SE544632C2 - A collector for froth flotation, a method for producing the collector and the use thereof - Google Patents

Abstract

The present invention relates to a collector for use in froth flotation comprising particles of lignin at a size of 5 μm or less.

Description

A collector for froth flotation, a method for producing the collector and the use thereof Technical field The present invention relates to a collector for froth flotation. Further, the invention relatesto a method for producing the collector, and the use of the collector in froth flotation forseparation of minerals.

Background Flotation is one of the most important and versatile enrichment processes in the miningindustry. lt is a process in which selective separation of especially complex sulphide ores canbe achieved.

The ore to be treated in froth flotation is in general reduced to fine particles by crushing andgrinding, collectively known as comminution, so that the various minerals to as high extent aspossible exist as physically separate grains. This process is known as liberation and isperformed prior to froth flotation process Flotation is done by adding a suspension including small particles ofthe ore material and waterto a container. Reagents are added to the container to, among other things, selectivelyincrease the hydrophobic properties of the mineral or minerals to be separated. Air is suppliedto the container while the content is agitated by mechanical or hydrodynamic means to keepthe mineral afloat. The mineral that has now been made hydrophobic and becomes connectedto the air bubbles and floats to the surface ofthe containerwhere the air bubbles form a froth,rich in the hydrophobic minerals. The froth is then removed from the container.

The reagents used in the flotation process are typically collectors, frothers, and variousregulators (activators, depressants, pH-regulators, etc). Collectors are used for modifying thehydrophobicity of the minerals by adsorption onto the mineral surface. There are differenttypes of collectors conventionally used, whereby xanthates are the most common for sulphideores. Regulators are used to modify the collectors' effect on the minerals and provide moreselectivity regarding which material should become hydrophobic. Frothers are used toincrease the stability of the froth and to prevent the air bubbles from breaking.

Typically, collectors are short alkyl chains (2-6 carbon atoms) terminated by a xanthate orthiocarbonate or other functional groups that will chemisorb or selectively physically adsorbonto the target particle surface. By lowering the surface energy, the collector facilitatesparticle adhesion to the air bubbles during flotation. Collectors are consumed, depending onore properties in the amount of approx. 30-500 g per ton of ore, with an annual consumptiongrowth rate of 2-3%, as a result of the annually processed quantities of ores (billions of tons)that must continuously increase to meet the growing demand for minerals and metals.Xanthates are the most used mineral collectors for sulphide ores, due to their high mineralselectivity and cost effectiveness. Sodium ethyl xanthate is classified as a 'Priority Existing Chemical' in Australia (NICNAS 2000), meaning that its manufacture, handling, storage, use ordisposal may result in adverse health or environment effects. Xanthates alongside otherpetrochemical products cause C02 emissions during production and use.

Using xanthates for the flotation of sphalerite (ZnS) requires the addition of copper sulphate.lf copper sulphate is not added, the result is less efficient flotation separation.

Summary lt is an aim of the present invention to at least partly overcome the above problems, and toprovide an improved collector for flotation. The object of the disclosure is to provide a moreenvironmentally friendly and biodegradable collector with low carbon footprint duringproduction.

This aim is achieved by a collector for use in froth flotation as defined in claim 1. i» The collector comprises particles of lignin at a size of 5 pm or less.

Lignin is an organic material found as a natural element in plants and trees. Lignin is one ofthe most abundant natural organic molecules on earth, exceeded only by cellulose. As anorganic material which can be found abundantly in nature, its use has little to no negativeimpact on the environment. Also, when using lignin, waste material from the forest industrycan be used. Surprisingly, it has been found that using lignin particles smaller than 5 pm as acollector results in an efficient flotation. Tests have shown that using lining particles smallerthan 5 pm as a collector results in a more efficient flotation than using lignin particles of largersizes. The reason for this can be that particles of lignin at a size of 5 pm or less, allows thecollector to be better distributed in the suspension and increases the probability for mineral-collector collision and adhesion, than using lignin of larger sizes.

According to some aspects, at least 50 wt-% of the particles of lignin have a size of 1 pm orless, wherein wt% are percentages of a total weight of lignin.

According to some aspects, at least 50 wt-% of the particles are nanoparticles having a size of600 nm or less, wherein wt% are percentages of a total weight of lignin.

According to some aspects, at least 50 wt-% of the particles have a size of 400 nm or less,wherein wt% are percentages of a total weight of lignin.

Tests have shown that using lignin nanoparticles as a collector results in even more efficientflotation.

According to some aspects, at least 20 wt-% of the collector consists of said particles of lignin,wherein wt% are percentages of a total weight of the collector.

According to some aspects, at least 40 wt-% of the collector consist of said particles of lignin,wherein wt% are percentages of a total weight of the collector.

According to some aspects, at least 60 wt-% of the collector consists of said particles of lignin,wherein wt% are percentages of a total weight of the collector.

Tests show that lignin can be successfully used alone or alongside conventional collectors, e.g.xanthates, depending on the treated ore. Depending on what conventional collector is mixedwith the lignin, different wt-% of the lignin may be preferred. At some wt-%, the efficiency ofthe mixed collectors may be reduced or be equal compared to using only the conventionalcollector. Since lignin is a much more environmentally friendly and cheaper material, it maybe advantageous to use a mixture of lignin and one or more conventional collector even atreduced efficiency compared to using only conventional collectors. Through routine testing,the optimum weight percentage of lignin in the collector mixture may be determined toprovide a collector mixture having the advantages of lignin while reducing the downside ofless environmentally friendly conventional collectors. The desired wt-% may also depend onthe particle size of the lignin particles and the type of/origin ofthe ore to be treated.

According to some aspects, the collector comprises or consists of between 20 and 90 wt% ofthe lignin collector as defined above, wherein wt% are percentages of a total weight of thecollector in admixture with another collector selected from the group comprising or consistingof xanthates, dithiophosphate, and nitrile or any mixtures thereof. ln one aspect, the collectorcomprises of a mixture of between 20 and 90 wt% of the lignin collector as defined above inadmixture with xanthates. These collectors are efficient in their respective applications. Testshave shown that a collector mixture comprising or consisting of lignin and xanthates (50:50wt%) can be more efficient than a use of either collectors alone. As explained above, theefficiency of the collector must be put in relation to the fact that many collectors are notenvironmentally friendly. By mixing the lignin particles with other collectors, it is possible toprovide an efficient collector mix which as a whole has a reduced environmental footprint withalmost equal flotation efficiency.

Lignin and any optionally other collector are present in admixture with each other and do notchemically react with each other.

According to some aspects, said particles of lignin are isolated from lignocellulosic material.Lignocellulosic material is composed ofcellulose, hemicellulose and lignin.

According to some aspects, lignin is extracted from lignocellulosic material using organosolvmethod, especially organosolv fractionation method/process. Successful tests have been doneusing organosolv lignin having particles at a size of 5 um or less as collector in froth flotation.With organosolv lignin is meant particles of lignin which have been isolated fromlignocellulosic material using an organosolv process.

According to some aspects, said particles of lignin are isolated from lignocellulosic material byany of alkali extraction, a kraft process, organosolv fractionation, and/or hydrolysis process.There is a wide range of lignin sources available, including for example jute, hemp, cotton, andwood pulp. The lignin's physical and chemical behaviour is different with respect to the originalsource and extraction method used. The above methods give lignin particles which areadvantageously used as a collector.

According to some aspects, lignin is isolated from any of wood or grass. Extracting lignin fromwood or grass is preferable since their content of lignin is high. Also, the extraction methodsare especially efficient when extracting lignin from wood or grass.

According to another aspect, the aim is achieved by a method for producing the collectoraccording to the invention, as defined in method defined below.

The method comprises or consists of isolating said particles of lignin from lignocellulosicmaterial by a lignin isolation process selected from alkali extraction, a kraft process,organosolv fractionation, and/or hydrolysis process.

According to some aspects, the method comprises isolating said particles of lignin fromlignocellulosic material by organosolv fractionation.

The invention also relates to a use of the collector according to the invention as a collectorduring froth flotation for separation of minerals.

Brief description of the drawings The invention will now be explained more closely by the description of different embodimentsof the invention and with reference to the appended figures.

Fig. 1 shows a SEM picture of microparticles of organosolv lignin.

Fig. 2 shows a recovery versus grade diagram using xanthates as a collector and using ligninparticles as a collector for recovery of copper.

Fig. 3 shows weight percentage recovery of Cu during flotation trials with xanthates and ligninparticles.

Detailed description Aspects of the present disclosure will be described more fully hereinafter with reference tothe accompanying drawings. The device and method disclosed herein can, however, berealized in many different forms and should not be construed as being limited to the aspectsset forth herein. Like numbers in the drawings refer to like elements throughout.

The terminology used herein is for the purpose of describing particular aspects of thedisclosure only and is not intended to limit the invention. As used herein, the singular forms ll H ll a , an” and ”the” are intended to include the plural forms as well, unless the context clearlyindicates otherwise.

Unless otherwise defined, all terms used herein have the same meaning as commonlyunderstood by one of ordinary skill in the art to which this disclosure belongs.

Froth flotation is a process for separating_hydrophobic materials fromxhydrophilic, which process is used in the mining industry for recovery of valuable minerals, such as for instancecopper, zinc and lead containing minerals. A collector is a material that selectively binds to thesurface of mineral particles and imparts hydrophobicity to the mineral particles, and thusenables separation of the mineral particles during the froth flotation.

A collector is a type of reagent used in froth flotation for increasing the hydrophobic propertiesof the mineral particles to be separated. The mineral particles are usually very much largerthan the particles of the collector. The particles of the collector connect to the surface ofminerals. Air bubbles then connect to the hydrophobic part of the collector. lf the surfacecoverage of the mineral particles by the collector particles is large enough, the mineralparticles connect to the air bubbles and lift the mineral to the froth.

A collector is disclosed, the collector being for use in froth flotation comprising particles oflignin at a size of 5 um or less. Lignin is an organic material found as a natural element in plantsand trees. Lignin is one of the most abundant organic polymers on earth, exceeded only bycellulose. As an organic material which can be found abundantly in nature, its use has little tono negative impact on the environment. Also, when using lignin, waste material from theforest industry can be used. Using lignin ofsmaller particle sizes, i.e. smaller than 5 um, allowsit to be better distributed in the target material than using lignin of larger sizes. Thus, smallerparticle sizes result in more efficient flotation.

Figure 1 shows microparticles of organosolv lignin having sizes of less than 5um. The lignin isadapted to be used as a collector in froth flotation. Since the lignin is relatively small, it can beefficiently distributed onto the surface of the mineral particles. The example lignin in figure 1has been isolated from lignocellulosic wood material. With organosolv lignin is meant particlesof lignin which have been isolated from lignocellulosic material using an organosolv process.The organosolv process is well known in industrial paper-making processes.

The organosolv fractioning method may be as follows. Organosolv pretreatment involves thetreatment of lignocellulosic biomass with mixtures of organic solvents in water for itsfractionation into distinct streams at temperatures 160-210 °C. During the process, solventseffectively solubilize lignin and hemicellulose from the lignocellulosic biomass, which areseparated into the pretreating liquor. This method has environmental advantages. A catalystmay be added.

Lignin fractionation may be performed by addition of water or by removal of the solvent inthe pretreatment liquid. after the organosolv method. The ore to be treated in froth floatation is in general reduced to fine particles by crushing and grinding, a process known as com minution, so that the various minerals exist as physically separate grains. This process is known as liberation and is performed before the froth f|oatation process. Depending on thesizes of the grains, as well as the type of ore to be treated, different particle sizes of the ligninmay be used advantageous. For example, 50 wt-% of the particles of lignin may have a size of1 um or less, or at least 50 wt-% of the particles are nanoparticles having a size of 600 nm orless, or a size of 400 nm or less, or any combination thereof, wherein wt% are percentages ofa total weight of the collector.

Lignin particle sizes may be measured by using SEM ”Scanning Electron I\/|icroscopy". Thepicture of figure 1 is an SEM picture.

Tests show that lignin can be successfully used alone or alongside conventional collectors, e.g.xanthates, depending on the treated ore. Depending on what conventional collector is mixedwith the lignin, different wt-% of the lignin may be preferred. At some wt-%, the efficiency ofthe mixed collectors may be reduced or be equal compared to using only the conventionalcollector. Since lignin is a much more environmentally friendly and cheaper material, it maybe advantageous to use a mixture of lignin and one or more conventional collector even atreduced efficiency compared to using only a conventional collector. Through routine testing,the optimum weight percentage of lignin in the collector mixture may be determined toprovide a collector mixture having the advantages of lignin while reducing the downside ofenvironmentally unfriendly conventional collectors. The desired wt-% may also depend on theparticle size of the lignin particles and the type of/origin ofthe ore to be treated.

Successful tests have been performed using a collector comprising 50 wt-% of lignin particlesat a size of 5 um or less and 50 wt-% of PAX (Potassium Amyl Xanthate) in a CuPb-step of theflotation of CuPb-ore, and using a collector comprising of 50 wt-% of lignin particle at a size of5 um or less and 50 wt-% of IBX (Iso bytul xanthate) in a Zn-step of the flotation of Zn-ore.Wt% are percentages of a total weight of the collector.

At least 20 wt-% of the collector consist of said particles of lignin, or at least 40 wt-% of thecollector consist of said particles of lignin, or at least 60 wt-% of the collector consist of theparticles of lignin at a size of 5 um or less. Wt% are percentages of a total weight of thecollector.

There are many conventional collectors. The collector may comprise particles of lignin asdefined anywhere above in admixture with one or more conventional collector, such asxanthates, dithiophosphate, nitrile. These collectors are efficient collectors. Tests have shownthat one effective collector is a collector comprising or consisting of lignin alone or incombination with xanthates, for example at a ratio between 1 to 2 and 2 to 1, or about 1 to 1.As explained above, the efficiency of the collector must be put in relation to environmentalperformance and health and safety aspects. The flotation performance of the mixture isexpected to be improved compared to using only xanthates as a collector.

The method for producing the lignin may have an impact on how well the lignin works as acollector. The particles of lignin may be directly isolated from lignocellulosic material, forexample, from organosolv treated lignocellulosic material. Lignocellulosic material is composed of cellulose, hemicellulose and lignin. The particles of lignin may be isolated from :i lignocellulosic material by any of alkali extraction, a kraft process, organosolv fractionation,and/or hydrolysis process. ln some cases, an additional post-treatment process can be used.

There is a wide range of lignin sources available, including for example jute, hemp, cotton, andwood pulp. The lignin's physical and chemical behaviour is different with respect to the originalsource and extraction method used. The above methods provide lignin particles which mayadvantageously be used as a collector or in a mixture with one or more other collector. Thelignin may be isolated from any of wood or grass. Extracting lignin from wood or grass ispreferable since of the content of lignin is high. Also, the extraction methods are especiallyefficient when extracting lignin from wood or grass.

The organosolv fractionation is a known method used in pulp and paper industry. Organosolvfractionation is, for example, described in an article ”Organosolv Fractionation of SoftwoodBiomass for Biofuel and Biorefinery Applications” written by Christos Nitsos, Ulrika Rova, andPaul Christakopoulos and published 27 December 2017 in Energies-11-00050 by MDPI.

Lignin may be chemically modified. The lignin discussed herein is preferably pure lignin andnot a derivate thereof. ln other words, the lignin is preferably substantially chemicallyunmodified.

The lignin is for example used as particles of a size less than 5 um in powder form or as a liquidsuspension form.

Experiments has been performed to compare the use of lignin particles equal or less than 5um with xanthates as a collector in copper concentrating fractions. The experiments werecarried out in flotation conditions optimized for xanthates. Figure 2 show a recovery versusgrade diagram showing the performance of lignin particle as a collector in comparison toxanthates. Table 1 shows the performance of lignin particle as a collector in comparison toxanthates. The recovery is the same for both collectors. The Cu grade is higher with xanthatesas collector, which were expected since the flotation conditions where optimized forxanthates.

Table 1: Accumulative weight percentage recovery of Cu during flotation trials with normaldoes and 1.5x dose of lignin-based collectors.

Flotation Cu recoveryfraction (% w/w of initial Cu content)Normal dose 1.5x Normal dose 1 0.5 1.02 2.1 4.03 12.6 23.34 47.2 52.15 63.9 67.Normal doses were (g/ton ore): 2-2-1-5-5-5 for the different fractions. pH was adjusted to10.5 prior to flotation and Nasfroth 350 (as a frother) was added during flotation.

Figure 3 shows weight percentage recovery of Cu during flotation trials with xanthates andlignin particles. The recovery was the same for xanthates and lignin particles.

The above discussed lignin is used in a collector for increasing hydrophobicity of ore duringfroth flotation.

Examples Organosolv lignin is used as a collector but the method is not limited to the use of organosolvlignin. The first step is a rougher flotation step. Rougher flotation comprises the step of addingpre-grinded base material into a mixer containing a liquid. The base material comprisesdifferent types of minerals and the goal is to separate some minerals from the base material.Small amounts of collectors comprising lignin particles are added to the mixer. The mixer isadapted to stir the content so that the collector is mixed with the base material. Lime, or otherreagents, is applied to adjust pH to a desired value. The content in the mixer is then moved toa container. Further reagents, such as regulators, activators and the like are then often addedto the container.

The container is adapted to rotate the content and supply gas, typically air, to the content sothat gas bubbles are formed. The gas bubbles then float to the surface of the container andform a froth. Some of the material made hydrophobic by the reagents attach to the gasbubbles and are lifted to the surface. At the surface, the air bubbles form a mineral rich frothwhile leaving hydrophilic particles in the suspension. The froth is then removed from thecontainer. ln this step, the aim is to float materials comprising a larger concentration of desiredmaterials, such as specific minerals or elements.

As another example, reversed flotation is used to float unwanted material to a greater extentand leave the desired material in the container. This can be used for example to removeimpurities, such as sulphur. For example, the concentrated second material is moved to a new container where similar steps are carried out in order to further float the desired materialsfrom the base material. Optionally, further reagents may be added at the new containers. ln order to improve the recovery of the desired material, reagents are added to increase thefloating of the desired material. Large mineral particles often contain a mixture of differentminerals in the same particle. These may still float because there is enough exposed surfaceof the floatable mineral. Regrinding of the larger particles is done to further liberate thevaluable minerals from such particles. Further flotation steps are then needed to separatethese and produce a cleaner final concentrate.

The flotated material is then moved through a sleeve, to separate large particles. The particleswith a satisfactory size are then moved to a new flotation step while the materials deemedtoo large is re-grinded. The concentrated desired material is then usually further floated,preferably at a pH adapted to float a specific material and to depress others, to furtherincrease the concentration ofthe desired material.

The above collector may be used in floatation processes in several fields; for example, mineralprocessing, wastewater treatment, and paper recycling, for example, to de-ink papers.

The present invention is not limited to the embodiments disclosed but may be varied andmodified within the scope of the following claims.

Claims (9)

1. CLAIMS wherein__between 20 and 90 wt% of thecollector consists of said particles of lignin, and wherein the particles are inadmixture with another collector selected from the group com prising xa nthates,dithiophosphate, and nitrile or any mixtures thereof, wherein wt% arepercentages of a total weight of the collector. The collector according to claim 1, where the other collector is xanthates. Use of a collector comprising particles of lignin at a size of 5 um or less as acollector during direct froth flotation for separation of minerals, whereby thelignin is chemically unmodified lignin, whereby reagents are used in the flotationprocess, such as frothers, and regulators. Use of a collector comprising particles of lignin at a size of 5 pm or less as acollector during direct froth flotation for separation of copper, whereby thelignin is chemically unmodified lignin. The use according to claim 3 or 4, wherein at least 50 wt-% of the particles oflignin has a size of 1 um or less or wherein at least 50 wt-% of the particles oflignin are nanoparticles having a size of 600 nm or less, and preferably a size of400 nm or less, wherein wt% are percentages ofa total weight of lignin. The use according to any of claims 3 to 5, wherein between 20 and 90 wt% ofthe collector consists of said particles of lignin, and wherein the particles are inadmixture with another collector selected from the group com prising xa nthates,dithiophosphate, and nitrile or any mixtures thereof, wherein wt% arepercentages of a total weight of the collector. The use according to claim 3, wherein at least 20 wt % consist of said particlesof lignin, preferably at least 40 wt% of the collector consist of said particles oflignin, and most preferably at least 60 wt-% of the collector consist of saidparticles of lignin, wherein wt% are percentages of a total weight of thecollector. The use according to any of claims 3 to 7, wherein lignin is extracted fromlignocellulosic material using organosolv. The use according to any claims 3 to 8, wherein the lignin is isolated from any ofwood or grass.