WO2012095815A1

WO2012095815A1 - Production of calcium carbonate

Info

Publication number: WO2012095815A1
Application number: PCT/IB2012/050174
Authority: WO
Inventors: John Ngoni ZVIMBA; Jean Lubilanji MULOPO; Mlawule Reginald MASHEGO
Original assignee: Csir
Priority date: 2011-01-14
Filing date: 2012-01-13
Publication date: 2012-07-19

Abstract

The invention relates to a process of producing precipitated calcium carbonate. The process includes, in a calcium dissolution step, contacting a solid particulate feedstock, which contains calcium as a calcium compound in a calcium-containing matrix, with hydrochloric acid. At least some of the calcium is thereby dissolved from the matrix and is recovered as an acidic calcium-containing solution. In a neutralisation step, a base is admixed with the acidic calcium-containing solution, thereby obtaining a basic calcium-containing solution. N-methyldiethanolamine is optionally added to the basic calcium-containing solution. In a carbonation step, the basic calcium-containing solution is contacted with carbon dioxide. The carbon dioxide reacts to form bicarbonate which, in turn, reacts with at least some of the dissolved calcium, thereby forming a calcium carbonate precipitate and a resulting calcium-depleted solution. In a calcium carbonate recovery step, the calcium carbonate precipitate is recovered from the calcium-depleted solution as a calcium carbonate product.

Description

PRODUCTION OF CALCIUM CARBONATE

FIELD OF THE INVENTION

THIS INVENTION relates to the production of calciunn carbonate, more particularly precipitated calcium carbonate. The invention provides for a process of producing precipitated calcium carbonate, and extends to a precipitated calcium carbonate product when produced in accordance with the process.

BACKGROUND TO THE INVENTION

Calcium carbonate is of commercial importance in the manufacture of a number of products, including, but not limited to, Portland cement, paint, toothpaste, paper, and pharmaceutical products.

Particularly when used in pharmaceuticals, toothpaste and paper, calcium carbonate having a high grade of purity and specific properties of colour, hardness, particle size and uniformity in shape is desired. It is seldom that naturally occurring calcium carbonate, in the form of limestone, is found to comply with the desired quality requirements, even in its refined form. Limestone also generally contains contaminants which also render it unsuitable for commercial use in relation to the abovementioned products. In fact, calcium carbonate in its natural form is generally considered to be a low value commodity, particularly for these reasons. Instead of using or refining naturally occurring calcium carbonate, precipitated calcium carbonate (PCC) can be manufactured for commercial use. One advantage which PCC holds over naturally occurring calcium carbonate is that it can be manufactured according to desired quality specifications and can therefore be tailored to suit the requirements of a specific application.

Generally, PCC may be manufactured by carbonation of limestone mineral, which is a so-called "direct" process (being applied directly to the mineral) and involves, broadly speaking, the steps of (i) calcination of the mineral (in a high temperature kiln) to obtain calcium oxide, (ii) hydration of the calcium oxide to obtain calcium hydroxide, and (iii) carbonation of the calcium hydroxide to obtain precipitated calcium carbonate.

Alternatively, PCC may be manufactured by means of the so-called "lime soda" process, which involves reacting calcium hydroxide with sodium carbonate to obtain sodium hydroxide and precipitated calcium carbonate. The lime soda process, however, is used mainly for commercial production of sodium hydroxide, with calcium carbonate being formed as by-product. As will be appreciated, the starting materials (or feedstocks) used in the abovementioned known processes for producing PCC respectively comprise calcium carbonate in natural mineral form (limestone) and a relatively pure calcium-containing compound (calcium oxide). The Applicant is, however, aware that large quantities of calcium containing wastes are produced by various industries, including steelmaking, pulp- and papermaking, and water purification industries. The calcium in such wastes is generally present as a calcium compound in a calcium-containing matrix of other materials and thus being of an impure nature. Thus, although these wastes provide a rich and economically attractive source of calcium, they are generally, due to their impurity, not regarded as suitable for use as a starting material for producing calcium carbonate.

The present invention seeks to provide for the production of calcium carbonate, more particularly precipitated calcium carbonate, which is suitable for commercial use, in a commercially viable manner, specifically from calcium-containing wastes in which calcium is contained as a calcium-compound in a calcium-containing matrix.

It is to be noted that, in this specification, the term "calcium" is used in a broad sense and should not be construed as referring only to elemental calcium (Ca), but also as including calcium in ionic (Ca²⁺) form.

SUMMARY OF THE INVENTION

In accordance with one aspect of the invention, there is provided a process of producing precipitated calcium carbonate (CaCOs), the process including

in a calcium (Ca) dissolution step, contacting a solid particulate feedstock, which contains calcium as a calcium compound in a calcium-containing matrix, with hydrochloric acid (HCI), thereby dissolving at least some of the calcium from the matrix, and recovering the dissolved calcium in an acidic calcium-containing solution; in a neutralisation step, admixing a base with the acidic calcium-containing solution, thereby obtaining a basic calcium-containing solution; optionally, adding N-methyldiethanolamine to the basic calcium-containing solution;

in a carbonation step, contacting the basic calcium-containing solution with carbon dioxide, such that the carbon dioxide reacts to form bicarbonate which, in turn, reacts with at least some of the dissolved calcium, thereby forming a calcium carbonate precipitate and a resulting calcium-depleted solution; and

in a calcium carbonate recovery step, recovering the calcium carbonate precipitate from the calcium-depleted solution. The calcium dissolution step, the neutralisation step, the carbonation step and the calcium carbonate recovery step may be carried out under conditions of atmospheric pressure and at ambient temperature. In other words, the process may, in its entirety, be carried out under conditions of atmospheric pressure and ambient temperature. By the phrase "conditions of atmospheric pressure and at ambient temperature" is meant that the process does not involve synthetic or deliberate application or control of pressure and/or temperature, e.g. pressurising, heating and/or cooling operations are not employed. Preferably, all of the steps are therefore carried out while exposed to and susceptible to be influenced freely by atmospheric conditions. It is important to note that changes in temperature and pressure occurring as a matter of course due to carrying out the process of the invention, e.g. generated heat of reaction, are not considered to constitute "synthetic or deliberate application or control of pressure and/or temperature". Typical atmospheric conditions include a pressure of between about 980 millibar (mb) and about 1050 mb and a temperature of between about 15°C and about 30°C, more typically between about 20°C and about 30°C. The calcium compound, i.e. as contained in the calcium-containing matrix, may, in particular, be calcium oxide (CaO).

The process may include, ahead of the calcium dissolution step, a prior step of comminuting the feedstock to a mean particle size <6mm, more preferably <75μηη.

The feedstock may typically comprise solid particulate material from an operation producing the calcium compound as a by-product. For example, the feedstock may be steelmaking slag, such as basic oxygen furnace (BOF) slag, in solid particulate form. Alternatively, the feedstock may comprise solid particulate calcium-containing materials, or wastes, produced by pulp- and papermaking and/or water-treatment operations. These are, of course, merely exemplary and the feedstock is not limited to materials from these sources. In the calcium dissolution step, the contacting of the feedstock with the hydrochloric acid may include admixing the hydrochloric acid with the feedstock.

The dissolved calcium may react with the hydrochloric acid to form a calcium salt, i.e. CaCl2, which results in the acidic calcium-containing solution being a calcium salt- containing solution. The calcium is therefore recovered to and present in the acidic calcium-containing solution as a dissolved calcium salt.

The hydrochloric acid may, in particular, be diluted hydrochloric acid and may have a concentration of between about 0.1 and about 2 mol/l (mol/litre). In accordance with the desired operating conditions of the process, the hydrochloric acid may be at ambient atmospheric temperature.

In the calcium dissolution step, sufficient of the hydrochloric acid may be used such that it is in stoichiometric excess to that which is required for complete dissolution of the feedstock calcium content. In other words, the hydrochloric acid that is contacted with the feedstock in the calcium dissolution step may be used in an acid/[feedstock Ca content] mass ratio which is higher than is stoichiometrically required for virtually complete dissolution of the feedstock calcium content. Typically, the excess to the stoichiometrically required amount of hydrochloric acid may be by about a 40%. In particular, sufficient of the hydrochloric acid may be used so that an acid/[feedstock Ca content] mass ratio of between about 2kg HCI/[kg feedstock Ca content] to about 4kg HCI/[kg feedstock Ca content] initially applies in the calcium dissolution step. The calcium-containing matrix may typically include other metallic compounds, such as magnesium oxide (MgO) and iron(lll)oxide (Fe₂Os). It will be appreciated that such compounds may then also react with and be dissolved by the acid. Such metals would thus typically also be extracted to the acidic calcium-containing solution, also in dissolved salt form. Thus, the acidic calcium-containing solution may comprise a mixture of dissolved calcium salt and other dissolved metallic salts, such as magnesium chloride (MgC^) and iron(lll)chloride (FeCIs).

The base may, in particular, be an alkali, or hydroxide, base. Preferably, the base is ammonium hydroxide (NH OH). A sufficient quantity of the base may be reacted, or admixed, with the acidic calcium- containing solution for the pH of the basic calcium-containing solution to be above 5. More preferably, a sufficient quantity of the base may be reacted, or admixed, with the acidic calcium-containing solution for the pH of the basic calcium-containing solution to be above 9, typically between about 9 and about 1 0, both values inclusive.

It will be appreciated that, when other metal salts than the calcium salt, such as magnesium chloride and iron(lll)chloride as mentioned above, are present in the acidic calcium-containing solution, the base may react, in the neutralisation step, also with at least some of such other metal salts. Such reaction may cause precipitation of the metals, typically as metal hydroxides when the base is ammonium hydroxide. Without wishing to be bound by theory, it is believed that calcium hydroxide (Ca(OH)₂), if formed, will not precipitate, considering that calcium hydroxide is more soluble in water, having a K_sp value of 5.5x1 0^"6, than other metal hydroxides which may be formed during neutralisation, such as Fe(OH)₃, having a K_sp value of 4x 10^"38, and Mg(OH)₂, having a K_sp value of 1 .8x 1 0^{"1 1}. A sufficient quantity of the base may therefore be reacted with the acidic solution for at least some, preferably virtually all of, such other metals which are present in the acidic solution, to precipitate from the acidic solution.

The addition of the N-methyldiethanolamine (MDEA), when it takes place, may be in a quantity which is stoichiometrically equivalent to the quantity of calcium that is present in the basic calcium-containing solution. Such addition may be to increase the efficiency of carbonation of the calcium in the basic calcium-containing solution and therefore control the morphology of the calcium carbonate precipitate. Instead, the MDEA may be added in a catalytically effective quantity, e.g. between about 5 and about 10ml per one litre of basic calcium-containing solution. Such a quantity of MDEA may be particularly effective as carbonation catalyst for a calcium-containing solution having a concentration of 26g per litre.

The contacting of the basic calcium-containing solution with the carbon dioxide may include passing gaseous carbon dioxide through the basic calcium-containing solution. Such passing may, for example, include bubbling the carbon dioxide through the basic calcium-containing solution. The gaseous carbon dioxide may typically be passed through the basic calcium-containing solution and a flow rate of between about 0.3 litres per minute and about 1 .2 litres per minute.

As indicated hereinbefore also, the carbonation step is preferably carried out at atmospheric pressure, as is the case when carbon dioxide is bubbled through the basic calcium-containing solution while the solution is exposed to the atmosphere. Similarly the carbonation step may be carried out at ambient temperature. Thus, the basic solution may typically be at ambient temperature, which is typically from about 15°C to about 30°C, more particularly about 20°C to about 30°C. The carbonation step may be carried out for between about 10 and about 90 minutes, both values inclusive. More preferably, the carbonation step is carried out for between about 20 minutes and about 40 minutes, both values inclusive.

During carbonation, the basic solution may be stirred or agitated, typically at a rate of between 40 rpm and 186 rpm. Recovering the precipitated calcium carbonate may include separating the calcium carbonate from the calcium-depleted solution, e.g. by filtering the calcium-depleted solution.

The process may also include recovering at least some other carbonation products than the precipitated calcium carbonate and, possibly, recycling the recovered products for use in some of the preceding steps. For instance, when the base comprises ammonium hydroxide, the process may further include recovering ammonia from the calcium-depleted solution. In such a case, the process may include an ammonia recovery step. The ammonia recovery step may include heating the calcium-depleted solution, typically by using heat recovered from other steps, thereby obtaining an ammonia-depleted solution. The process may also include recycling recovered ammonia for use in the neutralisation step.

When the process includes recovering ammonia from the calcium-depleted solution, it may further include recycling the ammonia depleted solution for use in the calcium dissolution step. The process may further include at least one of

(i) filtering the acidic calcium-containing solution ahead of the neutralisation step and then carrying out the neutralisation step on the acidic calcium-containing solution filtrate; and (ii) filtering the basic calcium-containing solution ahead of the carbonation step and then carrying out the carbonation step on the basic calcium-containing solution filtrate. Filtering the acidic calcium-containing solution may be required particularly to remove any residual undissolved solid feedstock therefrom.

Filtering the basic calcium-containing solution may be required to remove metal hydroxides, other than an intermediate calcium hydroxide, from the basic solution, which other hydroxides may have formed and precipitated as hereinbefore described.

In accordance with another aspect of the invention there is provided a process of producing precipitated calcium carbonate, the process including,

in a calcium dissolution step, contacting a solid particulate feedstock, which contains calcium as a calcium compound in a calcium-containing matrix, with an acid, thereby dissolving at least some of the calcium from the matrix, and recovering the dissolved calcium in an acidic calcium-containing solution;

in a neutralisation step, admixing a base with the acidic calcium-containing solution, thereby obtaining a basic calcium-containing solution;

optionally, adding N-methyldiethanolamine to the basic calcium-containing solution;

in a carbonation step, contacting the basic calcium-containing solution with carbon dioxide such that the carbon dioxide forms bicarbonate which reacts with at least some of the calcium, thereby forming a calcium carbonate precipitate and a calcium-depleted solution; and in a calcium carbonate recovery step, recovering the calcium carbonate precipitate from the calcium-depleted solution

wherein the calcium dissolution step, the neutralisation step, the carbonation step and the calcium carbonate recovery step are carried out under conditions of atmospheric pressure and at ambient temperature.

The calcium dissolution step, the neutralisation step, the carbonation step and the recovery step may be as is hereinbefore described, with the acid thus preferably being hydrochloric acid.

The invention extends to a precipitated calcium carbonate product, when produced in accordance with the process of the invention.

The invention extends to a precipitated calcium carbonate product when produced in accordance with the process of the invention.

DETAILED DESCRIPTION OF THE INVENTION

The invention will now be described in more detail with reference to the following non- limiting worked example and accompanying drawings.

In the drawings:

FIGURE 1 shows a flow diagram of an installation for carrying out the process of the invention;

FIGURE 2 shows a plot of data on the effect of hydrochloric acid concentration on calcium dissolution, or extraction, efficiencies, from a solid basic oxygen furnace (BOF) slag at ambient temperature and a mean slag particle size of <75μηη, performed in accordance with the calcium dissolution step of the process of the invention;

FIGURE 3 shows a plot of data on the effect of temperature on calcium dissolution, or extraction, from BOF slag having a mean particle size of <75μηη, performed in accordance with the calcium dissolution step of the process of the invention;

FIGURE 4 shows a plot of data on the effect of particle size on calcium and magnesium dissolution, or extraction, efficiencies, from BOF slag at a HCI/Ca ratio of 2.38 kg/kg and ambient temperature, performed in accordance with the calcium dissolution step of the process of the invention;

FIGURE 5 shows a three-dimensional plot of in line temperature and pH readings of BOF slag when contacted and reacted with 1 .69 M HCI at 25 °C (100g BOF slag added at time 0:00), performed in accordance with the calcium dissolution step of the process of the invention;

FIGURE 6 shows a plot of the mass of ammonium hydroxide required for the neutralisation of acidic calcium-containing solution obtained from the calcium- dissolution step (using 1 .17 M HCI) at ambient temperature (100g BOF slag added at time 0:00), performed in accordance with the neutralisation step of the process of the invention;

FIGURE 7 shows a plot of data on carbonation efficiency (%) and pH over time during carbonation of basic solution obtained from the neutralisation step at a CO₂ flow rate of 0.3 l/min and ambient temperature, performed in accordance with the carbonation step of the process of the invention; FIGURE 8 shows a plot of data on the effect of CO2 flow rate on carbonation efficiency (%) at a stirring rate of 186 rpm and ambient temperature, performed in accordance with the carbonation step of the process of the invention;

FIGURE 9 shows a plot of data on the effect of stirring rate on carbonation efficiency (%) at ambient temperature, performed in accordance with the carbonation step of the process of the invention;

FIGURE 10 shows four SEM images, designated A through D, of selected morphologies observed during carbonation experiments: (A) 1 .2 l/min CO2-25°C-180 rpm; (B) 0.3 l/min CO₂-25°C-180 rpm ; (C) 0.3 l/min CO₂-50°C-180 rpm ; (D) 0.3 l/min CO2-25°C-40 rpm, performed in accordance with the carbonation step of the process of the invention;

FIGURE 1 1 shows inline temperature and pH recordings during carbonation of the basic solution at a CO2 flow rate of 0.3 l/min and ambient temperature, performed in accordance with the carbonation step of the process of the invention; and

FIGURE 12 shows a plot of the pH evolution of 40 ml of water harvesting gas from heating of 300 ml of filtrate from the carbonation stage, performed in accordance with the ammonia recovery step of the process of the invention.

Referring to the drawings and particularly to Figure 1 , reference numeral 10 generally indicates an installation for carrying out the process of the invention.

The installation 10 includes a comminution stage 12, constituted by a crusher at 12.1 to which a calcium-containing solid feedstock material inlet line 14 leads. The installation 10 further includes a calcium dissolution stage 16 comprising a dissolution reaction vessel 16.1 that is open to the atmosphere. A comminuted feedstock material transfer line 18, leads from the crusher 12.1 to the dissolution vessel 16. An acid inlet line 20 also leads into the dissolution vessel 16.1 .

The installation 10 also includes a neutralisation stage 22 comprising a neutralisation reaction vessel 22.1 that is open to the atmosphere. A calcium-containing acidic solution transfer line 24 leads from the dissolution vessel 16.1 into the neutralisation reaction vessel 22.1 , passing through a first intermediate filtering stage 25 comprising a first filter 25.1 . An alkaline base feed line 26 also leads into the neutralisation reaction vessel 22.1 .

The installation 10 further includes a carbonation stage 28 comprising a carbonation reaction vessel 28.1 that is open to the atmosphere. A calcium-containing basic solution transfer line 30 leads from the neutralisation reaction vessel 22.1 to the carbonation reaction vessel 28.1 , passing through a second intermediate filtration stage 32 comprising a second filter 32.1 . A carbon dioxide feed line 34 also leads into the carbonation reaction vessel 28.1 . Additionally, an N-methyldiethanolamine feed line 36 leads into the carbonation reaction vessel 28.1 .

An intermediate product line 38 leads from the carbonation reaction vessel 28.1 to a third filtration stage 40 comprising a third filter 40.1 .

From the third filter 40.1 , a main product line 42 and a by-product line 44 respectively lead to a main product treatment operation (not illustrated) and to a regeneration stage 46 comprising a regenerator 46.1 . From the regenerator 46.1 , a regenerated gaseous product line 48 and a residual liquid product line 50 lead respectively to the alkaline base feed line 26 and to the acid feed line 20. In use, a calcium-containing feedstock, such as a BOF slag, is fed into the crusher 12.1 along a feedstock feed stream conveyed along feed line 14. In the crusher 12.1 , the feedstock is comminuted to a desired particle size, preferably being a mean particle size between 75μηη and 6mm, and is then transferred along transfer line 18, as a feedstock transfer stream, to the dissolution vessel 16.1 .

In the dissolution vessel 16.1 , the comminuted feedstock is contacted with an acid, preferably hydrochloric acid, preferably being diluted hydrochloric acid at a concentration of between about 0.1 mol/L and about 2 mol/L, with the acid being fed to the vessel 16.1 as an acid feed stream along feed line 20. Calcium contained in the feedstock is then dissolved and thus extracted from the feedstock in the dissolution reaction vessel 16.1 , being recovered as an acidic calcium-containing solution.

The acidic calcium-containing solution is then transferred along transfer line 24 as an acidic calcium-containing solution stream to the neutralisation reaction vessel 22.1 , first passing through the first filter 25.1 in which residual solid material which may be contained in the acidic calcium-containing solution is filtered therefrom and withdrawn from the filter 25.1 along a residue line 23 as a residue stream.

In the neutralisation reaction vessel 22.1 , the acidic calcium-containing solution is treated with an alkaline base, typically ammonium hydroxide, which is fed to the vessel 22.1 along feed line 26 as an alkaline base feed stream, and is admixed with the acidic calcium-containing solution. This causes the pH of the acidic calcium-containing solution to rise and thus become a basic calcium-containing solution. The basic calcium-containing solution is then transferred along transfer line 30, as a basic calcium-containing solution transfer stream, to the carbonation reaction vessel 28.1 , first passing through the second filter 32.1 in which any solid precipitates which may have formed in the neutralisation reaction vessel 22.1 (possibly in the manner hereinbefore described) are filtered out and withdrawn from the second filter along withdrawal line 33 as an undesired precipitate stream.

In the carbonation reaction vessel 28.1 , carbon dioxide, and optionally MDEA, is introduced into the basic calcium-containing solution, causing at least some of the calcium to become carbonated and precipitate as calcium carbonate, thus rendering a mixture of the calcium carbonate precipitate and a calcium-depleted solution.

The mixture of the calcium carbonate precipitate and the calcium-depleted solution is then passed along transfer line 38, as an intermediate product stream, to the third filter 40.1 where the calcium carbonate precipitate is filtered out of the mixture and the calcium-depleted solution is transferred to the regenerator 46.1 along transfer line 44 as a calcium-depleted solution stream. The calcium carbonate precipitate is withdrawn from the filter 40.1 as a main product stream along main product line 42 for further processing. In the regenerator 46.1 the calcium-depleted solution is heated so as to boil-off a gaseous product, typically ammonia if the alkaline base was ammonium hydroxide, which is recycled along recycle line 48 as a first recycle stream to the alkaline base flowing in feed line 26. Remaining liquid in the regenerator is recycled as a second recycle stream along recycle line 50 to the acid feed line 20.

EXAMPLE

A number of experimental trials of a number of the steps of the process of the invention were conducted, which trails also, it will be appreciated, constituted operations which would, in use, be carried out in the appropriate corresponding process units of the installation 10 of Figure 1 .

Conditions of ambient temperature in the trials were approximated, for reference purposes, at 25°C.

The experimental trials were conducted on a feedstock of basic oxygen furnace (BOF) slag that was provided by the South African steelmaking industry.

Feedstock Composition

The typical composition of BOF slag obtained from the South African steelmaking industry is presented in Table 1 below.

An X-ray diffraction (XRD) analysis of the BOF slag showed that it consisted mostly of Ca, Si (silicon), Mg (magnesium), Al (aluminium) in amorphous phases (see Table 1 ). According to this data, the iron and calcium concentration of the BOF slag are high, while concentrations of magnesium, silica and aluminium are moderate.

Table 1 : Typical BOF slag composition

All experimental calculations involving feedstock composition were based on these values.

Materials and Methods

Feedstock

The BOF slag was crushed, pulverized and classified to mean particle size fraction of 75 μιτι - 6 mm and minus 75 μιτι (< 200 mesh). The composition of the BOF slag was subsequently determined by X-ray powder diffraction (XRD), the results of which have been provided above. N-methyldiethanolamine (MDEA) (99 % and density of 1 .04), hydrochloric acid (HCI) (32% and density of 1 .12) and ammonium hydroxide (32 % and density of 0.88) were purchased from Sigma-Aldrich. Carbon dioxide (30%, balance nitrogen) was purchased from Air Liquide ©.

Equipment

The trial experiments were carried out batch-wise using a 3 litre Perspex reactor vessel.

The reactor vessel had four equally spaced baffles and was equipped with an overhead stirrer with radial turbine impellers for mixing.

A Hanna HI2829 multi-parameter meter was used to log reactor electrical conductivity, pH and temperature data.

A hypodermic syringe was used to draw out sample aliquots from the reactor.

A Metrohm 691 pH meter was used to measure pH of collected samples.

A Perkin Elmer AAnalyst 700 Atomic Absorption Spectrophotometer (AAS) and a Varian Inductively Coupled Plasma (ICP) spectrometer were used to analyse presence and concentration of Ca, Mg, Fe, Si and Al during the calcium dissolution process step. A PANalytical X'Pert Pro powder diffractometer with X'Celerator detector, having variable divergence and fixed receiving slits, and Fe filtered Co-K a radiation on a back loading preparation process, was used. Phases were identified using X'Pert High score plus software.

Scanning electron microscopy (SEM) data were obtained using a SEM microscope at an acceleration voltage of 5 KV.

Experimental procedure

Calcium dissolution step

In simulating the calcium dissolution step of the process, calcium extraction from BOF slag through dissolution of the calcium compound with HCI was carried out batch-wise under agitation in the reaction vessel. In each run, samples of between 50g and 150g were loaded into the reaction vessel. The molar ratio of HCI to calcium in the feed samples was fixed at respective values of 0.15 to 1 .0; 0.38 to 1 .0; 0.75 to 1 .0; 1 .50 to 1 .0 and 2.57 to 1 .0 for the respective runs.

After HCI addition and occurrence and completion of the dissolution reaction after a specified test time of 90 min, the dissolved samples were immediately filtered, thereby obtaining an acidic calcium-containing solution. The calcium ion concentration in the acidic calcium-containing solution was analyzed by Atomic Absorption Spectrophotometry (AAS). Neutralisation step

In simulating the neutralisation step, the acidic calcium chloride, iron chloride and magnesium chloride solution (i.e. the acidic calcium-containing solution from the dissolution step) was reacted with ammonium hydroxide in a stirred reactor vessel at ambient temperature. Ammonium hydroxide was added until the resultant pH of the mixture was about 10, thus obtaining a basic calcium-containing solution.

The reaction produced a mixture of ferric and magnesium hydroxide sludge which was subsequently filtered out. Virtually all of the calcium remained in solution.

Carbonation step

In simulating the carbonation step, the calcium rich filtrate (basic calcium-containing solution) from the neutralisation step was reacted with CO2. The amount of MDEA required, and used, was calculated on the basis of the stoichiometric Ca/MDEA molar ratio. 10-50 ml of MDEA was also added in order to assess the effect of MDEA on the carbonation reaction and quality of the calcium carbonate produced. The reaction was terminated by opening an outlet valve from the reaction vessel and thereby discharging the sample from the reaction vessel.

The produced calcium carbonate precipitate was separated out of the discharged sample by filtration and was then dried at 120°C to determine the carbonation conversion and the morphology thereof. The ammonium rich filtrate from the carbonation step was heated at moderate temperature (50 °C) to regenerate ammonia and acid rich gas.

Calculations

Correlative calculation of the calcium dissolution efficiency, carbonation efficiency and ammonia regeneration efficiency involved use of the following equations:

Calcium dissolution efficiency (%) = (Ca in extraction filtrate / Ca in feed sample) χ 100 Eq. 1

Carbonation efficiency (%) = (Ca in initial carbonation sample - Ca in carbonation filtrate) / (Ca in initial carbonation sample) χ 100 Eq. 2

Ammonia regeneration efficiency (%) = (ammonia regenerated / initial ammonium hydroxide used in neutralisation stage) χ 100. Eq. 3

Process and sequence of process step equations

The process and sequence of process steps can be expressed by the following simplified equations:

Calcium dissolution

CaSi0₃ (s) + 2HCI (aq) Ca^ (aq) + 20Γ (aq) + Si0₂ (s) + H₂0 (I)

Fe₂0₃ (s) + 6HCI (aq) 2Fe (aq) + 60Γ (aq) + 3H₂0 (I) [2]

MgO (s) + 2HCI (aq) Mg^ + 2CI^" (aq) + H₂0 (I) [3]

Neutralisation

CaCI₂(aq)+ FeCI₃(aq)+MgCI₂(aq)+5NH₄OH(aq)→CaCI₂(aq)+5NH₄CI +Fe(OH)₃(s)+Mg(OH₎₂(s) [4] Carbonation

OH^" (aq) + C0₂ (aq) => HC0₃ ^"(aq) [5]

(aq) + HC0₃ ^" (aq)→ CaCQ₃ (s) H⁺(aq) [6]

NH₄CI (aq) HCI (aq) + NH₃ (g)

Product characterization and analysis

Calcium determination was effected by using a Perkin Elmer AAnalyst 700 Atomic Absorption Spectrophotometer (AAS). X-ray diffraction measurements were made using a back loading preparation process or a zero background sample holder, depending on the amount of sample available. They were analysed with a PANalytical X'Pert Pro powder diffractometer with X'Celerator detector and variable divergence- and fixed receiving slits with Fe filtered Co-Κα radiation. The phases were identified using X'Pert Highscore plus software.

The relative phase amounts (weights %) were estimated using the Rietveld process (Autoquan Program). For Scanning electron microscopy (SEM) studies were obtained using an SEM microscope at an acceleration voltage of 5 KV.

Results and discussion

Calcium dissolution step

The effect of hydrochloric acid concentration on calcium extraction from BOF slag was investigated over a range of -0.1 to ~2 mol/L at ambient temperature using <75 μιτι BOF particle size. With particular reference to Figure 2, it can be seen that hydrochloric acid is an efficient mineral acid to extract calcium from BOF slag, with over 95 % of calcium being leached out from the BOF slag matrix within 90 min using about 1 .7 M HCI. Calcium extraction efficiency was found to increase with increasing hydrochloric acid concentration. The extent of calcium extraction after 20 min with 0.099 mol/L and 0.991 mol/L hydrochloric acid is 8.9 % and 36.1 % respectively. The calcium extraction efficiency slightly increases with further increase in hydrochloric acid concentration from 0.991 mol/L to 1 .694 mol/L.

The calcium efficiency data represented in Figure 2 show that the strength of the hydrochloric acid has a marked effect on the extraction efficiency of calcium. Water and the weakest hydrochloric acid solution extracted only a small proportion (2 and 5 % respectively) of the slag's calcium within 90 minutes, while the more concentrated 0.99 M and 1 .69 M hydrochloric acid solutions extracted about 60 % and 98 % of the calcium respectively, within 60 minutes after the slag was added. The weakest hydrochloric acid solution (0.099 M) contained only 0.278 kg of hydrochloric acid per kg of calcium, whereas already the 0.99 M solution contained more acid (0.3 kg HCI/kg Ca more) than would stoichiometrically have been required to dissolve and extract all the calcium from the slag. It therefore appears that a stoichiometric amount of hydrochloric acid vis-a-vis the calcium content is not enough for the complete calcium extraction from the BOF slag, probably due to the high proportion of iron and magnesium oxides which require hydrochloric acid for their dissolution as well. Taking into account the main extractable metal content in BOF, namely calcium, iron, aluminium and magnesium, the stoichiometric acid amount required would be 2.38 kg HCI/kg Ca. It was found, however, that the optimum amount of acid is about 3.3 kg HCI/kg Ca (0.86 kg HCI/kg BOF slag) which constitutes a 40 % stoichiometric excess.

The effect of temperature on calcium extraction from BOF slag was also studied using 62 g of hydrochloric acid and 100 g BOF slag of <75 μηη fraction size over an extraction time of 90 min. The results of this investigation are presented in Figure 3

The calcium extraction efficiencies obtained were 74%, 88%, and 89% respectively at 25°C, 50°C, and 80°C, showing a very slight increase in extraction at temperatures higher than 25°C at longer retention time. It is, however, clear that the effect of temperature is less significant compared with the effect of hydrochloric acid concentration. Tests at higher temperatures however would be less suitable due to increased corrosion rates and loss of HCI vapour. The effect of particle size on the extraction efficiencies of calcium and magnesium was also investigated in 1 .69 M HCI solution at ambient temperature, using two particle size fractions: <6mm and <75μηη. The results, presented in Figure 4 show that the dissolution extraction rates are inversely proportional to the average initial diameter of the particles.

In Figure 5, inline temperature and pH recordings of the dissolution step trial at ambient temperature (100 g BOF slag added at time 0:00) using 1 .69 M HCI are plotted against time. It can be seen that immediately after addition of the hydrochloric acid solution, the solution pH dropped due to the neutralisation of the alkaline oxides in the BOF by the mineral acid. The solution temperature of the HCI-BOF slag solutions also rose by about 15 - 25 °C, indicating the occurrence of an exothermic reaction.

Neutralisation step

The initial pH of the filtrate from the dissolution step was quite low, being in the order of -0.1-0.7. In order to promote carbonation in the carbonation step, alkaline conditions are required as dissolution of CO₂ into carbonate and bicarbonates ions occurs preferentially under such conditions. Ammonium hydroxide was used to increase the pH of 1 liter of the filtrate from the extraction process containing 10-12 g/l Ca to a pH in the order of 9 to 10. In Figure 6 it can be seen that 26 g of NH OH were used to increase the pH of the solution from 0.32 to 9.4 whereas 24 g of NH OH were used to increase further the pH from 9.4 to 10. The results further show that at a pH above 9 the buffer capacity of the solution increased and the rate of neutralisation is about nil.

From an operational point of view, it is preferable to stop the neutralisation at pH 9 rather than pH 10, thereby to reduce expected ammonium hydroxide consumption by about 50% (1 ,5 L NH₄OH (32%, p=1 .05)/kg BOF slag).

Visual observations also confirmed that most of the iron precipitation occurs between pH 7 and 9. It was also observed that the addition of ammonium hydroxide not only increased the pH of the solution, but in all investigated cases also increased the calcium content in solution by 40-60%. This is a key observation as it shows that it may be better not to optimize the individual processes (extraction or neutralisation) but rather their combination in order to leverage the use of the most expensive product between HCI and NH OH and still achieve the desirable calcium content and pH after the neutralisation stage. Carbonation step

(i) Effect of Reaction Time

With particular reference to Figure 7, carbonation of the solution from the neutralisation stage shows that in the first 10-15 min, over 80% of the calcium is carbonated, while only an extra 18% reacts when the reaction time is increased to 90 min. This pattern is similar to the fresh BOF slag calcium leaching in HCI where more than 80 % leached in the first 2-10 minutes depending on the particle size and acid concentration.

The declining carbonation reaction rate in the later stage of the reaction, as seen from Figure 7, shows that the precipitation of calcium carbonate is likely not the rate- determining step for this process. In that case, the calcium carbonate initially precipitated would have served as nuclei and would enhance further precipitation and subsequently also the carbonation efficiency. (ii) Effect of CO₂ Flow Rate

The effect of CO2 flow rate on the carbonation efficiency is shown in Figure 8. In both runs which were conducted (see Figure 8), the carbonation rate decreases with reaction time. The reaction rate increases significantly initially within the first 10 and 30 min, using a flow rate of 1 .2 L/min and 0.3 L/min respectively, and then attains equilibrium values. It is expected that increasing the CO2 flow rates can improve the carbonation reaction rate.

(iii) Effect of stirring rate

The trends in Figure 9 indicate that the Ca conversion decreases at a low stirring rate, probably due to inadequate mixing and a deficiency of dissolved CO2.

It therefore appears that the carbonation reaction can be divided into two parts: (1 ) diffusion and dissolution of CO2 into alkaline solution to generate carbonate ions; (2) reaction of calcium and carbonate ions to produce calcium carbonate.

It was shown above that the declining carbonation reaction rate at long retention time is an indication that the precipitation of calcium carbonate is likely not the rate- determining step during carbonation. However the moderate influence of the stirring rate on the conversion suggests that mass transfer of CO2 into the solution may be comparatively a more important factor. Therefore the diffusion and dissolution of CO2 into the solution is the likely reaction step that determines the overall carbonation rate as given in reaction 5 above. (iv) Calcium carbonate analysis

Calcium carbonate exists in one of three different polymorphs, or spatial arrangements, of its calcium and carbonate ions. Calcium carbonate polymorphs are calcite, aragonite and vaterite. The vaterite polymorph of calcium carbonate often exhibits nearly perfect spherical shapes. Although these spheres may appear smooth, they can be quite porous. Vaterite may also exhibit a platy structure, and when this happens the plates tend to self-assemble into larger structures that may be spheroidal. Vaterite is metastable meaning that atoms in the crystals have a natural tendency to rearrange, usually to a calcite structure. For this reason, vaterite is not commonly found in nature and is not a commercially significant form of PCC.

The aragonite polymorph generally exhibits needle-shaped orthorhombic crystals. The needle shape is called acicular, and the ratio of length-to-diameter of the crystals is called aspect ratio. High aspect ratio aragonite is useful in many applications. In paper coatings, this morphology tends to produce high gloss finishes and is better at covering substrates at lower coating thicknesses. A high aspect ratio also can improve strength or impact resistance in polymeric materials that employ this form of calcium carbonate as an additive. Sometimes, clusters of needle-shaped crystals are observed, and these can be very efficient at scattering incident light. Calcite is the most common polymorph of calcium carbonate and the most stable and can be found in rhombohedral (cubic), prismatic (barrel-shaped) and scalenohedral (triangular) morphologies. The rhombohedral and prismatic forms are useful in paper coating applications and as strength enhancers in polymer matrixes.

All the precipitates formed from the carbonation of calcium chloride solution (derived from BOF slag by adding hydrochloric acid and ammonium hydroxide) contained calcium carbonate phases as a mixture of calcite and vaterite, according to XRD analyses. Carbon dioxide was bound entirely as calcite and vaterite (100% calcium carbonate content). The purity of commercially available precipitated calcium carbonate (PCC) is typically about 97 %, indicating that calcium carbonate produced by the process in this study is most suitable for use as PCC. According to XRD and SEM analyses, the precipitates obtained through the experimental trial runs were calcium carbonate in a form of calcite and vaterite with respectively a rhombohedral and hexagonal particle shapes. Particular reference in this regard is made to Figure 10. The CO2 flow rate did not appear to have an impact on the shape of calcium carbonate, as the calcite and vaterite content of the precipitates was calculated as being respectively 94.52 and 5.4 wt% when the CO2 flow rate is 0.3 l/min and 95.52 and 4.48 wt% when the CO2 flow rate is 1 .2 l/min. It is probable that below a certain value the carbon dioxide diffusion does not control the nucleation process and the appearance of calcium carbonate crystals.

The SEM analysis further shows that the temperature has a more pronounced effect on the shape of calcium carbonate produced. It was found that spherical particles were formed in the carbonation system (see Figure 10C) at high temperature.

The XRD results confirm that the calcite and vaterite content of the precipitates shown in Figure 10C was determined to be respectively 1 .29 and 96.39 wt-% at 50°C whereas the calcite and vaterite content of the precipitates was determined to be respectively 94.52 and 5.4 wt-% at ambient temperature. The effect of the stirring rate is shown in Figure 10D. The SEM micrographs reveal that the stirring rate during carbonation impacts on the shape and structure of calcium carbonate precipitate. As the stirring rate decreases, the vaterite content of the calcium carbonate precipitates slightly increases. The calcite and vaterite content of the precipitates in Figure 10D was determined to be respectively 88.15% and 1 1 .85 wt-% at 40 rpm, whereas the calcite and vaterite content of the precipitates was determined to be respectively 94.52 and 5.4 wt-% at 180 rpm as shown in Figure 10B

Figure 10 also shows the aggregation phenomena of the precipitate. A great number of individual crystals aggregate to reach a large size (see Figure 10A). The observed aggregation phenomena are due to the high supersaturation in the aqueous solution.

Raw material consumption and implications

The results discussed above show that when using 50 g of BOF slag with 120 ml HCI (32%, p=1 .12) and 880 ml of water one is able to prepare 0.8-0.9 I of a calcium-rich solution (6-9 g Ca/I) with pH in the range 0.2-0.5. For neutralisation of this leaching solution in order to increase the pH and precipitate iron and magnesium, an average of 50-75ml of NH OH (32%, p=1 .05) is required. With these amounts of chemicals, about 24 g of 100 % pure calcium carbonate should precipitate per litre of calcium rich solution produced from the extraction stage, requiring ~ 9.6 g CO₂. This means that, for 1 ton of BOF slag, 1000 dm³ of prepared calcium chloride solution is needed with 0,86 ton of HCI and 0.5 ton of NH₄OH, producing ~ 0.48 ton of 100 % pure CaCO₃. Production of this calcium carbonate would consume approximately 0.2 ton of CO₂. Post-carbonation products regeneration

After filtration of the precipitated calcium carbonate the remaining solution is rich in ammonium chloride. The precipitation of calcium carbonate produces heat (per kilogram of carbon dioxide consumed). The inline temperature recording of the carbonation reaction shows the occurrence of an exothermic reaction, as shown in Figure 1 1 . It is submitted, for clarity, that the thicker and darker shaded of the two lines depicts temperature, whilst the thinner and lighter shaded of the two lines depicts pH. The ammonium hydroxide used in the neutralisation step, which is converted to ammonium chloride in that step, is suitable for regeneration. In order to recover ammonia from the ammonium chloride solution, the filtrate from the carbonation was heated. Figure 12 shows the pH profile over time of 40 ml of water (initially at pH 7) harvesting gas from the heating of 300 ml of ammonium chloride-containing filtrate. Ammonia as recovered in the region CD of Figure 12 and the pH of the 40 ml of water rose to 10.3.

The temperature required for ammonia recovery is moderate, and it is likely that some of the required heat could be acquired by process integration using waste heat from either or both of the exothermic dissolution and carbonation stages. An initial estimate shows that for every ton of BOF slag treated, about 20% of ammonia used for neutralisation can be recycled. Conclusions and general discussion

In the experimental trials, an indirect BOF slag carbonation system, with optional recycling of ammonium hydroxide, was tested in accordance with the process of the invention.

Laboratory conditions of calcium extraction from BOF slag through dissolution thereof with HCI and calcium carbonate precipitation were investigated. Calcium extraction from BOF slag dissolution was accelerated by hydrochloric acid, and the calcium carbonate yield increased with increasing pH and calcium concentration with ammonium hydroxide neutralisation.

Ammonium hydroxide used was found to be capable of regeneration through ammonia recovery by means of a temperature process.

The calcium carbonate particles obtained in the carbonation step were found to comprise a mixture of calcite and vaterite, which could be used commercially as a pigment or filler in materials.

The Applicant regards it as a particular advantage of the invention as described that commercially advantageous use can be made calcium-containing materials which would of otherwise be regarded mainly as inutile wastes which accumulate at various industries. Particularly BOF slag, which, to the Applicant's knowledge, is available in voluminous stockpile quantities at steelmaking operations, can now, through the process of the present invention, be beneficially employed or processed in a commercially beneficial manner, whilst also achieving reduction of the slag volumes which are stockpiled.

The Applicant is aware of a number of other processes which also seek to process BOF slag in a commercially viable manner. However, to the Applicant's knowledge, a number of such processes have fallen into disfavour due to inherent disadvantages associated therewith.

In some cases, BOF slag is recycled into the steel making process to recover the fluxing compounds CaO, MgO in the slag as well as the iron units. However the effectiveness of this process is restricted because of the high P₂O₅ content as most of the phosphorous in the BOF slag reverts to the hot metal in the blast furnace. This limits the quantity of BOF slag that can be charged back to the blast furnace and does not provide a long term solution to slag accumulation.

The use of the BOF slag as an aggregate in road ballast is well-known as it provides excellent anti-skid properties. However, in recent years its use has fallen out of favour because the free lime which is present in steelmaking slag which can subsequently hydrate, causing expansion and disintegration of the roadbed. This has forced the implementation of methodologies to stabilize the "free" lime by aging or by steam curing which render this option of slag processing commercially unattractive. The Applicant therefore believes that the present invention provides for commercially viable and commercially beneficial processing of calcium-containing wastes. The Applicant further regards it as another advantage of the invention that it allows for carbon dioxide sequestration, particularly in operations which produce both carbon dioxide and calcium-containing wastes (or at least have access to both) in an economically attractive manner.

Claims

1 . A process of producing precipitated calcium carbonate (CaCO₃), the process including

in a calcium (Ca) dissolution step, contacting a solid particulate feedstock, which contains calcium as a calcium compound in a calcium-containing matrix, with hydrochloric acid (HCI), thereby dissolving at least some of the calcium from the matrix, and recovering the dissolved calcium in an acidic calcium-containing solution; in a neutralisation step, admixing a base with the acidic calcium-containing solution, thereby obtaining a basic calcium-containing solution;

in a calcium carbonate recovery step, recovering the calcium carbonate precipitate from the calcium-depleted solution.

2. The process according to Claim 1 , wherein the calcium dissolution step, the neutralisation step, the carbonation step and the calcium carbonate recovery step are carried out under conditions of atmospheric pressure and at ambient temperature.

3. The process according to Claim 1 or Claim 2, wherein the calcium compound is calcium oxide (CaO).

4. The process according to any of claims 1 to 3 inclusive, which includes, ahead of the calcium dissolution step, a prior step of comminuting the feedstock to a mean particle size <6mm.

5. The process according to any of claims 1 to 4 inclusive, wherein the hydrochloric acid is diluted hydrochloric acid having a concentration of between about 0.1 and about 2 mol/L (mol/litre).

6. The process according to any of claims 1 to 5 inclusive, wherein, in the calcium dissolution step, sufficient of the hydrochloric acid is used so that it is in stoichiometric excess to that required for complete dissolution of the feedstock Ca content.

7. The process according to Claim 6, wherein, in the calcium dissolution step, sufficient of the hydrochloric acid is used so that an acid / [feedstock Ca content] mass ratio of between about 2 kg HCI / [kg feedstock Ca content] to about 4 kg HCI / [kg feedstock Ca content] initially applies in the calcium dissolution step.

8. The process according to any of claims 1 to 7 inclusive, wherein the base is ammonium hydroxide (NH OH).

9. The process according to any of claims 1 to 8 inclusive, wherein a sufficient quantity of the base is admixed with the acidic calcium-containing solution for the pH of the basic calcium-containing solution to be above 5.

10. The process according to Claim 9, wherein a sufficient quantity of the base is admixed with the acidic calcium-containing solution for the pH of the basic calcium-containing solution to be above 9.

1 1 . The process according to any of claims 1 to 10 inclusive, wherein the addition of the N-methyldiethanolamine takes place, and wherein it is used in a quantity which is stoichiometrically equivalent to the quantity of calcium in the basic calcium-containing solution.

12. The process according to any of claims 1 to 10 inclusive, wherein the addition of the N-methyldiethanolamine takes place and wherein it is used in a catalytically effective amount of between about 5 and about 10 ml per 1 L of basic calcium-containing solution.

13. The process according to any of claims 1 to 12 inclusive, wherein the contacting of the basic calcium-containing solution with the carbon dioxide includes passing gaseous carbon dioxide through the basic calcium-containing solution at a flow rate of between about 0.3 L/min and about 1 .2 L/min.

14. The process according to any of claims 1 to 13 inclusive, wherein the carbonation step is carried out for between about 10 and about 90 minutes.

15. The process according to any of claims 1 to 14 inclusive, which includes at least one of (i) filtering the acidic calcium-containing solution ahead of the neutralisation step and then carrying out the neutralisation step on the acidic calcium-containing solution filtrate; and

(ii) filtering the basic calcium-containing solution ahead of the carbonation step and then carrying out the carbonation step on the basic calcium-containing solution filtrate.