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flotation cells for profitable minerals concentration - flsmidth

flotation cells for profitable minerals concentration - flsmidth

The ideal flotation machine for your mining operations will deliver high recovery and grade with easier start-up, simpler operation, lower reagent consumption, longer mechanism life and less required maintenance. TheWemco 1+1 is just that solution.

At the heart of each Wemco 1+1 cell is a patented rotor-disperser that delivers intense mixing and aeration. Ambient air is drawn into the cell uniformly distributed throughout the pulp, providing optimum air/particle contact. In larger cells, a false bottom and draft tube channel slurry flow, ensures high re-circulation and eliminates your need for sanding. Also available as the InertGas design for use in copper/moly separation resulting in lower reagent consumption and eliminating the need for nitrogen supply.

A simple and rugged construction make the Wemco 1+1 flotation mechanism extremely reliable and keeps your maintenance to a minimum. A heavy-duty bearing stand supports the motor driven V-belt and shaft assembly. Twin oversized antifriction bearings mounted in cast iron housing maintain accurate shaft alignment, ensuring you get smooth, trouble free rotor operation. The stand pipe, disperser, and hood are suspended beneath the base plate.

The basic module of the Wemco 1+1 flotation system is a cell consisting of a single tank and mechanism. Several cells are bolted together to form a flotation machine with feed, connection and discharge boxes as required. Pulp level in the tanks is controlled by automatically operated dart valves in the connection and discharge boxes. All boxes are elastomer-lined for maximum abrasion resistance.

FLSmidth provides sustainable productivity to the global mining and cement industries. We deliver market-leading engineering, equipment and service solutions that enable our customers to improve performance, drive down costs and reduce environmental impact. Our operations span the globe and we are close to 10,200 employees, present in more than 60 countries. In 2020, FLSmidth generated revenue of DKK 16.4 billion. MissionZero is our sustainability ambition towards zero emissions in mining and cement by 2030.

forced-air flotation cell | flsmidth

forced-air flotation cell | flsmidth

Flotation is about creating the proper energy dissipation rate in the cells to obtain optimal contact between the air bubbles and the particles for extracting the minerals. The function of the rotor/stator is to make bubbles from the forced air, suspend the particles, and create an environment for bubbles and particles to make contact and rise to the top as froth for concentration and collection.

Our forced-air flotation design features a streamlined, high-efficiency rotor that works as a very powerful pump. Working together the stator, these components generate an energy-intensive turbulence zone in the bottom of the cell. The forced-air design allows for control of the air flow. The well-defined turbulence zone results in multiple passes of unattached particles through the highest energy dissipation area of the cell where fine particles are driven into contact with the air bubbles.

The stator design, in addition to providing good separation of the cell zones, also serves to redirect the rotor jet uniformly across the tank. This allows dispersion, or distribution, of the maximum amount of air into the cell without disturbing the surface an important consideration for fine particle recovery. The air dispersion capabilities of our Dorr-Oliver cell design exceed all competitive forced-air designs.

By containing the intense circulation energy at the bottom of the cell, the upper zones of the cell remain quiescent, or passive, to maximise recovery of marginally attached coarse particles and minimise the carriage of undesired material.

We have equipped our forced-air flotation tank cells with a uniquely designed, high-efficiency radial launder system that accelerates froth removal as it reaches the surface. Bubble-particle aggregates travel vertically through the froth lattice. The high-efficiency radial launder is shaped to receive the froth uniformly from the cell surface, as well as from the typically heavy-loaded area near the centre of a forced-air machine. On passing over the lip, the froth accelerates to the perimeter of the cell. This unique design rarely requires launder water.

The two factors having the strongest impact on a flotation circuits performance are metallurgical recovery and flotation cell availability. Our forced-air flotation machines provide superior performance in both of these important areas, while offering additional, distinct advantages.

Superior metallurgical performance: Intense recirculation in a well-defined mixing zone multiplies the chances of contact between mineral particles and air bubbles, providing for greater mineral recoveries and higher concentrate grades.

Greater availability: Non-clogging design of the rotor reduces maintenance requirements, minimising failure, and increases availability. Our flotation mechanisms also can be removed for maintenance without process interruption.

Low reagent costs: Air is a natural reagent in the flotation process. Having a wide air dispersion capability permits you to fine-tune your flotation plant to deliver the optimum value for your process.

FLSmidth provides sustainable productivity to the global mining and cement industries. We deliver market-leading engineering, equipment and service solutions that enable our customers to improve performance, drive down costs and reduce environmental impact. Our operations span the globe and we are close to 10,200 employees, present in more than 60 countries. In 2020, FLSmidth generated revenue of DKK 16.4 billion. MissionZero is our sustainability ambition towards zero emissions in mining and cement by 2030.

focus on: optimising deink plant quality and yield - tissue world magazine

focus on: optimising deink plant quality and yield - tissue world magazine

Tissue World Magazine is operated by a business or businesses owned by Informa PLC and all copyright resides with them. Informa PLC's registered office is 5 Howick Place, London SW1P 1WG. Registered in England and Wales. Number 8860726.

Office sources of high quality fibre have been hit by increased work-from-home across the world. Here Mark Christopher, global market development manager tissue, Buckman, explains how deink plants can turn lower quality fibre into a viable option.

With the recent global shutdowns and work from home edicts being applied across vast swaths of the global economy, the sorted office pack (SOP) waste stream which is preferred by tissue makers has been significantly impacted. The reduction in office-based work has resulted in much less SOP being created and available for collection.

This reduction in supply is tempered somewhat by the concurrent reduction in demand for AfH tissue products, which is the predominant user of recycled fibre streams like SOP. Many companies began a significant stockpiling of SOP furnish during the early stages of the shutdowns in anticipation of potential shortages which created a tighter supply early and is now resulting in an easing of demand as they begin to draw down their inventory.

The situation is highly variable and other producers are finding it difficult to source SOP or they can only do so at elevated prices. This then forces them to consider less uniform and more highly contaminated sources of recycled fibre. Moving to lower quality recycled furnishes puts pressure on the deink plant to maintain brightness, dirt and stickies levels without sacrificing yield.

Doing so relies on three key functions of the deink plant: detachment of ink from the fibre, separation of the ink and contaminants from the fibre stream, and finally rejection of the ink and contaminants. This paper will examine these key process steps and provide guidance on how to optimise them.

Ink detachment predominately takes place in the pulper. The bales of collected paper are placed into the pulper along with clarified process water, and mechanical energy is applied to create rotor-to-fibre and fibre-to-fibre friction that separates them from each other as well as separating the coatings and inks from the fibre surface. This is critical because it is not possible to remove ink through the rest of the process unless it has been first detached from the fibre. Along with fibre exiting the pulper, there are two types of ink:

It stands to reason that the more ink we can detach from the fibre, the higher the potential final brightness that can be achieved. Driving detachment in the pulp is not a linear benefit curve due to two detrimental impacts of longer pulper times:

2. More time in the pulper does not just detach more ink but serves to break down the ink, coating and contaminants into smaller and smaller particle sizes. This in turn makes the removal of the particles across screening and flotation more difficult. The water soluble and the very smallest of these ink particles can actually be absorbed into the lumen of the fibres along with the water as they hydrate and swell. (1) This results in a darkening of the fibre that is irreversible.

1. It acts as a penetrant drawing water with it. This speeds up disruption of the surface coatings and the hydrogen bonding structure of the base sheets, facilitating the breakdown of the bales into a pulp slurry.

Having detached the ink from the fibre, we must now separate it, and other contaminants, from the fibre stream. Deinking plants are designed with various processes to remove all types and sizes of contaminants.

They are typically set up in cascading banks such that rejects are re-screened/re-cleaned in several steps to further ensure concentration of the contaminants and reduce yield loss. Particles that have a specific gravity greater or lower than water can be effectively rejected with cleaning stages, but this does not include ink and many stickies.

Low consistency slotted screening is considered to be the best approach for stickies removal by screens, and this is the approach most mills employ (3). These screens will be able to separate most contaminants including stickies which are larger than the nominal slot size, but ink and stickies smaller than this threshold will pass through. Even larger stickies particles can extrude through screen holes and slots that are smaller than the stickies particle itself (6).

Another methodology often applied to address these stickies in the deink plant is with enzymatic technology that breaks down and changes the surface characteristics of the stickies, rendering them more hydrophilic.

This approach has been successfully applied for over 20 years in the recycled operations of pulp, packaging and tissue mills. Because enzyme technology is specific, other chemistries and operations are not impacted. Operational benefits include increased ink detachment and significantly reduced stickies counts in the pulp as shown in the mill application below (Firgure 2).

The flotation deinking process works by using air bubbles floated through a pulp suspension. Hydrophobic contaminants like ink and stickies within the pulp slurry preferentially attach to these bubbles as they rise to the top of the suspension. This results in a concentration of the undesirable contaminants within a foam layer at the top of the flotation cell.

Let us consider the bubble/contaminant interaction: As a bubble approaches a particle to within a certain minimum distance, which we denote as the critical gap, gc, then it is assumed that strong attractive forces take over via a range of methods whereby bubble/particle attachment takes place as depicted in the Figure 3 (4).

Proper optimisation and operation of the air system and its injectors is important in order to ensure high specific air volume. Bubble structure and stability in the cell froth, which is rejected, impacts yield as well. Highly stable foam bubbles do not allow floated fibre to drop down through the foam froth and back into the cell for acceptance. When no surfactant is employed, the air bubbles still float ink, but the resulting froth is much more stable. This often has the effect of driving lower cell level set points which reduces ink rejection or more fibre carryover which reduces yield.

Modern deinking surfactants are intended to not just improve the repulping process but carry through to the flotation cells where they contribute to more efficient collection of ink by the air and a foam froth that breaks down more easily. The effect is the ability to get higher brightness at lower equivalent yield loss as shown in the Figure 4.

An often ignored benefit of flotation is the highly effective removal of stickies from low quality pulp. We have already explained how the basic mechanism of flotation is one driven by hydrophobic forces, and stickies are highly hydrophobic particles. Application of the correct deinking surfactant in the cells improves stickies collection and rejection in these systems. It has been shown by Tinna Sarja that upwards of 80% of microstickies can be removed via flotation (7).

Specific air volume (SAV) is defined as the volume of air per unit mass of solids in the flotation cell feed, generally expressed as cm3/gms or L/Kg. We can see that changing either the air volume or the cell consistency will result in a change in the SAV. Depending on the floatation equipment design, the operator may or may not have the ability to impact air volume, but all operations can dictate targeted consistency of operation of the cells.

The importance of this will become apparent as we examine how to most effectively concentrate the ink and other contaminants in the reject stream such that we can maintain brightness at lower overall yield loss.

It has been shown by Peters and Romigio (5) that improved concentration of ink can be obtained at high flotation cell operating consistencies as long as the SAV can be increased along with it. In other words, maintaining and improving ink removal at higher flotcell consistencies requires ever larger volumes of air.

Although ash losses via flotation are not as sensitive to cell consistency, applying the approach above, which increases SAV, will increase ash loss via flotation. This is generally less of a concern for the tissue maker because it is overridden by the benefits of the overall pulp quality, and the reduced ash content in itself provides benefits.

One or more washing stages may be present within a deink plant layout. A washing stage in a deink plant is technically just a thickening stage, although depending on the plant configuration, a clarified water source may be used to dilute the stock just prior to the thickening stage. In washing, ink and contaminant removal efficiency is a function of the percent increase in consistency across the stage. A washerthat takes a pulp slurry from 1% consistency to 2% consistency has removed 50% of the water. If we assume the ink was evenly dispersed in the water and no mat filtering occurred, we would expect a 50% ink removal. Of course, depending on the design of the equipment, different amounts of mat filtering are exhibited, and as such larger particles of ink and contaminants are trapped within the mat and not rejected here.

1. Reducing inlet consistency. Almost invariably, lower inlet consistency into a washing stage will result in greater consistency increase across the stage (greater fraction of ink separated) as well an improvement in the removal at the larger particle end of the removal spectrum.

Having detached and separated the contaminants from the pulp substrate, we must now reject them from the system. Some stages, like secondary flotation rejects, may be sent directly to effluent. Other reject streams may employ a thickening stage(s) to maximise water recovery and reuse and minimise disposal costs of the rejects fraction. This is commonly done via a DAF operation treated with a polymeric program to maximise the quality of the reclaimed clarified water stream.

The DAF rejects themselves may be sent to a further thickening stage such as a screw press. These systems are often referred to as the kidneys of the deink plant. It is important to monitor and control the quality of the clarified water streams as they will be reintroduced into your system.

When water systems are contaminated with ink or stickies, problems with quality and operational efficiency will result. The objective is for the contaminants to leave mill with minimal loss of fibre or water.

With careful attention to detachment, separation and removal of contaminants, lower quality fibre sources are now a viable option thus easing the burden of the constant search for higher quality fibre. A properly operated deink plant relies on a good design, proper operation of the equipment and the proper use of chemistry.

3. I Jokinen H, mml A, Virtanen JA, Lindroos K & Niinimki J (2006) Effect of bar geometry on screen plate performance a laboratory study on pressure screening. Nordic Pulp and Paper Research Journal 21(4): 451-459.

4. RONGGEN PAN, FRITZ G. PAULSEN, DONNA A. JOHNSON, DOUGLAS W. BOUSFIELD, AND EDWARD V.THOMPSON. Global model for predicting flotation efficiency Part 1: model results and experimental studies. VOL. 79: NO. 4 TAPPI JOURNAL.

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