WO2020165224A1

WO2020165224A1 - Method for dosing quicklime in at least one dosing point in a processing unit

Info

Publication number: WO2020165224A1
Application number: PCT/EP2020/053557
Authority: WO
Inventors: Chris Du Plessis; Hugues LAMBERT
Original assignee: S.A. Lhoist Recherche Et Developpement
Priority date: 2019-02-12
Filing date: 2020-02-12
Publication date: 2020-08-20

Abstract

The present invention relates to a method for dosing quicklime in at least one dosing point (34) of a processing unit (1) wherein a process requiring calcium hydroxide which is to be provided by the dosed quicklime is carried out or wherein calcium hydroxide is consumed as a reactant reagent. The quicklime is dispersed into a flow of aqueous liquid to produce a dispersion. The dispersion is discharged in the processing unit (1) when it still contains available lime in the form of calcium oxide and this calcium oxide is allowed to hydrate further in the processing unit (1). The quicklime can thus be dosed easily and accurately into the processing unit since it does not need to be dosed in dry form into the processing unit (1). Moreover, no voluminous slaker is required to first convert the quicklime into milk of lime before dosing it into the processing unit.

Description

"Method for dosing quicklime in at least one dosing point in a processing unit"

The present invention relates to a method for dosing quicklime in at least one dosing point of at least one processing unit in which calcium hydroxide is consumed as a reactant reagent within the process. The processing unit may be a unit for treating or producing materials. The processing unit is not a slaker wherein the quicklime is only mixed and slaked with water to produce a milk of lime (or slaked lime). Instead, when the quicklime is being hydrated in the processing unit at least a process requiring calcium hydroxide as a reactant occurs therein. This process is thus different from a process for producing milk of lime or slaked lime. The calcium hydroxide present in an aqueous medium of the processing unit reacts in particular with a chemical compound present in the processing unit either in a dissolved or in a solid form to produce another chemical compound different from calcium hydroxide.

The present invention therefore relates to a method for dosing quicklime into units other than slaking units. Many applications in water treatment, ore refining, metallurgical mineral processing circuits, and selective precipitation of ions, require alkalinity which can be provided through the addition of quicklime. Mostly the quicklime is not directly added to the processing unit but it is first converted, in a slaker, into milk of lime. Milk of lime is generally produced by slaking quicklime with a certain amount of water in certain conditions to obtain an aqueous slurry of calcium hydroxide, having a certain solid content. The lime solids content is expressed as the amount of quicklime (expressed as oxide, and including not only the suspended fraction of the quicklime but also the dissolved fraction thereof) on the total amount of quicklime and water. Milk of lime produced by a slaker has always a lime solids content which is higher than 5% by weight of the lime slurry/suspension and which is generally comprised between 5 and 40% by weight of the slurry. Quicklime is prepared from calcining limestone in kilns at temperatures exceeding 900°C. The production of quicklime is well known in the art. After calcination, the quicklime is generally in the form of pebbles. Pebble lime is generally milled before slaking. A milk of lime can be produced by slaking milled quicklime with water in a tank provided with means of agitation. Many processes of slaking quicklime to provide a milk of lime are known in the art.

Users of milk of lime have a choice between buying pre prepared milk of lime or buying quicklime and preparing milk of lime at, or within close proximity of their own plant. Both choices have their drawbacks and advantages and depend for example on the quality of the milk of lime that is required for the specific application, on the availability of water having the required quality for the slaking operation and on logistic constraints.

A sulphide mineral separation unit is a first example of a processing unit wherein lime is used. Sulphide mineral processing circuits typically include an ore crushing and milling stage followed by a mineral separation stage by flotation. The efficiency of the separation process is often pH dependent and lime, in various forms, is typically used as a cost effective pH control agent for such applications.

A base metal hydrometallurgical leaching unit is another example of a metallurgical processing unit which is used for ore types that are not suitable for mineral separation methods. In such processing units, crushed or milled ore is leached using a lixiviant (chemical leaching agent) of which sulfuric acid is the most commonly used. The leaching process is intended to dissolve the mineral elements and then selectively recover valuable metals. Base metal hydrometallurgical leaching units (e.g. copper, cobalt and nickel) typically make use of acid-based leaching processes, from which selective recovery of valuable metals are targeted, followed by subsequent downstream treatment methods to neutralize the acid and remove, by precipitation, gangue mineral elements from solution. The latter often involves the use of lime-based reagents.

A gold leaching unit is another example of a processing unit wherein use is made of lime. In the case of gold cyanidation leaching, after crushing and milling, leaching with cyanide requires a pH control (to a pH higher than 9 and up to 1 1.5), mostly using lime reagents, to maintain the pH at an effectively high level to ensure cyanide is present in the form of CN , both for reagent efficiency and for safety reasons.

Alumina refining from bauxite is another example wherein lime is used for numerous purposes. One processing unit within alumina refineries is used for the causticization reaction of Na2C03 into NaOH. In another processing unit the lime is used to react with alumina liquor to form tri-calcium aluminate used as filter aid.

In many plants or processing units thereof, lime-based reagents are added in several stages of the processing units. For example, in base metal sulphide flotation scenarios lime-based reagents are added directly with the mill feed [e.g. either to the SAG (semi-autogenous grinding) mill or to the ball mill, or to both], and/or during the initial rougher flotation step, and/or at various stages of subsequent flotations steps (i.e. cleaner/scavenger flotation). The lime reagent addition, typically as milk of lime, is split into various dosing points in the processing circuit. This staged addition is used for a number of reasons: to allow for sufficient reaction time of the reagent within the circuit, to avoid too large dosing requirements at any individual dosing point in the circuit, and to increase the precision of the pH control.

Similar split lime-based reagent addition regimes are also often used in gold processing units, with a portion of the lime being added with the ore feed to the milling circuit and subsequent additions immediately prior to, or within the cyanide leaching circuit. Addition of lime in different dosing points also occur in alumina Bayer process refineries, with lime additions being made to various process components within the plant, such as: the pre-desilication unit, the digestion unit, the causticization unit, the filter aid generation unit, the oxalate removal unit, and the filtration unit. In some alumina refineries a portion of the quicklime is added to process units in dry particulate unslaked form, while in other process units lime is added in slaked format as milk of lime.

In hydrometallurgical processing units, staged split dosing of lime is also used in the downstream processing circuit for increasing the pH in different stages to induce precipitation reactions of the various elements that are successively removed from solution in various parts of the circuit.

Lime based reagents are often preferred for use in mineral and hydrometallurgical processing units, over other alkali reagents because of the comparatively lower cost of lime. Calcium oxide (CaO), also called quicklime, does not directly react in most aqueous target applications. In aqueous solutions, CaO is converted into calcium hydroxide Ca(OH)2, by a hydration reaction with water. This hydration reaction causes the decomposition of the quicklime particles and results in a suspension or slurry of calcium hydroxide solid particles, Ca(OH)2 in equilibrium with dissolved Ca²⁺ and OH . In most industrial applications of quicklime, it is dissolved Ca²⁺ and OH that participate in the target reactions. As dissolved Ca²⁺ and OH are consumed by the target reaction, more of the solid Ca(OH)2 dissolves to re-establish the equilibrium between solid Ca(OH)2 and dissolved Ca²⁺ and OH .

Industrial users, as described above (where dosing of the reagent occurs into aqueous reactors/processing units), have a choice of either adding dry quicklime (CaO) directly into the processing unit, or using the reagent in pre-hydrated calcium hydroxide (Ca(OH)2) form. The hydration of CaO to Ca(OH)2 can be conducted either in a hydrator (generating a hydrated product, but with effectively no free water) or in a slaker (generating a milk of lime suspension with typically a water content of 60 - 90 wt.%).

Slaking of CaO quicklime is typically conducted on-site at large industrial scale plants due to the fact that hydrated forms of lime have a significantly larger volume and weight, which negatively impacts the costs of logistics and transport to, often remote, metallurgical processing units. For this reason lime is transported to metallurgical processing application sites in the form of dry particulate quicklime. End-users then either use the particulate lime as such or in a slaked form, after an on-site slaking. On-site hydration (to form a dry Ca(OH)2 hydrate product) is typically not done, mainly due to the high cost of hydrator equipment compared to the cost of conventional slaking equipment, and due to the more complex operation and management of hydrators.

In some processing units, dry particulate quicklime is directly dosed as such. Dosing dry quicklime directly into the processing units is however generally too inaccurate to enable a precise control of the pH. Moreover, dry particulate quicklime is often difficult to be dispersed homogeneously into the processing units.

Since dry particulate quicklime is difficult to be dispersed in water or processing solutions, dry quicklime is sometimes supplied into a metallurgical processing unit by adding it to the ore which is subjected to a milling process. In such a milling process, ore and process water (containing impurities) are mixed and are milled in order to reduce the particle size of the ore before being further processed. Dosing of dry quicklime at this location may cause slaking of the quicklime and thus a raise of the pH of the water in the mill. The pH increase thus occurs at the start of the metallurgical process whilst a precise pH control is required in downstream stages of the process. Dry lime addition at the start of the process circuit makes precise pH control in subsequent parts of the circuit very difficult or even impossible.

The direct addition of particulate quicklime to the water contained in a processing unit also has other disadvantages. Apart from the fact that it is difficult to disperse in the aqueous phase, dry lime dosing directly into aqueous reactor tanks often causes clumping and blockages at the location where the dry quicklime comes into contact with the aqueous phase contained in the processing unit or with water vapour escaping therefrom.

Dry quicklime moreover causes fine lime dust emissions, posing health and safety hazards. As a result of these drawbacks, dry quicklime is generally not dosed directly in processing units that are present in a plant.

Despite these problems, dry quicklime is dosed for some applications into aqueous reactors or processing units, such as in causticization processing units in alumina refineries, wherein a reduced input of water is essential to reduce the overall energy cost of evaporation that would otherwise be required for such applications.

Slaking of lime, and using the resulting milk of lime, overcomes the problems associated with dry quicklime dosing, as described above. Slaking, however, requires significant capital investment into equipment to allow sufficient residence time for the hydration or slaking reaction to occur. In addition, resources are necessary to operate the slakers; and significant power consumption may be incurred when using slakers (e.g. detention slakers, drum slakers, ball mill slakers) and agitated tanks to maintain lime particles suspended in large quantities of water at typical residence times required for slaking, i.e. 10 - 60 minutes.

The use of milk of lime has the disadvantage that both the minimum and the maximum content of the solids contained therein is limited. The upper limit for the solids-to-water ratio in milk of lime is typically determined by the maximum operating temperature of the slaker. The slaking reaction is exothermic and as the quicklime solids-to-water ratio is increased, the slaking temperature increases and eventually reaches the boiling point, with associated operational constraints and safety hazards. Slakers are, for this reason, typically operated at a maximum operational temperature of 85 - 90°C. An additional limit on the quicklime solids-to-water ratio is the viscosity or consistency (pumpability) of the milk of lime. A high quicklime solids-to-water mass ratio (up to 1 parts by weight of solids / 3.5 parts by weight of water) increases both the temperature and the viscosity or consistency of the lime slurry. A high viscosity or consistency causes in particular problems of blockage of grits screens, pumps and pipes. Moreover, a milk of lime with a high consistency can no longer be pumped through piping. Moreover the incorporation of a high viscosity fluid (i.e. high solid content milk of lime) into a low viscosity fluid (slurry of pyrite and chalcopyrite minerals for example) is known to be difficult.

The lower limit for the solids-to-water ratio in milk of lime is determined by the required size of the slaking reactors for the slaking reaction. Since a specific residence time is required for the slaking reaction, the solid content should be sufficiently high to avoid the costs of a too large slaker volumes to avoid after slaking. In practice, in most industrial slaking processing units, the amount of lime solids is limited to 13 to 25 parts by weight per 100 parts by weight of the final milk of lime (13 to 25 wt.% lime solids, expressed as oxide and including also the dissolved lime fraction, on the total weight of the milk of lime).

Finally, the particle size distribution of the slaked lime product, Ca(OH)2, is usually dependent upon the slaking conditions such as the slaking temperature. Since, the particle size distribution is generally not controllable independently of the slaking conditions or the quicklime reactivity, unless ball milling is used to reduce the particle size of the slaked product, the temperature or in other words the maximum solid content of the produced slaked lime slurry, is therefore also limited by the required particle size distribution of the slaked lime.

An object of the present invention is to provide a new method for dosing quicklime in at least one dosing point of at least one processing unit wherein a process requiring hydrated, and more particularly dissolved, quicklime is carried out or wherein, in other words a reaction occurs that consumes calcium hydroxide (either in insoluble or more typically in soluble form, as the ultimate hydration product of quicklime in aqueous solution), in which method the quicklime does not need to be dosed in dry form into the processing and no voluminous slaker is required in the new method to first convert the quicklime into milk of lime before dosing it into the processing unit.

To this end, the method according to the present invention for dosing quicklime in at least one dosing point of at least one processing unit, wherein in particular a process requiring calcium hydroxide which is to be provided by the dosed quicklime is carried out, is characterised in that it comprises the steps of:

- providing a supply of particulate quicklime which comprises available lime as determined in accordance with EN 459-2:2010, including available lime in the form of calcium oxide;

- providing a flow of aqueous liquid through a piping;

- feeding an amount of said quicklime from said quicklime supply at a dispersing location in said flow of aqueous liquid;

- dispersing said amount of said quicklime into this aqueous liquid to produce a dispersion of said quicklime in said aqueous liquid;

- pumping said dispersion into a first pipe of said piping leading to said at least one dosing point; - partially hydrating the available lime comprised in said quicklime in the form of calcium oxide in said piping;

- discharging the said dispersion which still contains available lime in the form of calcium oxide in said at least one dosing point out of said piping in said processing unit; and

- further hydrating the available lime in the form of calcium oxide contained in the discharged dispersion in said processing unit.

Milk of lime is a suspension of calcium hydroxide in water which is in equilibrium with dissolved calcium hydroxide in this water or aqueous media. Producing such a milk of lime requires a relatively large slaker installation, not only to produce lime of milk having a high solids content but especially also to produce lime of milk having a lower solids content. A certain residence time in the slaker is indeed required to hydrate all the available lime in the lime suspension. The processing unit, described this invention, wherein the quicklime is dosed is not a slaker since this processing unit is used to carry out a process which requires calcium hydroxide as reactant (i.e. since the calcium hydroxide is consumed in this processing unit) whilst the hydration process carried out in a slaker only requires quicklime and water and results in calcium hydroxide as the product of the reaction which is not consumed in the slaker.

Instead of dosing dry quicklime directly into a processing unit or instead of producing and dosing a milk of lime in a processing unit with the drawbacks associated to these methods, a dispersion of the quicklime is made in the method of the present invention without hydrating the quicklime completely. In this way, no minimum residence time is needed in the dosing installation. It has been found that without a complete hydration of the quicklime, the quicklime dispersion can be quickly and easily made, without requiring a voluminous and complex slaker, and that the obtained dispersion can also be dosed easily and accurately into the processing unit. Moreover, any quicklime that has not yet been hydrated in the dispersion discharged into the processing unit was found to be further hydrated rapidly and easily in the processing unit to provide the required calcium hydroxide for reaction in the processing unit since the quicklime is already dispersed finely in the dispersion dosed into the processing unit.

The method according to the present invention provides a safer and easier way to dose quicklime into the processing unit with accurate control of the amount of quicklime required for the process carried out in the processing unit. Consequently, the quicklime can be dosed more easily and accurately in said dosing point of a processing unit, or even in different dosing points of a same processing unit or in different dosing points of different processing units, without having to slake the quicklime, and can be transported easily through piping leading to the different dosing points and/or processing units, in contrast to dry powdery quicklime.

In an embodiment of the method according to the invention, the quicklime dispersion is discharged in said processing unit and the available lime in the form of calcium oxide contained in the discharged dispersion is further hydrated in said processing unit without producing milk of lime in said processing unit which has a lime solids content of at least 5 wt.% and preferably without producing milk of lime in said processing unit which has a lime solids content of at least 3 wt.%.

In this embodiment, milk of lime (i.e. fully slaked lime) does not need to be produced, neither in the dosing installation nor in the processing unit itself. The quicklime is thus only dosed in a relatively small amount in the processing unit, and is thus quickly diluted therein, or is consumed immediately by the chemical reaction carried out in the processing unit.

In an embodiment of the method according to the present invention said quicklime has a reactivity characterized by the too, as determined in accordance with EN 459-2:2010 and expressed in units of time, and said dispersion has a residence time in said piping which is less than 3 times the ΐbo of said quicklime, preferably less than twice the ΐbo of said quicklime, more preferably less than 1 .5 time the ΐbo of said quicklime and most preferably less than the ΐbo of said quicklime.

With such short residence times, the available lime present in the quicklime in the form of calcium oxide is only partially hydrated before the dispersion is discharged into the processing unit. This embodiment therefore enables design of the dosing installation as a function of the reactivity of the quicklime which is to be dosed and/or to select a quicklime, based on its reactivity, which is suitable to be dosed with a particular dosing installation.

In an embodiment of the method according to the present invention or according to any one of the preceding embodiments, the amount of quicklime fed into said flow of aqueous liquid and said flow of aqueous liquid are selected to achieve a lime solid content of less than 15 wt.%, preferably less than 12 wt.% in the dispersion which is discharged out of said piping.

An advantage of producing such a suspension with the method according to the present invention is that the dosing installation does not require large slaking tanks to provide the necessary residence time to produce milk of lime which is completely slaked.

In an embodiment of the method according to the present invention or according to any one of the preceding embodiments, the amount of quicklime fed into said flow of aqueous liquid and said flow of aqueous liquid are selected to achieve a lime solid content of at least 12 wt.%, preferably of at least 15 wt.% in the dispersion which is discharged out of said piping.

Producing milk of lime having such high lime solids contents generates a lot of heat during the exothermic slaking reaction and result in a milk of lime which has a relatively high viscosity. An advantage of the method according to the invention is that the quicklime isn’t hydrated completely before being dosed in the processing unit, but is preferably discharged in the form of a mainly quicklime (CaO) dispersed slurry as soon as possible after being dispersed in the flow of aqueous liquid. In this way, the dispersed slurry does not become too viscous in the piping and excessive heat release is prevented due to the uncomplete reaction of quicklime with water in the pipe. Due to the fact that the quicklime does not have to be fully slaked before being discharged out of said piping, an installation for performing the method according to the invention can be kept compact. In particular such an installation does not need to comprise any cumbersome tank wherein a milk of lime has to be stirred vigorously to prevent sedimentation of said milk of lime. Feeding the quicklime into the flow of aqueous liquid and dispersing the quicklime therein can instead be done in a closed in-line mixing/dispersing device so that no lime dust is released.

In an embodiment of the method according to the present invention, or according to any one of the preceding embodiments, said at least one processing unit is a continuous processing unit.

An advantage of dosing the quicklime in a continuous installation is that the lime slurry can be continuously dosed so that the flow of the lime slurry in the dosing installation does not have to be interrupted and no or less regular purging or cleaning steps are required.

In a second aspect the present invention relates to a method for dosing quicklime into at least two dosing points of at least one processing unit, wherein in particular a process requiring calcium hydroxide which is to be provided by the dosed quicklime is carried out. The processing unit may be comprised in particular be a processing circuit. Such a processing circuit is a processing plant comprising several processing units, often arranged in series.

In the known methods slaked lime is produced and stored in a reservoir in a slaker and is continuously circulated through a ring piping to which a number of dosing points are connected. The reservoir acts as buffer to enable to start or stop dosing the milk of lime in a number of these dosing points.

An object of the present invention is however to provide a new method for dosing quicklime into at least two dosing points of at least one processing unit, in particular of a processing circuit, which does not require a slaker for producing first slaked lime which is then dosed in the form of milk of lime in the different installations and which also does not require the dry particulate quick lime to be dosed directly, as dry lime, into the dosing points. Moreover, the new method should enable the ability to vary the amount of lime suspension produced whilst still assuring a good dispersion of the quicklime in the lime suspension.

To this end, the method according to the present invention is characterised in this second aspect of the invention in that it comprises the steps of:

- providing a supply of particulate quicklime comprising available lime as determined in accordance with EN 459-2:2010, including available lime in the form of calcium oxide;

- providing a flow of aqueous liquid through a piping;

- dispersing said amount of said quicklime by means of a centrifugal pump into the aqueous liquid to produce a dispersion of said quicklime in said aqueous liquid;

- pumping said dispersion into a first pipe of said piping leading to said at least two dosing points;

- providing a further flow of aqueous liquid; and

- diluting said dispersion by mixing it with said further flow of aqueous liquid before the dispersion is discharged in said at least two dosing points out of said piping. The use of a centrifugal pump to disperse the quicklime into the flow of aqueous liquid and to pump the obtained suspension into the first pipe enables to obtain immediately a homogeneous dispersion of the quicklime into the aqueous liquid. A drawback of such a centrifugal pump is however that it functions only optimally within a certain range of flow rates. Consequently, the amount of lime slurry produced thereby can only be varied within certain limits. In the method according to this second aspect of the invention this problem is solved by diluting the suspension produced by means of the centrifugal pump before it is discharged into said dosing points. Although the flow rate through the centrifugal pump can only be varied to a limited extent, it is possible to vary the amount of quicklime supplied to the centrifugal pump. It is thus possible to produce lime suspension with no or with a relatively large lime solids content. An advantage of the diluting step is that the suspension produced by the centrifugal pump may have a lime solids content which is considerably higher than the maximum lime solids content of usual milk of lime. Indeed, the lime suspension is diluted shortly after being produced, i.e. before it becomes too viscous or before too much heat is generated.

In case a larger amount of diluted lime suspension is required, the flow rate of the further flow of aqueous liquid can be increased and at the same time the amount of quicklime supplied to the centrifugal pump can be increased to maintain the same solids content in the produced diluted lime suspension, at least if a constant solids content is required. In case a smaller amount of diluted lime suspension is required, for example in case one processing unit or dosing in one of said dosing points is stopped, the flow rate of the further flow of aqueous liquid can be decreased and at the same time the amount of quicklime supplied to the centrifugal pump can be decreased to maintain the same lime solids content in the produced diluted lime suspension. In an embodiment of the method according to the second aspect of the invention, said at least two installations are continuous installations and, when the dosing of quicklime in any one of said installations is stopped, the flow rate of said further flow of aqueous liquid and the amount of quicklime metered into said flow of aqueous liquid is reduced but the flow rate of said flow of aqueous liquid is maintained between said minimum and said maximum value.

In an embodiment of the method of the invention, or according to any one of the preceding embodiments, the quicklime is dispersed in said aqueous liquid by means of a mixing and dispersing device to produce said dispersion. The mixing and dispersing device is preferably an inline mixing and dispersing device. The action of the mixing and dispersing device provides a dispersion of quicklime in aqueous liquid free of agglomerates. By“agglomerate”, is meant particles or wet particles having a particle size superior than 1 mm.

In this way, the quicklime particles are instantly wetted with the aqueous liquid and can thus be dispersed in an efficient way.

In an embodiment of the process according to the present invention or according to any one of the preceding embodiments, said quicklime is dispersed in said aqueous liquid by means of a mixing and dispersing device which comprises a centrifugal pump by means of which said dispersion is pumped into said first pipe of said piping.

The centrifugal pump does not only provide the required pumping effect but it also provides the dispersion of the quicklime particles in the aqueous liquid.

In an embodiment of the method of the invention, the method comprises an additional step of diluting said dispersion before the said step of providing a further flow of aqueous liquid; and the step of diluting said dispersion by mixing it with said further flow of aqueous liquid before the dispersion is discharged in said at least one dosing point out of said piping. An advantage of this embodiment is that the flow of aqueous liquid wherein the quicklime is dispersed can be kept constant. The amount of quicklime can be adjusted by feeding more or less quicklime into this flow of aqueous liquid. The solid content in the final dispersion discharged in the processing unit can be varied by controlling the flow rate of the further flow of aqueous liquid. The required amount of quicklime can also be accurately dosed in each of the dosing points even when dosing to one of the other dosing points is stopped or started.

Preferably, said dispersion is diluted with a dilution factor, expressed as the ratio of the weight of the diluted dispersion over the weight of the dispersion of at least 1 .2, preferably of at least 1 .5, more preferably of at least 2.0, most preferably of at least 2.5 and even more preferably of at least 3.0.

The higher the dilution factor, the more the amount of quicklime dosed in the different dosing points can be finely and precisely varied. A diluted dispersion will also reduce the well-known pH hot spot effect. Those hot spot can locally induce higher pH than expected, thus resulting in the precipitation of some dissolved metals that would normally stay in solution.

Preferably, said further flow of aqueous liquid is introduced in a vessel provided in said piping downstream said first pipe to produce the diluted suspension in said vessel, which vessel preferably has a gas outlet, the diluted suspension being pumped out of said vessel at a predetermined flow rate to each of said dosing points.

The required amount of quicklime can be accurately dosed in each of the dosing points by pumping it out of said vessel even when dosing to one of the other dosing points is stopped or started.

Preferably, said quicklime has a reactivity characterized by the too, as determined in accordance with EN 459-2:2010 and expressed in units of time, and said diluted dispersion has a residence time in said vessel which is less than twice the ΐbo of the said quicklime, preferably less than 1.5 times the ΐbo of the said quicklime, more preferably less than the t6o of the said quicklime and most preferably less than 0.7 times the ΐbo of the said quicklime.

Preferably, the level of said diluted dispersion in the said vessel is controlled by a level sensor to not exceed a predetermined level. The role of the said vessel is to remove any residual gas, to provide dilution of the dispersion pumped out from the dispersion location, and to dispatch the dispersion to various dosing points connected to said vessel through further piping. The purpose of said vessel is not to complete the slaking of said quicklime in the aqueous liquid and therefore the control of the volume level within said vessel is preferably such as to minimize the residence time of said dispersion within said vessel.

Preferably, said piping comprises a plurality of further pipes extending from the said vessel to a plurality of dosing points, said further pipes comprising valves and pumps, said pumps and valves being controlled for dosing said quicklime to at least one selected dosing point.

In an embodiment of the method according to the present invention, or according to any one of the preceding embodiments, said flow of aqueous liquid is led into a mixing chamber wherein said amount of quicklime is fed and mixed with said aqueous liquid, the quicklime being preferably mechanically mixed in said mixing chamber with said aqueous liquid.

Preferably, the liquid contained in said flow of aqueous liquid is pressurised and is sprayed under pressure into said mixing chamber onto the quicklime which is being fed therein.

In this way, wetting of the dry quicklime can be accelerated.

Also preferably, said mixture of quicklime and aqueous liquid is pumped by means of a centrifugal pump out of said mixing chamber into said first pipe and is dispersed by means of said centrifugal pump to produce said dispersion.

The quicklime, which has already been premixed and wetted in the mixing chamber, can be more effectively and better dispersed in the aqueous liquid.

Also preferably, the method comprises the step of diluting said dispersion produced by means of the centrifugal pump, with the flow rate of said flow of aqueous liquid being selected between a minimum and a maximum value, and being preferably kept substantially constant, and the lime solid content, expressed in percent by weight of calcium oxide, in the diluted suspension, being controlled by adjusting the flow rate of said further flow of aqueous liquid and/or by adjusting the amount of quicklime fed into said flow of aqueous liquid.

A centrifugal pump operates optimally within a certain range of flow rates. Consequently, the amount of quicklime in the dispersion pumped by means of the centrifugal pump into the piping is preferably only varied within certain limits. Variation of the flow rate of the flow of aqueous liquid supplied to the centrifugal pump can be avoided or limited in this embodiment by diluting the dispersion pumped the centrifugal pump before it is discharged in said dosing points into said processing unit or units. Although the flow rate through the centrifugal pump is not varied or only to a limited extent, it is possible to vary the amount of quicklime supplied to the dispersing location. It is thus possible to produce dispersion pumped by the centrifugal pump having a solid content selected within a broad range.

In an embodiment of the method of the invention the amount of quicklime fed and said flow of aqueous liquid are selected to achieve a lime solid content of at least 12 wt.%, preferably at least 15 wt.% in the dispersion which is discharged out of said piping prior to the completion time of the slaking reaction of the said quicklime. By“lime solid content” is meant the solid content of (partially hydrated) quicklime fed in the aqueous liquid and including thus both the suspended quicklime and the dissolved quicklime (hydrated quicklime), the amount of quicklime being expressed as calcium oxide.

In an embodiment of the method of the invention said dispersion has a residence time in said piping, optionally including a vessel as described hereabove, which is less than 3 times the ίbo of said quicklime, preferably less than twice the ίbo of said quicklime, more preferably less than 1.5 time the ΐbo of said quicklime and most preferably less the ΐbo of said quicklime.

The smaller the residence time of the dispersion in the piping of the dosing installation, the smaller the viscosity built up and the less heat is generated in the piping. In other words, at high ratios of quicklime feed solids to water, conveyance to a dosing point out of said piping is achieved by ensuring a sufficiently short contact time of water and quicklime, to avoid complete hydration conversion of CaO to Ca(OH)2, thereby preventing a too significant increase in viscosity. Since a dispersion of CaO in water is significantly less viscous than a slurry in which the CaO is converted to Ca(OH)2, the avoidance or prevention of the hydration slaking reaction enables a higher solids content of substantially unreacted CaO to be conveyed in water.

In an embodiment of the method according to the present invention, or according to any one of the preceding embodiments, process solution from said processing circuit or unit is used to provide said flow of aqueous liquid.

An advantage of this embodiment is that no additional water is fed into the processing unit in order to dose the quicklime therein. This is especially advantageous in processing circuits or units where water added to the circuit or unit has to be removed by evaporation. Less water has thus to be evaporated when using the process water for dosing the quicklime into the processing circuit or unit.

Other particularities and advantages of the method according to the present invention will become apparent from the following description of some particular embodiments thereof. The reference numerals used in this description refer to the drawings wherein:

Figure 1 is a schematic drawing showing the different components of an embodiment of a dosing installation for dosing quicklime in a processing unit according to the method of the present invention.

Figure 2 is a schematic drawing of a cross sectional view of an embodiment of an assembly comprising a screw conveyor, a mixing and dispersing device and a centrifugal pump used in the dosing installation for performing the method according to the present invention.

Figure 3 is a schematic drawing showing the different components of another embodiment of a dosing installation for dosing quicklime in accordance with a particular embodiment of the method according to the present invention in different dosing points of different processing units.

Figure 4 is a schematic drawing of a metering installation for providing metered amount of quicklime to the assembly of Figure 2.

The present invention generally relates to a method for dosing quicklime in at least one dosing point 34 of a processing unit 1 . The term processing unit embraces all types of processing units, different from a slaker or a hydrator, wherein a process is carried out and wherein quicklime is consumed, or in other words wherein reactions occur which require calcium hydroxide as reactant. The processing unit 1 can thus be a chemical reactor, a mineral flotation cell, a settling tank, a sedimentation tank, a retention basin, a tailings dam, storage pond, discharge pond, a heap leaching liquor pond, a water treatment or purification installation, etc. Different types of processes can be carried out in the processing unit, in particular the known processes described hereabove. The dispersion of quicklime in aqueous liquid can be used in those processes for a chemical reaction, for example for the causticization reaction of Na2CC>3 into NaOH and CaCC>3, or for controlling the pH of the aqueous medium present in the processing unit.

The quicklime involved in the method according to the invention is characterized by different parameters. A first parameter of the quicklime is the amount of available lime (CaO) in the quicklime, determined in accordance with the standard EN 459-2:2010. According to this standard the available lime content present in the quicklime in the form of calcium oxide (and optionally for a small part in the form of calcium hydroxide when some of the calcium oxide might have been hydrated, for example by contact with the moisture contained in the atmosphere) is determined by putting 0.5 g of the dry quicklime into a sugar solution containing 15 g of sugar in 150 cm³ of deionized water. The sugar solution will hydrated and dissolve the available lime (i.e. the calcium oxide and/or calcium hydroxide) contained in the sample. The resulting mixture is agitated for at least 10-15 minutes to insure complete dissolution of the lime and is then titrated with a solution of hydrochloric acid (HCI 0.5N), phenolphthalein being used as indicator. The alkalinity measured by this titration is then expressed as percent by weight of CaO on the total dry weight of the quicklime, with the dry weight being the dehydrated weight in case some calcium hydroxide is present in the quicklime. The amount of available lime is thus equal to this percentage by weight of the amount of dry, dehydrated quicklime, which is being dosed. Preferably for the method according to the invention, the available lime is superior or equal to 80 wt.% of the quicklime.

The quicklime is supplied in the form of a dry particulate material, in particular a powder. Its dsio value is preferably smaller than 3mm, preferably smaller than 1000 pm. The quicklime is further characterized by its reactivity with water. Such reactivity is characterized by the teo, expressed in units of time, and determined in accordance with EN 459-2:2010. The ΐbo value is the time required for a predetermined amount of quicklime slaked into a predetermined amount of water to reach a temperature of 60°C in minutes. Reactive quicklimes have a ΐbo value smaller than 2 minutes, more preferably smaller than 1 .5 minutes and most preferably smaller than 1 minute. Less reactive quicklimes can have a ΐbo up to 10 minutes. Preferably, quicklime having a ΐbo less or equal than 5 minutes, more preferably less or equal than 3 minutes is used in the method according to the invention. Preferably, quicklime should have a ΐbo of at least 30 seconds.

Figure 1 shows an embodiment of a dosing installation for dosing quicklime in a processing unit 1 according to the method of the present invention. The dosing installation comprises a supply of quicklime 2, a supply of aqueous liquid 21 which can be water or process solution, in particular water removed from processing unit 1 or any other processing unit. By the term“process solution” is meant water or aqueous solution directly taken or recycled from any processing unit within the plant circuit. The dosing installation comprises a piping 24 including a mixing and dispersing device 10 through which the aqueous flow is circulated and wherein quicklime is fed through a screw conveyor 12. The mixing and dispersing device 10 comprises a centrifugal pump 14. The piping 24 comprises a first pipe 23 extending from the mixing and dispersing device to a dosing point in the processing unit 1 . The centrifugal pump 14 has an outlet 19 (shown in Figure 2) through which the centrifugal pump 14 pumps the produced dispersion into the first pipe 23 of the piping 24.

The mixing and dispersing device 10 is shown more into detail in Figure 2 in a cross sectional view. In an embodiment of the invention, a screw conveyor 12 and the mixing and dispersing device 10, including the centrifugal pump 14, form an assembly wherein the components of the mixing and dispersing device 10 are mounted on a same shaft 1 1 driven by a single motor 22.

The quicklime is fed, in particular metered, through an opening, in particular through a small hopper 8 into the mixing and dispersing device 10. The small hopper 8 comprises an outlet connected to the upper part of the assembly which comprises a screw conveyor 12 comprising a helicoidal blade 12a arranged on a first portion 1 1 a of the shaft 1 1 which by rotation brings the quicklime from the hopper 8 into the mixing and dispersing device 10.

The mixing and dispersing device 10 comprises a mixing chamber 13 formed by the inner walls of the mixing and dispersing device 10. The mixing and dispersing device comprises a second portion 1 1 b of the shaft 1 1 on which are arranged a set of radially extending blades 17 providing mixing of the quicklime in the flow of aqueous liquid 15 entering in the mixing chamber 13 through an inlet 16 arranged in the vicinity of the entrance point 12b of the quicklime in the chamber 13 and preferably oriented radially relative to the said shaft 1 1 . Rotation of the blades 17 provides a wetting and mixing of the quicklime in the aqueous liquid coming from the inlet 16 connected to an aqueous liquid supply pipe 33 of the piping 24. Advantageously, the blades 17 extend radially from the second portion 1 1 b of the shaft 1 1 to an extremity at a distance relative to the inner wall of the mixing and dispersing device 10 inferior to a few millimetres, preferably inferior to 2 mm, more preferably inferior to 1 mm or within the tolerance limits to allow rotation of the shaft within the chamber 13 and such as to provide an additional shearing effect on the quicklime particles crossing the mixing and dispersing device 10. In order to add further shearing effect on quicklime particles, the inner walls of the chamber 13 can also be provided with static blades arranged between the blades provided on the shaft (embodiment not represented). The mixing and dispersing device 10 can comprise an inner cylindrical sleeve (35) arranged within the mixing chamber 13 in between the inner wall thereof and the blades 17, the inner cylindrical sleeve being provided with lateral openings (36) through which the flow of aqueous liquid 15a entering in the device 10 through the inlet 16 thereof is sprayed under pressure onto the quicklime which is being mixed in the mixing chamber 13.

In the mixing chamber 13, the blades 17 arranged on the shaft 1 1 can be inclined to draw the mixture of quicklime and water away from the conveying screw 12.

The quicklime is thus mixed and optionally dispersed to some extent in the mixing chamber 13 into the aqueous liquid.

The centrifugal pump 14 is arranged in line and downstream of the mixing chamber 13, opposite to the screw conveyor 12. The centrifugal pump 14 comprises a third portion 1 1 c of the shaft 1 1 which is provided with an impeller 18, i.e. a rotor, comprising blades extending radially from the third portion 1 1 c of the shaft and arranged in a casing 20. The casing 20 is provided with an outlet 19 arranged radially to the shaft 1 1 , in the plane of the impeller 18 and connected to the first pipe 23. In an embodiment, the casing 20 is a volute casing. In the embodiment presented in Figure 2, the casing comprises a diffuser 20a formed by a stator provided with slits through which the mixture is pumped out of the mixing chamber 13. The rotation of the impeller 18 exerts a suction of the dispersion in the mixing chamber 13 of the mixing and dispersing device 10 and a centrifugal force pushing the dispersion against the inner wall of the diffuser 20a which forces the dispersion 15b out of the mixing and dispersing device 10 through the said outlet 19 of the casing 20.

The tip speed of the impeller 18 is preferably comprised between 5 and 50 m/s, more preferably between 10 and 40 m/s. Advantageously, the impeller 18 (rotor) extends radially from the third portion 1 1 c of the shaft 1 1 to an extremity at a distance relative to the inner wall of the diffuser 20a (stator) inferior to a few millimetres, preferably inferior to 2 mm, more preferably inferior to 1 mm or within the tolerance limits to allow rotation of the rotor within the stator and such as to provide a high shearing effect on the quicklime particles which are pumped through the slits of the diffuser 20a.

Preferably, the inner diameter of the mixing chamber 13 is comprised between 0.7 and 5 times, preferably between 1 and 3 times the diameter of the first pipe 23 of the piping through which the dispersion is pumped towards the processing unit 1 . The size of the mixing and dispersing device and the size of the assembly are therefore not cumbersome and can be adapted easily in various locations.

Various assemblies including a screw conveyor, a mixing blades and a centrifugal pump are available commercially such as those provided by the company IKA under the name MHD 2000 and which are available in different models allowing various nominal flow rates, maximum powder feed rates, base power and solid/liquids outlets connections diameters. A similar assembly is disclosed in DE-A-196 29 945, which is included herein by way of reference. Those assemblies are not limitative for the present invention and other providers supply similar assemblies.

In the embodiment of the dosing installation described in relation with the figure 1 , and comprising an assembly such as presented above in relation with the Figure 2, the flow rate of the dispersion 15b supplied to the processing unit 1 should then be within the flow rates for which the centrifugal pump 14 associated with the mixing and dispersing device 10 is designed.

The amount of quicklime supplied to the processing unit 1 can however be accurately controlled by metering the required amount of quicklime by means of a metering installation such as presented in Figure 4. In the metering installation illustrated schematically in Figure 4, the powdered quicklime is stored in a quicklime storage tank 2. The bottom of this tank 2 is provided with a quicklime outlet which is connected to a fall pipe 3 ending in a funnel 4 through which the quicklime falls onto a weighing belt 5. The fall pipe 3 is provided with a shut-off valve 6 and with a rotary feeder 7 which controls the flow of quicklime through the fall pipe 3. Alternatively, the rotary feeder 7 could be replaced for example with a screw feeder or a circle feeder mechanism directly from the tank or silo.

The weighing belt 5 meters the required amount of quicklime into the hopper 8 in line with the screw conveyor 1 2 of the assembly including the screw conveyor 12 and the mixing and dispersing device 10 including the centrifugal pump 14. By controlling the rotation speed of the weighing belt 5, by means of an adjustable motor 9, the required amount of quicklime can be metered accurately into the hopper 8. The hopper 8 could be replaced by a pipe.

An advantage of the use of the centrifugal pump 14 for pumping the dispersion is that the amount of quicklime metered in the flow of liquid can be varied within wide limits, in particular within wider limits than with eductors which only function based on the Venturi effect.

Another embodiment of a dosing installation is presented in Figure 3 and comprises a supply of quicklime 2, a source of aqueous liquid 21 , an assembly including a screw conveyor 12 and a mixing and dispersing device 10 including a centrifugal pump 14 such as presented with the description of the embodiment of Figure 2. The assembly is connected to a piping 24 extending to at least one dosing point 34A, 34B, 34C of at least one processing unit 1 A, 1 B, 1 C. In order to be able to adjust the amount of quicklime supplied to the processing unit 1 A, the dispersion pumped out from the centrifugal pump 14 can be diluted by means of a further flow of aqueous liquid, in particular of water or process water. The piping 24 of the dosing installation shown in Figure 3 further comprises a vessel 25 for diluting the dispersion with a further flow of aqueous liquid provided from a dilution pipe 26 connected to a source of aqueous liquid, for example the same source of aqueous liquid as used for providing the aqueous flow through the mixing and dispersing device 10. The vessel 25 is provided with a gas outlet 27 to remove any residual gas brought during the step of feeding quicklime or produced during the mixing of quicklime with the aqueous flow. The vessel 25 is provided with a mechanical stirring device 28 for assisting the dilution of the dispersion in the further flow of aqueous liquid. The piping 32A between the vessel 25 and the processing unit 1 A is provided with a shut-off valve 30A and with a pump 31 A. The pump 31 A is used to pump the required amount of dispersion out of the vessel 25 into the processing unit 1 A. The vessel 25 itself is further provided with a level sensor 29 by means of which the level of liquid in the vessel does not exceed a predetermined level, in particular by controlling at least the flow rate of the further flow of aqueous liquid supplied to the vessel 25 through the dilution pipe 26, and preferably by further controlling the flow of diluted dispersion flowing out of the vessel 25 and/or the flow of dispersion entering in the said vessel 25.

As discussed here above, the amount of quicklime supplied to processing unit 1 A can be easily adjusted by metering the required amount of quicklime accurately by means metering installation presented with the embodiment of Figure 4. The lime solids content in the diluted dispersion can be controlled by adjusting the flow rate of the further flow of liquid supplied via the pipe 26 and/or by adjusting the amount of quicklime metered into the mixing and dispersing device 10.

In the dosing installation illustrated in Figure 3, the vessel 25 is connected to further processing units 1 B and 1 C by additional piping or conduits 32B, 32C, over shut-off valves 30B and 30C and pumps 31 B and 31 C. By means of the pumps 31 A to 31 C, the dispersion can be pumped at the required flow rate to each of the processing units 1 A to 1 C. Each of the processing units 1 A to 1 C illustrated in Figure 3 has only one dosing point 34A to 34C wherein the dispersion is dosed in the unit. However one or more of the processing units 1 A to 1 C may be provided with more than one dosing point. In that case an additional conduit 32, shut-off valve 30 and an additional pump 31 extends from the said vessel 25 to each additional dosing point in order to be able to dose also quicklime into the additional dosing points.

All of the processing units can be operated continuously, with some possible interruptions for example at night or during the weekends or for maintenance. When the dosing of quicklime in one of the processing units, or in one or more of the dosing points thereof, has to be stopped, the piping can simply be flushed by stopping the supply of quicklime to the mixing and dispersing device 10 whilst continuing providing the flow of aqueous liquid through the piping until the remaining quicklime is evacuated from the piping 24.

As a function of the lime requirement in the process occurring in the processing unit, the dispersion of quicklime can be diluted with a dilution factor of at least 1.2, preferably of at least 1.5, more preferably of at least 2.0 and most preferably of at least 2.5 or even at least 3.0. The dilution factor is defined as the ratio of the weight of the diluted dispersion over the weight of the dispersion as produced by the mixing and dispersing device 10.

In an embodiment of the method, the dosing of quicklime can be varied based on a parameter measured in one or more of the processing units. For example, when the quicklime is used to control the pH in the processing unit, a pH sensor can be provided in the processing unit and the quicklime dosing can be adjusted based on the measured pH value. A pH probe can be connected to a control system which can be configured such as to allow the user to set a desired pH in at least one of the processing units, to monitor the pH in the at least one of the processing units and to control the pH in the processing units by adapting the quicklime feed rate, the flow of aqueous liquid and the dilution factor.

In an embodiment of the method according to the invention, the amount of quicklime fed and the flow of aqueous liquid are selected to achieve a lime solid content of at least 12 wt.%, preferably at least 15 wt.% in the dispersion at the exit of the dispersion location (i.e. at the outlet 19 of the centrifugal pump 14). At high ratios of quicklime feed solids to water, conveyance is achieved by ensuring a sufficiently short contact time of water and quicklime, to avoid complete hydration conversion of CaO to Ca(OH)2, thereby preventing a too significant increase in viscosity. Since a dispersion of CaO is significantly less viscous than a slurry of Ca(OH)2, the avoidance or prevention of the hydration slaking reaction enables a higher solids content of substantially unreacted CaO to be conveyed in water. In this case, the dispersion has a residence time in said piping, optionally including a vessel as described hereabove, which is preferably less than 3 times the ΐbo of said quicklime, preferably less than twice the ΐbo of said quicklime, more preferably less than 1 .5 time the ΐbo of said quicklime and most preferably less the ΐbo of said quicklime. Also in case the dispersion has a lower lime solids content, the residence time in the dosing installation is preferably also relatively short so that the dosing installation can be kept compact. It mainly only requires tubes for guiding the water and the quicklime to the processing units and an inline mixing and dispersing device in the piping formed by the tubes. Dilution can be achieved in a vessel but it can simply also be obtained by two parallel tubes which come together, one of the tubes supplying the further aqueous liquid and the other the dispersion produced by the mixing and dispersing device.

The residence time of the dispersion in the piping 24 can be minimized by keeping the length of the piping sufficiently short or by increasing the linear velocity of the dispersion in the piping 24. This linear velocity can be increased by selecting a pump of high power or by reducing the inner diameter of the piping 24. In order to keep the quicklime dispersed in the aqueous liquid when the dispersion flows through the piping 24 whilst avoiding too much turbulence, the linear velocity of the dispersion and/or the diluted dispersion in the piping is preferably comprised between 1 and 4 m/s. The linear velocity is preferably superior to 2 m/s. These linear velocities apply in particular to the part of the piping 24 extending between the vessel 25 and the different dosing points 34A to 34C of the processing unit or units 1 A to 1 C.

Examples

1 .0. description of the processing unit and quicklime parameters

Some examples are presented here below for a method for dosing quicklime into one or more processing units using a dosing installation according to the embodiment described herein above with reference to Figure 3.

The assembly including the screw conveyor 12, the mixing and dispersing device 10 and the centrifugal pump 14 such as presented in fig. 2 and used therein is the MHD 2000/20 device of IKA which can disperse at most 2.8 m³/h of dry powder in the flow of aqueous liquid. The quicklime is metered in the assembly with the metering installation described in relation with Figure 4. The maximum total flow rate of lime powder and aqueous liquid is equal to 7 m³/h. The maximum suspension feed to the vessel 25 (SFV) is thus equal to 7 m³/h. For a bulk density of the quicklime of 1 100 kg/m³, the maximum throughput of powdery quicklime that can be dispersed by the mixing and dispersing device 10 into the first pipe 23 is equal to 2545 kg/h.

In this process, the aqueous liquid provided to the mixing and dispersing device 10 and to the vessel 25 is water. Powdered quicklime having a dsio smaller than 3 mm is provided in the storage tank 2 and metered by means of the weighing belt 5 into the mixing and dispersing device 10. The produced dispersion is pumped by the centrifugal pump 18 of the assembly into the vessel 25. This vessel 25 has an inner volume of 0.5 m³ but contains only at most 0.1 m³ of (diluted) dispersion, i.e. the maximum volume of dispersion in the vessel 25, VSmax, is set to 0.1 m³, thus limiting the residence time in the vessel. This volume is dependent upon the feed capacity of the dosing installation, so in this case it applies to a system that has quicklime feed intake of about 2 metric tons per hour (maximum 2.8 m³/h).

The vessel 25 is connected to :

- the first processing unit 1 A by a first conduit 32A;

- the second processing unit 1 B by a second conduit 32B;

- the third processing unit 1 C by a third conduit 32C.

Each of the first conduit 32A, second conduit 32B, and third conduit 32C has the same inner diameter d of 40 mm whilst the first pipe 23 has an inner diameter of 65 mm.

Advantageously, the target range of the lime solid content in the dispersion in the first conduit 32A, second conduit 32B and third conduit 32C is comprised between 3 and 30 wt.%, typically around 10 to 18 wt.%.

Advantageously, the target range of linear velocities of the dispersion in the conduits, 32A, 32B, 32C is comprised between 1 and 4 m/s, preferably between 2 and 4 m/s.

In the following examples, the quicklime used has a bulk density BD of 1 100kg/m³, a specific gravity SG of 3300 kg/m³ and a particle size distribution d90 smaller than 1 mm, more preferably smaller than 500pm, more preferably smaller than 200pm. The quicklime has an available lime content of 92 wt.% of CaO.

In the following example, the density of water is approximated to 1000 kg/m³.

1 .1 Process wherein the lime requirement is the same in all of the processing units 1 A, 1 B and 1 C ln a first example of a process wherein lime is provided in the processing units 1 A to 1 C using the dosing installation as described in section 1 .0. , the following input process parameters are introduced in a user interface of the dosing installation:

- the lime requirement R1 in the first processing unit 1 A is 500 kg/h expressed in dry quicklime equivalent per hour;

- the lime requirement R2 in the second processing unit 1 B is 500 kg/h expressed in dry quicklime equivalent per hour; and

- the lime requirement R3 in the third processing unit 1 C is 500 kg/h expressed in dry quicklime equivalent per hour;

- the solid content SCR in the dispersion to be provided in all of the processing units 1 A to 1 C is set to 5 wt.%.

Therefore the total quicklime feed rate required, noted QFR is

1500 kg/h

Knowing that the maximum throughput noted SFV (suspension feed to vessel) of the mixture of water and quicklime lime through the mixing and dispersing device 10, is 7 m³/h, the output parameter of the maximum water flow rate (WFy) of the flow of water introduced into the mixing and dispersing device 10 is calculated by the equation:

Therefore, the calculated maximum flow rate of the water introduced into the mixing and dispersing device 10 WFy = 5.63 m³/h.

Taking into account the required solid content SC in the diluted dispersion provided in the processing units as input parameter, the calculated output parameter of the total water flow rate WFtot to be provided in the piping 24, will be the sum of :

- the maximum water flow rate (WFy) to be supplied to the mixing and dispersing device 10, and - the water flow rate (WFz) to be fed to the vessel for diluting the dispersion according to the solid content SC required in the processing units.

WFtot is calculated from the equation:

rnru _!u 100 - 5 _ _{nn )} 100 - 5 _{c j} 100 - 5

500 kg/h * - g - 500 kg/h * - g - 500kg/h * - g -

1000 kg/m^{3 +} 1000 kg/m^{3 +} 1000 kg/m³

= 28.5 m³/h

Therefore, the water flow rate (WFz) to be provided to dilute the dispersion in the vessel 25 is equal to:

WFtot - Wfy = 28.5m³/h - 5.63 m³/h = 22.87 m³/h.

The suspension volumetric flow rate SFRi to be delivered to a processing unit through each of the conduits extending from the vessel to an processing unit is given by the equation:

In this example, since all of the lime requirements R1 , R2, R3 in each of the processing units are the same,

The total suspension flow rate to be delivered to all of the processing units is given by the equation:

9.65 m³/h + 9.65 m³/h + 9.65 m³ /h = 28.95m ³/h The time of residence of the suspension in the vessel 25 is given by the equation :

3600^*VSmax/SFRtot = 3600^*0.1 m³/28.95m³/h = 1 2.43 s

Knowing the inner diameter d of the conduits extending from the vessel to their respective processing unit, and knowing the suspension volumetric flow rate SFRi for each conduit, it is possible to calculate the velocity of the suspension VSi in each of the conduits between the vessel and the processing unit by the equation:

₄ £££i

Vs =

¹ n d ²

Since all the diameters d of the conduits and the suspension volumetric flow rate to be delivered in the application reactors are the same for this example,

VSi - - 2.13 m/s which is in the target range of linear velocities

of suspension in the conduits 1 A, 1 B and 1 C. For a total flow rate of 28.95 m³/h through the first pipe 23, the linear velocity in the first pipe is equal to 2.4 m/s.

Knowing the linear velocities of the dispersion in the conduits 1 A to 1 C, the time of residence of the dispersion in the vessel 25, and the volumetric flow rate of the dispersion in the first pipe 23 and the cross- section thereof, it is possible to calculate the time of residence of quicklime in the entire piping 24, including the vessel 25, and check if it is sufficiently small compared to the ΪQO value of the quicklime.

1 .2 Process wherein the lime requirement is the same in all of the processing units 1 A, 1 B and 1 C: interruption of lime dosing to some of the processing units

In Example 1 , the lime was dosed in an amount of 500 kg/h to each of the processing units in the form of dispersion having a solid content of 5 wt.%. The dispersion prepared per hour by the mixing and dispersing device 10 contained 1500 kg CaO and 5630 kg water, in total 7130 kg, and had therefore a lime content of 21 .04 wt.%. The dispersion was diluted with 22 870 kg/h of water to produce 30 000 kg/h of diluted dispersion (= 28 500 kg water + 1500 kg of quicklime). The dilution factor was therefore equal to (30 000 kg/h)/(7130 kg/h) = 4.2.

When stopping the dosing of lime in one of the processing units, the amount of quicklime metered into the mixing and dispersing device has to be reduced to 1000 kg/h resulting in 6630 kg/h of dispersion having a lime content of 15.08 wt.% when leaving the mixing and dispersing device. This dispersion has then to be diluted with only 13 370 kg/h of water to produce 20 000 kg/h of diluted dispersion. The dilution factor was therefore equal to (20 000 kg/h)/(6630 kg/h) = 3.0.

When stopping the dosing of lime in two of the processing units, the amount of quicklime metered into the mixing and dispersing device has to be reduced to 500 kg/h resulting in 6130 kg/h of dispersion having a lime content of 8.16 wt.% when leaving the mixing and dispersing device. This dispersion has then to be diluted with only 3870 kg/h of water to produce 10 000 kg/h of diluted dispersion. The dilution factor was therefore equal to (10 000 kg/h )/(6130 kg/h) = 1 .6.

An advantage of diluting the dispersion is thus that quicklime can always be optimally dispersed into the flow of liquid, in particular in a continuous flow of liquid, even when stopping dosing of the quicklime in one or more of the processing units.

2.1 Process wherein the lime requirement is different for each of the processing units 1 A to 1 C

In a second example of a process wherein lime is provided in processing units using a dosing installation as described in section 1 .0. , the following input process parameters are introduced in the user interface of the dosing installation:

- the lime requirement R2 in the second processing unit 1 B is 900 kg/h expressed in dry quicklime equivalent per hour; and

- the lime requirement R3 in the third processing unit 1 C is 1 100 kg/h expressed in dry quicklime equivalent per hour;

- the solid content SCR in the suspension to be provided in all of the processing units 1 A to 1 C is set to 10 wt.%. Therefore the total quicklime feed rate required, noted QFR is

2500 kg/h

Therefore, the calculated maximum flow rate of the water introduced into the mixing and dispersing device 10 WFy = 4.7 m³/h.

- the maximum water flow rate (WFy) to be supplied to the mixing and dispersing device 10, and

- the water flow rate (WFz) to be fed to the vessel for diluting the dispersion according to the solid content SC required in the processing units.

WFtot is calculated from the equation:

22.5 m³ /h

WFtot - Wfy = 22.5 m³/h - 4.7 m³/h = 17.8 m³/h.

The suspension volumetric flow rate SFRi to be delivered to a processing unit through each of the conduits extending from the vessel to an processing unit is given by the equation: 1100 kg/h

10.23 m³//!

3300 kg/m^{3 +} 1000 kg/m³

4.65 m³/h + 837 m³/h + 10.23 m³/h = 23.26 m³/h

The time of residence of the suspension in the vessel 25 is given by the equation :

3600^*VSmax/SFRtot = (3600^*0.1 m³)/(23.26 m³/h) = 15.5 s

In this example the diameter of the conduits have been reduced to the following values in order to achieve a sufficiently high linear velocity:

Conduit 32A: 25 mm

Conduit 32B: 30 mm

Conduit 32C : 40 mm.

In this example, 2.63 m/s

3.29 m/s 2.26 m/s

'

which is in the target range of linear velocities of suspension in the conduits 32A to 32C.

Knowing the linear velocities of the dispersion in the conduits 32A to 32C, the time of residence of the dispersion in the vessel 25, and the volumetric flow rate of the dispersion in the first pipe 23 and the cross- section thereof, it is possible to calculate the time of residence of quicklime in the entire piping 24, including the vessel 25, and check if it is sufficiently small compared to the t6o value of the quicklime.

2.2 Process wherein the lime requirement is different for each of the processing units 1 A to 1 C: interruption of lime dosing to some of the processing units

In Example 2, the lime was dosed in an amount of 500 kg/h, 900 kg/h and respectively 1 100 kg/h to the different the processing units in the form of dispersion having a solid content of 10 wt.%. The dispersion prepared per hour by the mixing and dispersing device 10 contained 2500 kg CaO and 4700 kg water, in total 7200 kg, and had therefore a lime content of 34.72 wt.%. The dispersion was diluted with 17 800 kg/h of water to produce 25 000 kg/h of diluted dispersion (= 22 500 kg/h water + 2500 kg/h of quicklime). The dilution factor was therefore equal to (25 000 kg/h)/(7200 kg/h) = 3.5.

When stopping the dosing of lime in the first processing unit, the amount of quicklime metered into the mixing and dispersing device has to be reduced to 2000 kg/h resulting in 6700 kg/h of dispersion having a lime content of 29.85 wt.% when leaving the mixing and dispersing device. This dispersion has then to be diluted with only 13 300 kg/h of water to produce 20 000 kg/h of diluted dispersion. The dilution factor was therefore equal to (20 000 kg/h)/(6700 kg/h) = 3.0.

When stopping the dosing of lime in the second processing unit, the amount of quicklime metered into the mixing and dispersing device has to be reduced to 1600 kg/h resulting in 6300 kg/h of dispersion having a lime content of 25.4 wt.% when leaving the mixing and dispersing device. This dispersion has then to be diluted with only 9700 kg/h of water to produce 16 000 kg/h of diluted dispersion. The dilution factor was therefore equal to (16 000 kg/h)/(6300 kg/h) = 2.5.

When stopping the dosing of lime in the third processing unit, the amount of quicklime metered into the mixing and dispersing device has to be reduced to 1400 kg/h resulting in 6100 kg/h of dispersion having a lime content of 22.95 wt.% when leaving the MHD device. This dispersion has then to be diluted with only 7900 kg/h to produce 14 000 kg/h of diluted dispersion. The dilution factor was therefore equal to

(14 000 kg/h)/(6100 kg/h) = 2.3.

When stopping the dosing of lime in the first two processing units, the amount of quicklime metered into the MHD device has to be reduced to 1 100 kg/h resulting in 5800 kg/h of dispersion having a lime content of 18.97 wt.% when leaving the MHD device. This dispersion has then to be diluted with only 5200 kg/h of water to produce 1 1 000 kg/h of diluted dispersion. The dilution factor was therefore equal to

(1 1 000 kg/h)/(5800 kg/h) = 1 .9.

When stopping the dosing of lime in the last two processing units, which require the largest amount of quicklime, it has first to be checked whether the amount of water supplied to the MHD device is not larger than the amount of water contained in the dispersion to be dosed in the first processing unit. This amount of water is determined by the following equation:

loo-io

This flow rate is smaller than the amount of water supplied to the MHD device, i.e. smaller than 4700 kg/h. When maintaining the water flow rate of 4700 kg/h, the lime content of the slurry discharged in the first processing unit would be equal to 9.6 wt.% instead of 10%, without any further dilution in the vessel 25.

If such a lower solid content is not allowable, the flow rate of the water supplied to the MHD device can be lowered to 4700 kg/h. With no further dilution, the dispersion discharged in the first processing unit has in this way the required solid content of 10 wt.%.

An advantage of diluting the dispersion is again that the quicklime can always be optimally dispersed into the flow of liquid, even when the flow rate thereof has somewhat to be reduced.

In conclusion, in the described dosing installation and in the described process the dry particulate quicklime is rapidly dispersed into a flow of aqueous liquid that is used as a conveying substance for quicklime. In an embodiment of the process according to the invention, the requirement for high purity slaking water becomes facultative as process water can be used, especially for the water supplied to the vessel 25 for diluting the dispersion. The process water may arise from one or more of the processing units. Some of the aqueous phase contained therein can thus be recirculated over the quicklime dosing installation, and can in particular be recirculated over the vessel 25 thereof. Liquids other than water can be used as the conveyance media, for example alumina refinery liquors containing a mixture of sodium, alumina, caustic and carbonates.

Conveyance of quicklime according to the process of the invention is achieved by maintaining pumpability of the mixed slurry both at high and low quicklime solids to water ratios. A high quicklime content is in particular a quicklime content, expressed as CaO, of at least 12 wt.% whilst a low quicklime content is in particular a quicklime content, expressed as CaO, of less than 12 wt.%. Especially at the high solids contents, the quicklime needs to be discharged sufficiently quickly to avoid too high viscosities.

The hydration reaction of CaO into Ca(OH)2 can be deliberately delayed to keep the dispersion longer pumpable so that the residence time thereof in the dosing installation may be longer. Delaying the reaction can be achieved via a number of mechanisms including the use of additives such as siloxanes or surfactants (such as laurylsulfonate or dodecylsulfonate) or certain carbohydrates (such as gluconates). It may also be achieved by decreasing the reactivity of the quicklime, either through the selection of limestone that yields lower reactivity quicklime or by manipulating the limestone calcination conditions (the conversion of CaCC>3 to CaO) to produce a quicklime product with lower reactivity, as is known in the art.

Preferably, according to the process of the invention, completion of quicklime hydration (and subsequent application reaction) occurs after discharge into the processing unit in which the lime-based reagent is required, rather than being conducted prior to discharge into the processing unit.

Some advantages provided by the invention are summarized here below.

Quicklime, introduced into a water (or aqueous solution or process liquor) conveyance stream, is delivered into a processing unit wherein it is significantly diluted by the larger volume of aqueous phase present therein.

Conveyance of quicklime in this method allows accurate pumping and dosing and avoids dust emissions. Dosing accuracy and ease - precise mass feed of quicklime, combined with water flow rate, allows for accurate dosing in various locations in a plant. No slaking equipment or milk of lime storage tanks is required.

Power consumption, maintenance costs, and thus overall operating costs can be reduced, compared to processes that make use of a slaker or of a hydrator, because there is very little residence time in the piping of the dosing installation, including any vessel contained therein, and no energy or less energy is used to maintain the lime particles suspended in the water after mixing and dispersing.

Health and safety risks are reduced due to the above two points.

The process according to the invention also provides the ability to accurately dose quicklime into a liquid stream at the desired lime rate, irrespective of the liquid flow rate.

Increased reliability is achieved due to the use of individual pipes from the dilution vessel to the separate processing units or dosage points, which means ease of flushing individual pipes and less chance of all dosage points being offline at the same time (as would be the case for a ring main). The dosing installation can thus function continuously so that especially the MHD device need no regular flushing or cleaning, except when the dosing installation is shut off for planned maintenance or shut down cycles.

The MHD device or any assembly comprising a screw conveyor 12 coupled to a quicklime supply, a mixing and dispersing device 10 arranged downstream said screw conveyor and a centrifugal pump 18 arranged downstream said mixing and dispersing device 10, can also be used in combination with a ring main, preferably at low ratios of quicklime solids- to-water, preferably comprised between 3 and 12 wt.%. Such an assembly requires less space, cost and maintenance than a slaker used in combination with a ring main. The ring main starts and ends in particular in the dilution vessel. In this case, the flow rate of the diluting water can be controlled to keep the level of the dispersion in the vessel constant, with the amount of quicklime metered in the MHD being adjusted based on the amount of dilution water to keep a constant solid content in the circulating dispersion. The amount of water supplied to the MHD or assembly being preferably kept constant, or being varied only within certain predetermined limits.

A diluted dispersion will also reduce the well-known pH hot spot effect. Such hot spots can locally induce higher pH than expected, thus resulting in the precipitation of some dissolved metals that would normally stay in solution.

With the present invention, lime utilization efficiency is enhanced, particularly when dosing quicklime conveyed in a dilute format, as this increases the proportion of soluble to solid calcium and thus the utilization efficiency and kinetics in the processing unit.

Claims

1 . A method for dosing quicklime in at least one dosing point (34) of at least one processing unit (1 ), wherein in particular a process requiring calcium hydroxide which is to be provided by the dosed quicklime is carried out or wherein calcium hydroxide is consumed as a reactant reagent, which method comprises the steps of:

- providing a supply of particulate quicklime (2) which comprises available lime as determined in accordance with EN 459-2:2010, including available lime in the form of calcium oxide;

- providing a flow of aqueous liquid (15a) through a piping (24);

- feeding an amount of said quicklime from said quicklime supply (2) at a dispersing location in said flow (15) of aqueous liquid (15a);

- dispersing said amount of said quicklime into the aqueous liquid (15a) to produce a dispersion (15b) of said quicklime in said aqueous liquid;

- pumping said dispersion (15b) into a first pipe (23) of said piping (24) leading to said at least one dosing point (34);

- partially hydrating the available lime comprised in said quicklime in the form of calcium oxide in said piping (24);

- discharging said dispersion (15b) which still contains available lime in the form of calcium oxide in said at least one dosing point (34) out of said piping (24) in said processing unit (1 ); and

- allowing further hydration of the available lime in the form of calcium oxide contained in the discharged dispersion, to occur in said processing unit (1 ).

2. A method according to claim 1 , characterized in that said dispersion is discharged in said processing unit (1 ) and the available lime in the form of calcium oxide contained in the discharged dispersion is further allowed to hydrate in said processing unit (1 ) without producing milk of lime in said processing unit (1 ) which has a lime solids content of at least 5 wt.% and preferably without producing milk of lime in said processing unit (1 ) which has a lime solids content of at least 3 wt.%.

3. A method according to claim 1 or 2 wherein said quicklime has a reactivity characterized by the teo, as determined in accordance with EN 459-2:2010 and expressed in units of time, characterized in that said dispersion has a residence time in said piping (24) which is less than 3 times the ΐbo of said quicklime, preferably less than twice the ΐbo of said quicklime, more preferably less than 1.5 time the ΐbo of said quicklime and most preferably less than the ΐbo of said quicklime.

4. A method according to any one of the preceding claims, characterized in that said at least one processing unit (1 ) is a continuous processing unit (1 ).

5. A method according to any one of the preceding claims, characterized in that it comprises the additional steps of:

- providing a further flow of aqueous liquid; and

- diluting said dispersion by mixing it with said further flow of aqueous liquid before the dispersion is discharged in said at least one dosing point (34) out of said piping (24).

6. A method for dosing quicklime in at least two dosing points (34) of at least one processing unit (1 ), wherein in particular a process requiring dissolved calcium hydroxide which is to be provided by the dosed quicklime is carried out, which method comprises the steps of:

- providing a supply of particulate quicklime (2) comprising available lime as determined in accordance with EN 459-2:2010, including available lime in the form of calcium oxide;

- providing a flow (15) of aqueous liquid (15a) through a piping (24);

- feeding an amount of said quicklime from said quicklime supply (2) at a dispersing location in said flow of aqueous liquid (15a); - dispersing said amount of said quicklime by means of a centrifugal pump (14) into the aqueous liquid (15a) to produce a dispersion (15b) of said quicklime in said aqueous liquid;

- pumping said dispersion (15b) into a first pipe (23) of said piping (24) leading to said at least two dosing points (34);

- providing a further flow of aqueous liquid; and

- diluting said dispersion by mixing it with said further flow of aqueous liquid before the dispersion is discharged in said at least two dosing points (34) out of said piping (24).

7. A method according to claim 5 or 6, characterized in that said dispersion is diluted with a dilution factor, expressed as the ratio of the weight of the diluted dispersion over the weight of said dispersion before the diluting step, of at least 1 .2, preferably of at least 1 .5, more preferably of at least 2.0, most preferably of at least 2.5 and even more preferably of at least 3.0.

8. A method according to any one of the claims 5 to 7, characterised in that said further flow of aqueous liquid is introduced in a vessel (25) provided in said piping (24) downstream said first pipe (23) to produce the diluted suspension in said vessel (25), which vessel (25) preferably has a gas outlet (27), the diluted suspension being pumped out of said vessel (25) at a predetermined flow rate to each of said dosing points (34).

9. A method according to any one of the claims 6 to 8, wherein said quicklime has a reactivity characterized by the ΐbo, as determined in accordance with EN 459-2:2010 and expressed in units of time, characterized in that said diluted dispersion has a residence time in said vessel (25) which is less than twice the ΐbo of the said quicklime, preferably less than 1 .5 times the ΐbo of the said quicklime, more preferably less than the ΐbo of the said quicklime and most preferably less than 0.7 times the ΐbo of the said quicklime.

10. A method according to any one of the preceding claims, characterized in that the said quicklime is dispersed in said aqueous liquid (15a) by means of an inline mixing and dispersing device (10) to produce said dispersion (15b).

1 1. A method according to any one of the preceding claims, characterized in that said quicklime is dispersed in said aqueous liquid (15a) by means of a mixing and dispersing device (10) which comprises a centrifugal pump (14) by means of which said dispersion (15b) is pumped into said first pipe (23) of said piping (24).

12. A method according to any one of the claims 1 to 9, characterised in that said flow of aqueous liquid (15a) is led into a mixing chamber (13) wherein said amount of quicklime is fed and mixed with said aqueous liquid, the quicklime being preferably mechanically mixed in said mixing chamber (13) with said aqueous liquid.

13. A method according to claim 12, characterised in that the liquid contained in said flow of aqueous liquid (15a) is pressurised and is sprayed under pressure into said mixing chamber (13) onto the quicklime which is being fed therein.

14. A method according to claim 12 or 13, characterised in that said mixture of quicklime and aqueous liquid is pumped by means of a centrifugal pump (14) out of said mixing chamber (13) into said first pipe (23) and is dispersed by means of said centrifugal pump (14) to produce said dispersion (15b).

15. A method according to any one of the claims 11 to 13, characterised in that it comprises the additional steps of:

- providing a further flow of aqueous liquid; and

- diluting said dispersion (15b) by mixing it with said further flow of aqueous liquid into said piping (24), before the dispersion is discharged out of said piping (24), and in that the flow rate of said flow of aqueous liquid is selected between a minimum and a maximum value, and is preferably kept substantially constant, and the lime solid content, expressed in percent by weight of calcium oxide, in the diluted suspension, is controlled by adjusting the flow rate of said further flow of aqueous liquid and/or by adjusting the amount of quicklime fed into said flow of aqueous liquid.

16. A method according to claim 15 when dependent on any one of the claims 6 to 9, characterised in that said quicklime is dosed in at least two processing units (1 ), which are in particular continuous processing units (1 ) and, when the dosing of quicklime in any one of said processing units (1 ) is stopped, the flow rate of said further flow of aqueous liquid and the amount of quicklime metered into said flow of aqueous liquid is reduced but the flow rate of said flow of aqueous liquid is maintained between said minimum and said maximum value.

17. A method according to any one of the claims 1 to 16, characterised in that the flow rate of said flow of aqueous liquid (15a) is kept substantially constant.

18. A method according to any one of the preceding claims, characterized in that the amount of quicklime fed into said flow of aqueous liquid (15a) and said flow of aqueous liquid are selected to achieve a lime solid content of at least 12 wt.%, preferably at least 15 wt.% in the dispersion which is discharged out of said piping.

19. A method according to any one of the preceding claims, characterized in that process solution from said processing unit (1 ) is used to provide said flow (15) of aqueous liquid (15a).