PASTE & FILTERED TAILINGS
accurate and realistic values are typically obtained from these pilot units which contribute in establishing the underflow solids concentrations at specific bed retention times / bed depths required . Pilot tests although more extensive and involved , do provide significantly more confidence in thickener sizing requirements , and expected thickener results .
Rheology Test Work A rotational viscometer such as the Haake Viscotester ® 550 is used for vane yield stress measurements . To measure the yield stress , the speed of the viscometer is held at a constant speed ( RPM ), resulting in a constant shear strain rate ( s-1 ). The torque is measured , and the maximum is used to calculate the yield stress .
Thickeners are classified by their underflow slurry rheology , not by their underflow suspended solids concentration . Therefore , it is important to measure both the unsheared rheology , which is a measurement of the material in the bottom of the thickener at the underflow discharge , and the sheared rheology , which is a measurement of the material after being sheared in the underflow pumps or a specialised shear thinning system . Typically , the vane yield stress for a sample is tested over a range of test concentrations . However , for a given particle size distribution , mineralogy , and flocculation there is a strong relationship between suspended solids concentration and rheology .
Haake Viscotester ® 550 rotational viscometer
Coagulation and Flocculation Basic Mechanisms
Coagulation A slurry consists of fine and coarse solids in a liquid media . The coarser particles settle faster according to Darcy ’ s Law , so the main issue is capturing the fine particles that are typically carried to the thickener overflow . Fine particles
in suspension can either form a stable or unstable suspension . The stable suspension is where the fine particles remain suspended , are dispersed within the solution , and are not naturally coagulating . The unstable suspension is where the fine particles coagulate , forming larger particle agglomerates without assistance and settle out naturally . This can be attributed to the different forces acting on the fine particles . There are repulsive electrostatic forces and there are attractive Van der Waals forces and whichever force dominates in the system will dictate if the fine particles settle or remain suspended . If the attractive Van der Waals forces dominate and overcome the repulsive electrostatic forces , the fine particles will naturally coagulate and form an agglomerate with enough mass to settle . If the repulsive electrostatic forces dominate , the fine particles will remain suspended .
The fine particles need to be captured in an agglomerate to increase their mass and settle efficiently . This is achieved through either coagulation and / or flocculation . The key aspect of coagulation and flocculation is to destabilize these suspensions and promote the settling of particles and thereby allow solids-liquid separation . Coagulation is the process of destabilising a suspension through the addition of salts or polymers by reducing , neutralizing , or inverting of the electrical repulsion between particles . Flocculation is the process of agglomeration of smaller particles into larger size floccules that can settle out of the suspending liquid more rapidly through the addition of polymers which adsorb onto particles and form bridges between particles . In other words , chemistry can be used to collapse the repulsive forces and destabilize the colloidal suspensions to promote settling .
In some cases , coagulation is sufficient to achieve fast enough settling however , most cases require a further step of flocculation which binds the particles together through long chain polymers to form larger structures with greater mass . Most polymer coagulants are positive cationic and adsorb onto the surface of the negatively charged particles . This reduces the surface potential of the particles by cancelling out charges . Flocculation typically uses a negative anionic flocculant which has long chains to capture many particles into floccules . The flocculants attract the positive cations of the fine particles .
Flocculation In thickeners , many factors influence flocculation of a slurry including slurry solids concentration , flocculant concentration , polymer / flocculant type , mixing energy ( shearing ), addition location , number of addition locations , pH , and temperature . Proper feed dilution and flocculant concentration are important as the objective is to achieve equal distribution of the polymer within the slurry . It is counterintuitive that water addition generates faster settling and higher underflow solids concentration . However , through feed concentration , which may require dilution to be at an optimum and flocculant dilution , an optimal environment for flocculation is achieved by increasing the probability of solids-flocculant interaction . A typical undiluted thickener feed slurry concentration of 30 % m is typically diluted to approximately 10 % m for optimum flocculation , but the actual diluted concentration depends on the application and particle size distribution . Slurry feed dilution can be achieved by either diluting in the thickener feed tank , diluting in the feed pipe or trough , or diluting in the thickener feedwell . The flocculant system typically prepares the flocculant to ~ 0.5 % concentration and it should then be further diluted to ~ 0.025 % inline at the edge of the thickener prior to introduction into the feedwell . The polymer type ( molecular weight , charge ) for specific applications is typically established in test work .
An important factor in flocculation is the mixing energy experienced in the system . There has to be sufficient mixing energy to promote interaction but not excessive so that the floccule chains are broken or sheared . If the flocculant is sheared , it loses its effectiveness in forming agglomerates and releases fine particles into suspension again . Performance improves with increasing mixing energy until too much agitation is introduced and the floccules begin to shear and break up . Flocculant addition is typically recommended in the feedwell however , can also be introduced in the thickener feed tank or the feed pipe / trough or in multiple locations . It depends on the application and system . In other words , if the floccules are robust or prone to shearing easily or if a longer amount of time is required for polymer-solids interaction to flocculate and capture the fines effectively .
Conclusion
Thickeners are highly involved and complicated , more so than most people appreciate . Understanding the test work , process chemistry , and thickening basics upfront is critical in selecting the thickener type and size , its design features , its operating philosophy , and establishing its performance . Thickeners are usually the last unit operation people address during process design and are treated as a ‘ black box ’. Thickeners are more crucial than thought of in the industry and their design and operation are necessary for consistent and stable plant control .
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P14 International Mining SUPPLEMENT | JULY 2022