentrate filtration

What are minerals? 

Minerals are solids that contain valuable elements, such as metals. They have a certain chemical composition as well as a crystal structure. Minerals also occur naturally and can be exploited profitably – these are called ores. The term is purely economical, and just like minerals, ores will never have the exact same composition – there will always be differences even within one ore body. This means that when processing them, one needs to be very aware of what the composition is now, how it can change, and then optimize the process accordingly. 

Concentrator plant  

A Concentrator plant is a facility that separates required minerals from non-essential ones. 

Comminution is the part of the concentration process where the ore is crushed and ground into small particles. Usually, it happens in three main phases: Blasting, crushing, grinding, and classification. Each of these phases serves a specific purpose sequentially in order to ensure the slurry that is fed into the beneficiation stage has the optimal particle size distribution. 

Beneficiation  

Beneficiation is the part of the concentration process where particles are separated from one another by using their properties. In this phase, valuable minerals are separated from non-valuable minerals. 

The end-product of this phase is almost pure ore, called concentrate, which can be further processed to obtain metals. The common methods of beneficiation include flotation, gravity separation and magnetic separation. Thus, the beneficiation is a wet process where the particles are suspended in water. 

Dewatering 

Dewatering is a solids-liquid separation, where the excess liquid is removed from the slurry in order to prepare the concentrate for smelting and transportation. In concentration, process dewatering is a two-part process where the thickening comes before the filtration. 

Thickening removes as much liquid as possible to obtain a thick underflow. This is done as a continuous process that feeds slurry, flocculant and dilution into the feedwell. Solids and liquids settle as different layers by gravity separation and raking. These layers are called underflow and overflow respectively, with both layers being collected for further processing. Most thickeners used for concentration process are called High Rate Thickeners (HRT). 

The filtration area, such as filters, mainly works with the underflow solids from the thickener. The thickened solids can be as high as 68-70% solids by weight, averaging 63-65% solids feed. 

Figure 1 below shows the process flowsheet of the concentrator plant, and how the process before the dewatering is done, as well as after. 

Filtration of mineral concentrates produced in flotation concentrators is an area where Metso has significant experience, with hundreds of filters and successful customer cases delivered over 45 years. Along with the large number of installations is a wide range of product offering, with each innovation reflecting changes and upgrades within industry requirements. The delivered filters include vacuum filters, ceramic disc filter, traditional and fast opening membrane filter presses and Larox tower press filters generally as per the figure below. 

Figure 2 shows how the different filter technologies can be selected through moisture as well as the production requirements. The figure is for a general guideline – the results as well as the technology should always been selected through the filtration tests. 

Mineral concentrates require filtration targeted to transportable moisture limits and reducing transport costs. The Transportable Moisture Limit (TML) is a necessity for the product to be saleable, and filtration technologies are selected based on their ability to reliably achieve this limit.  

The term TML is derived from the solid bulk cargo industries where cargoes such as metal concentrates may appear to be in a relatively dry granular state when loaded. However, they may still contain sufficient moisture to become fluid under the stimulus of the compaction and vibration that occurs during a voyage. 

Cargoes that contain a certain proportion of small particles and a certain amount of moisture may liquefy when the moisture content exceeds the Transportable Moisture Limit (TML). In the resulting viscous fluid state, cargo may flow more easily, increasing the risk of the vessel capsizing. 

It’s crucial to determine if the material is safe for shipping before loading and to provide mariners with accurate information on the Transportable Moisture Limit (TML) and moisture content. The TML is determined by sampling the cargo appropriately and applying one of the IMO-approved test methods.  

To achieve this level of moisture content with different filter technologies, the solids will retain on a filter cloth or ceramic membrane allowing the liquid to pass through.  This concentration process continues until the cake reaches a suitable thickness, bringing the product close to its incompressible limit, and then further reduces the water content in the cake by air displacement. 

Grade 

Low grade has a negative effect from several perspectives. The lower solids Specific Gravity (SG) dilutes the cake and requires lower cake moisture to achieve the TML. The gangue may, in the case of clays, have flake shape particles that create tighter pores, and the surface properties can have a lower wetting angle which also increases the pore pressure.  

Surface chemistry 

The pH required for the separation of concentrates usually means that they gave a high surface charge. This is generally detrimental to filtration, as the best results are obtained when the surface charge is minimized. Where it is not possible to change the surface chemistry, coagulants and flocculants can be effective in improving filtration rates. 

Parameters which can affect filter performance 

There are several parameters that will affect the filter performance, such as mechanical or operational. When the process is upstream, these process conditions have the biggest impact on the filter performance. An example of these conditions include: 

• Particle size distribution (PSD) 

• Solid content in feed 

• Mineralogy: density, shape, surface properties (stability of slurry) 

• Feed temperature 

• Liquid viscosity - composition and temperature 

• Liquid salinity - salt crystals 

• pH 

• Filter aid, e.g., diatomaceous earth, flocculants etc. (from the thickener) 

These parameters, even by themselves, can significantly affect the filter results. It should also be considered that parameters such as mechanical, operational and process working together can make the biggest difference in filtration results. 

During the filtration process, adjusting parameters such as times, pressure, vacuum level, belt speed, rotation speed and more can optimize filtration results. 

As discussed throughout the guide, achieving the optimal moisture content in mineral concentrate filtration is essential for safe and cost-effective transportation. Metso's extensive portfolio of filtration technologies, backed by decades of industry experience, offers reliable solutions tailored to the specific needs of concentrator plants. By carefully selecting and optimizing filtration methods, plants can ensure compliance with transportable moisture limits while reducing costs and improving overall performance. That’s why, together, we are the partner for positive change.