WO 2010/096862 Al

(12) INTERNATIONAL APPLICATION PUBLISHED UNDER THE PATENT COOPERATION TREATY (PCT)

(19) World Intellectual Property OrganizationInternational Bureau

(10) International Publication Number(43) International Publication Date2 September 2010 (02.09.2010) WO 2010/096862 Al

(51) International Patent Classification: (81) Designated States (unless otherwise indicated, for everyCOlG 9/02 (2006.01) COlG 9/00 (2006.01) kind of national protection available): AE, AG, AL, AM,

AO, AT, AU, AZ, BA, BB, BG, BH, BR, BW, BY, BZ,(21) International Application Number: CA, CH, CL, CN, CO, CR, CU, CZ, DE, DK, DM, DO,

PCT/AU20 10/000206 DZ, EC, EE, EG, ES, FI, GB, GD, GE, GH, GM, GT,(22) International Filing Date: HN, HR, HU, ID, IL, IN, IS, JP, KE, KG, KM, KN, KP,

24 February 2010 (24.02.2010) KR, KZ, LA, LC, LK, LR, LS, LT, LU, LY, MA, MD,ME, MG, MK, MN, MW, MX, MY, MZ, NA, NG, NI,

(25) Filing Language: English NO, NZ, OM, PE, PG, PH, PL, PT, RO, RS, RU, SC, SD,

(26) Publication Language: English SE, SG, SK, SL, SM, ST, SV, SY, TH, TJ, TM, TN, TR,TT, TZ, UA, UG, US, UZ, VC, VN, ZA, ZM, ZW.

(30) Priority Data:2009900804 24 February 2009 (24.02.2009) AU (84) Designated States (unless otherwise indicated, for every

kind of regional protection available): ARIPO (BW, GH,(71) Applicant (for all designated States except US): AUSZ- GM, KE, LS, MW, MZ, NA, SD, SL, SZ, TZ, UG, ZM,

INC METALS & ALLOYS [AU/AU]; Lot 2 Shellhar- ZW), Eurasian (AM, AZ, BY, KG, KZ, MD, RU, TJ,bour Road, Port Kembla, New South Wales 2505 (AU). TM), European (AT, BE, BG, CH, CY, CZ, DE, DK, EE,

(72) Inventors; and ES, FI, FR, GB, GR, HR, HU, IE, IS, IT, LT, LU, LV,MC, MK, MT, NL, NO, PL, PT, RO, SE, SI, SK, SM,(75) Inventors/Applicants (for US only): PERRY, MichaelTR), OAPI (BF, BJ, CF, CG, CI, CM, GA, GN, GQ, GW,[AU/AU]; 1/1 Coolgardie Street, East Corrimal, NewML, MR, NE, SN, TD, TG).

South Wales 25 18 (AU). OWEN, Philip [AU/AU];Mackeys Lane, Robertson, New South Wales 2577 (AU). Published:SHORT, Stephen, A [AU/AU]; 9 Sunninghill Court,

— with international search report (Art. 21(3))Mount Ousley, New South Wales 25 19 (AU).

(74) Agent: PHILLIPS ORMONDE FITZPATRICK; Level21, 22 & 23, 367 Collins Street, Melbourne, Victoria3000 (AU).

(54) Title: ZINC OXIDE PURIFICATION

(57) Abstract: The present invention is directed to a method of purifying a zinc sulphate solution in the production of zinc oxide.In the preferred methodology there is a series of processes ranging from converting a zinc oxide/zinc metal residue material tozinc sulphate solution by leaching within acid, for example sulphuric acid, reacting the zinc sulphate solution with an alkalinemetal carbonate to produce zinc carbonate, and then heating the zinc carbonate, for example, by calcining to produce zinc oxide ina highly pure form.

ZINC OXIDE PURIFICATION

FIELD OF THE INVENTION

The present invention relates broadly to a method of purifying a zinc sulfate solution in the

production of zinc oxide.

BACKGROUND OF THE INVENTION

Zinc oxide is widely used as an additive in numerous materials and products including, for

example, plastics, ceramics, glass, cement, rubber (e.g. car tyres), lubricants, paints,

ornaments, adhesives, sealants, pigments, foods, batteries, ferrites, fire retardants. For

industrial use, zinc oxide is generally produced by one of three main processes:

(i) the French Process, where metallic zinc is melted in a graphite crucible and

vaporized to react with the ambient oxygen to give zinc oxide;

(ii) the Direct Method, in which zinc ores or roasted sulfide concentrates are mixed

with coal in a reduction furnace, wherein the ore is reduced to metallic zinc and

the vaporized zinc is allowed to react with oxygen to form zinc oxide; and

(iii) the American Process, in which zinc ore (zinc ash) is dissolved in, for example,

hydrochloric acid (as ZnCI2) and precipitated with alkali to give "active" zinc

oxide.

SUMMARY OF THE INVENTION

According to one aspect of the invention there is provided a method of purifying a zinc

sulfate solution comprising mixing the zinc sulfate solution together with an aqueous metal

carbonate to produce a zinc carbonate slurry.

According to another aspect of the present invention there is provided a method of

purifying a zinc sulfate solution in the production of zinc oxide, said method comprising the

steps of:

mixing the zinc sulfate solution together with an aqueous metal carbonate to

produce a zinc carbonate slurry;

separating zinc carbonate from the zinc carbonate slurry; and

heat treating the zinc carbonate to produce the zinc oxide.

Preferably the step of mixing the zinc sulfate solution together with an aqueous metal

carbonate comprises controlling the rate of addition of the aqueous metal carbonate. More

preferably the step of mixing comprises controlling the rate of addition of the aqueous

metal carbonate so that the mixing occurs at a pH of between 5 to 9 . Even more

preferably the step of mixing comprises controlling the rate of addition of the aqueous

metal carbonate so that the mixing occurs at a pH of between 6 to 8 . Still more preferably,

the step of mixing comprises controlling the rate of addition of the aqueous metal

carbonate so that the mixing occurs at a pH of between 7 to 7.2.

Preferably the step of mixing the zinc sulfate solution together with an aqueous metal

carbonate comprises controlling the rate of addition of the aqueous metal carbonate so

that the mixing occurs at a temperature of between 5OO to 6OO and at a pH of between

7 to 7.2.

Preferably the aqueous metal carbonate comprises one or more types of alkali metal

carbonates. More preferably, the one or more types of alkali metal carbonates include

those of sodium, potassium.

Preferably the method of purifying a zinc sulfate solution further comprises a preliminary

step of removing one or more contaminants from the zinc sulfate solution. More preferably

the step of removing one or more contaminants from the zinc sulfate solution includes the

addition of an alkaline solution and an oxidant in a temperature range of between 5OO to

100O. Even more preferably the alkaline solution com prises one or more types of alkali

metal cations, and the oxidant is selected from the group consisting of: hydrogen

peroxide, sodium hypochlorite, potassium permanganate, oxygen. Still more preferably,

the one or more types of alkali metal cations include those of sodium or potassium and

the oxidant is potassium permanganate.

Preferably the one or more contaminants includes nickel, aluminium, iron, calcium or

manganese. More preferably the step of removing nickel from the zinc sulfate solution

comprises adding a chelating agent material. Even more preferably the chelating agent

material is dimethyl glyoxime.

Preferably the step of separating zinc carbonate from the zinc carbonate slurry involves

centrifuging the zinc carbonate slurry.

Preferably the step of heat treating the zinc carbonate to produce the zinc oxide involves

calcining the zinc carbonate at a temperature of between 400O to 44OO. More

preferably the step of calcining the zinc carbonate occurs at a temperature of 420O.

BRIEF DESCRIPTION OF THE FIGURES

In order to achieve a better understanding of the nature of the invention a preferred

embodiment of a method of purifying a zinc sulfate solution in the production of zinc oxide

will now be described, by way of example only, with reference to the accompanying

figures in which:

Figure 1 is a schematic illustration of one embodiment of a method of purifying zinc

sulfate solution in the production of zinc oxide in accordance with the present invention.

Figure 2 is a schematic flow diagram of an exemplary plant for the production of zinc

oxide in accordance with the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention in one embodiment contemplates a method of purifying a zinc

sulfate solution in the production of zinc oxide. Briefly, the preferred method involves a

series of processes ranging from converting a zinc oxide/zinc metal residue material to

zinc sulfate solution by leaching with an acid, for example sulfuric acid, reacting the zinc

sulfate solution with an alkali metal carbonate to produce zinc carbonate, and then

heating the zinc carbonate, for example, by calcining, to produce zinc oxide in a highly

pure form.

The zinc sulfate solution may be obtained by processing a zinc bearing material such as a

zinc oxide/zinc metal residue material sourced as a primary feed from industrial or other

processes. A preferred method for recovering and purifying zinc oxide disclosed herein is

carried out using zinc oxide/zinc metal residue material sourced as a primary feed from a

galvanizing plant. It will be appreciated by persons skilled in the art that using zinc

oxide/zinc metal residue material sourced as a primary feed from other sources may

require the process circuit to be modified depending on the contaminants present. In the

preferred method, prior to forming the zinc sulfate solution, the primary feed may undergo

one or more pre-treatment stages to reduce the size of the fragments in the feed and to

remove certain contaminants that may hinder the purification of zinc oxide. Such pre-

treatment stages may include but are not limited to any one or more of the following:

crushing, milling, roasting, size- and density-selective separation, filtering, washing.

Leaching Stage

After processing, the zinc may be leached out of the zinc oxide fraction using any suitable

leachant. The leaching stage may be performed using wet or dry zinc oxide fractions. In a

preferred embodiment, the leaching stage is performed using a zinc oxide/water slurry.

Any suitable amount of water may be envisaged to produce a zinc oxide slurry appropriate

for use in the leaching stage. Good results have been obtained using a slurry formed from

a 50% zinc oxide/water mixture.

Figure 1 outlines a preferred embodiment of the zinc oxide purification process using a

zinc oxide slurry feed produced by any suitable process.

The zinc is preferably leached from the zinc oxide slurry using any suitable acid. Good

results have been obtained using sulfuric acid as the leachant. Dilute sulfuric acid

digestion of the zinc oxide in the slurry produces a zinc sulfate solution.

In a preferred embodiment as shown in Figure 2 , the zinc oxide slurry is continuously fed

to a leach mixing tank 1, to which sulfuric acid is added continuously to achieve a leach

solution with a measured finishing pH of 1.8. Good results have also been obtained using

leach solutions with a pH value in the range of 1 to 3.5. However, it will be appreciated by

persons skilled in the art that a finishing pH value greater than 1.8 (a more basic pH) may

lengthen the reaction time and may reduce the leaching efficiency. In addition, the

finishing pH also affects the dosage of alkali that may need to be added to the zinc sulfate

solution in the later purification stage. The concentration of acid used in the leaching

stage may depend on the amount of zinc in the slurry. Good results have been obtained

using a leach concentration of 80 g/L zinc (corresponding to 9% v/v sulfuric acid).

The continuous addition of sulfuric acid to the leach solution may cause an initial rise in

temperature due to exothermic reaction. Preferably, the temperature for the leaching

stage may be maintained using any suitable heating means. Such heating means may

include but are not limited to any one or more of the following: a hot plate, a heat

exchanger, direct steam injection, insulation, gas burner, electrical coil. In the preferred

embodiment, the temperature of the leaching stage is maintained around 8OO via steam

heating using an external heat exchanger (not shown). Good results have also been

obtained using a temperature in the range of between 50 to 100O. It will be appreciated

by persons skilled in the art that the leach time may vary with temperature. To ensure

maximum leaching efficiency (i.e. to extract the highest possible amount of zinc from the

zinc oxide slurry), the leach solution may be transferred from the main leach mixing tank 1

to one or more smaller leach tanks 2 . The use of the one or more smaller leach tanks 2 ,

preferably maintained at a temperature of around 8OO, may reduce the leach time.

Desirable leach conditions are as set out in Table 1 for a 1 tonne feed.

Table 1 Leach Conditions

Metallic zinc in the zinc oxide slurry may be converted to zinc oxide during the leaching

stage by adding an oxidant to the leach solution. The presence of an oxidant in the leach

solution may increase the leach efficiency. Any suitable oxidant may be used. Suitable

oxidants may include but are not limited to one or more of the following: hydrogen

peroxide, sodium hypochlorite, potassium permanganate, oxygen. Good results have

been obtained using potassium permanganate (KMnO4) as an oxidant. The oxidative state

of the leach solution may be monitored using any suitable measuring means, for example,

an oxidation/reduction potential (ORP) meter. Good results have been obtained where a

sufficient amount of oxidant is added to the leach solution to maintain an oxidative state of

around 200 to 900 mV.

In a preferred embodiment, good zinc dissolution may be achieved in (0.5 hours) by

maintaining the temperature and oxidative state of the leach solution around 8OO and

300 mV, respectively.

Once the leaching stage is complete, most or all of the zinc oxide in the leach solution will

have been digested by the sulfuric acid to produce the desired zinc sulfate solution.

Purification Stage

Following the leaching stage, it may be necessary to remove a number of low level

contaminants from the zinc sulfate solution. For example, contaminants may include but

are not limited to those containing one or more of the following metals: aluminium, iron,

manganese, calcium, nickel.

Raising the pH of the zinc sulfate solution and increasing the oxidation/reduction potential

(ORP) may create conditions within the solution that result in the formation of insoluble

compounds of the unwanted contaminants, which may subsequently be removed from

solution by any suitable means including but not limited to any one or more of the

following: filtration, decantation, centrifugation. As such, these one or more contaminants

may be removed to acceptable levels through controlled addition of an alkali metal cation

solution containing a suitable oxidant. Suitable solutions may be prepared comprising: (i)

an alkali metal cation including but not limited to any one or more of the following: sodium,

potassium, .and (ii) an oxidant, including but not limited to one or more of the following:

sodium hypochlorite, potassium permanganate. From at least a cost and availability

perspective, sodium hydroxide solution may be preferable over alkaline solutions

prepared from other alkali metals.

In a preferred embodiment, a solution comprising sodium hydroxide and potassium

permanganate (KMnO4) may be added to the zinc sulfate solution in the precipitation tank

3 to achieve a pH of 3.5 and an ORP of >850 mV. Any suitable means of heating may be

used to maintain the temperature of the zinc sulfate solution in the precipitation tank 3

around 8OO. Preferable forms of heating may include but are not limited to any one or

more of the following: steam heating, heat exchanger, direct steam injection, insulation,

gas burner, electrical coil.

Desirable purification conditions are shown in Table 2 .

Table 2 Purification Conditions

It will be apparent to persons skilled in the art that solution pH values above 3.7 may

cause zinc to precipitate from solution in the form of zinc diiron (III) tetraoxide. Hence,

maintenance of the pH during the purification stage may be important to maximize zinc

oxide recovery. Diluting the concentration of the sodium hydroxide solution may allow any

precipitated zinc hydroxide to re-dissolve into solution.

Aluminum as a contaminant may be removed from the zinc sulfate solution by forming a

potassium-alunite compound which precipitates from solution. The maximum conversion

to this insoluble compound occurs as the pH is raised to a finishing pH of 3.7. It will be

appreciated that adding a suitable flocculent to the zinc sulfate solution or filtering the

solution at temperatures of >60O may prevent the alu minium compound from re-

dissolving into solution. In a preferred embodiment, a concentration of O. g.L of K+ in

solution may ensure a high degree of aluminium removal from solution.

Iron as a contaminant may be removed from the zinc sulfate solution via oxidation of the

soluble ferrous (Fe2+) to give the more insoluble ferric (Fe3+) form. In a preferred

embodiment, oxidation coupled with a solution pH of >3.0 may ensure a high degree of

iron removal from solution.

Manganese as a contaminant may be removed from the zinc sulfate solution via oxidation

of the Mn2+ to Mn4+ and the subsequent formation of MnO2. It will be appreciated that

sufficient concentrations of either KMnO or sodium hypochlorite (NaCIOs) may achieve

an ORP >850 mV to yield MnO2.

Calcium as a contaminant may be present in the zinc sulfate solution in the form of

calcium sulfate (CaSO4) or gypsum. Gypsum may be removed from the zinc sulfate

solution by virtue of exploiting the solubility character of gypsum in the solution, which is

around 0.650 g/L calcium. In a preferred embodiment, the leaching stage may be modified

to increase the concentration of the leach solution from 80 g/L to 300 g/L zinc. This may

be achieved by increasing the rate of feed and acid, and reducing the volume of make-up

water added to the leach solution. This approach has the desirable effect of increasing the

equivalent calcium concentration for example, to around 10.7 g/L, while the gypsum

solubility remains the same (around 0.650 g/L calcium). In this example, the balance of

gypsum (around 0.05 g/L) in the leach solution may be removed by filtration. The filtered

leach solution may then be diluted to a desirable concentration of, for example 80 g/L

zinc, with a lower calcium concentration.

Nickel as a contaminant may be removed from the zinc sulfate solution through the

addition of any one or more ligands that have a good affinity towards the metal or metal

ion. Such ligands may include, for example, chelating agents. Chelating or sequestering

agents form chelate complexes with metals or metal ions through the formation of multiple

bonds with the metal or metal ion.

Preferably, the chelating agent is dimethyl glyoxime (DMG). As is known from gravimetric

analysis, DMG is widely used as a chelating agent in the gravimetric determination of

nickel, for example, in ores. The complex formed between nickel and DMG (Ni-DMG) is

poorly soluble in water at pH 5-8 and so precipitates from solution, thereby simplifying the

removal of the nickel contaminants. Solid DMG has low solubility in water. The solubility of

DMG in water may be increased by, for example, dissolving DMG in an alkali metal cation

solution. The alkali metal cation may be selected from but is not limited to any one or

more of the following alkali metals: sodium, potassium. From at least a cost and

availability perspective, sodium hydroxide solution may be preferable over alkaline

solutions prepared from other alkali metals. DMG may be dissolved in sodium hydroxide

solution to form a soluble sodium-DMG compound, which may be dosed directly into the

zinc sulfate solution. Good levels of nickel removal from the zinc sulfate solution may be

achieved using the sodium-DMG compound at a solution pH of >5.0. Advantageously,

DMG has little or no affinity for zinc; hence, addition of sodium-DMG to a zinc sulfate

solution preferentially forms a water-insoluble complex with the nickel contaminant.

The insoluble contaminants may be removed from the treated zinc sulfate solution using

any suitable means. For example, the insoluble contaminants may be removed from

solution by filtration, decantation, centrifugation. The removal of insoluble contaminants,

particularly, contaminants that are suspended in solution, may be enhanced through the

use of flocculation. Flocculents may be anionic or cationic in character and may cause

particles suspended in solution to aggregate into clumps or floe, which may then float to

the top of the solution, or settle to the bottom leaving a clear solution. For example, to

remove one or more of the abovementioned insoluble low level contaminants from the

zinc sulfate solution, it may be necessary to introduce a suitable flocculent into the

solution. Suitable flocculents may include but are not limited to any one or more of the

following anionic flocculents: Nalco Core® Shell 71301 , Nalco 82205, Ciba® Magnafloc®

338 It will be appreciated by persons skilled in the art that in the embodiments of the

present invention, the flocculent concentration and dosage required to flocculate the zinc

sulfate solution may depend on the zinc concentration of the solution to which the

flocculent is added. For example, flocculation of a zinc sulfate solution with a high zinc

concentration of say 300 g/L may form aggregates that settle more slowly than say a

solution containing 80 g/L zinc.

Desirable flocculents are shown in Table 3 and Table 4 .

Table 3

Table 4

Removal of the suspended floe from the zinc sulfate solution produces a clean zinc sulfate

solution, which may pass to the next stage in the zinc oxide purification process for further

treatment, and a settled residue. The suspended floe may be removed from solution by

decanting or filtering the solution using any suitable means. Preferably, the suspended

floe may be separated from the zinc sulfate solution by decanting, filtering or by a

combination of decanting and filtering of the solution at a temperature above 6OO to

avoid undesirable crystallization of zinc sulfate compounds from solution.

In a preferred embodiment (see Figure 2), the zinc sulfate solution may be flocculated

through a filter feed tank 4 and filtered through one or more pressure leaf filters 5 that

have been selected to achieve a relatively low moisture level of, for example, around 10%.

Optionally, the zinc sulfate solution may be flocculated through a thickener (not shown)

equipped with an in-line flocculent addition (not shown). The resulting slurry is then

transferred into a second thickener (not shown) where the solids may form aggregates

and settle. Any overflow from the second thickener may be fine filtered through any

suitable process filter (not shown) to remove any suspended floe, and then possibly

diluted to an appropriate zinc concentration desirable for processing in the subsequent

zinc carbonate precipitation stage.

Further processing of the settled residue may be beneficial if it is deemed to still contain

zinc sulfate solution or if it contains materials that may be extracted using other

processes, for example, lead/zinc smelting.

Optionally, it may be desirable to extract any remaining zinc sulfate solution from the

settled residue by transferring the residue to a plate and frame filter (not shown) to form a

filter cake, which may be air dried and furnaced to reduce the cake moisture to, for

example, <10%. The zinc sulfate solution filtrate, on the other hand, may be treated to

remove any unwanted contaminants, fine filtered, for example, using a polishing filter (not

shown), and then diluted to an appropriate zinc concentration desirable for further

processing in the subsequent zinc carbonate precipitation stage.

In a preferred embodiment, as shown in Figure 2 , nickel as a contaminant may be

removed from the zinc sulfate solution by adding sodium-DMG compound to the solution

in tank 6 , where the mixture is stirred at 8OO. A high concentration of sodium hydroxide

solution in the zinc sulfate solution may result in the undesirable formation of a basic zinc

sulfate compound. To avoid the formation of this basic zinc sulfate compound, the stirred

solution is passed through a heat exchanger 7 to raise the solution temperature to 950C.

The solution is then transferred to a further tank 8 , where more sodium hydroxide is added

to raise the solution pH to 3.8. The solution is then passed through a second heat

exchanger 9 to reduce the temperature to <40'C. The pH is then increased to >5.0

through addition of more sodium hydroxide to precipitate the insoluble Ni-DMG complex.

The zinc sulfate solution is then filtered using a plate and frame filter 10 and the filtrate

transferred to a reactor feed tank 11.

Zinc Carbonate Precipitation

A preferred embodiment of this aspect of the zinc oxide purification process involves

reacting the filtered zinc sulfate solution with an aqueous metal carbonate to form zinc

carbonate. Any suitable aqueous metal carbonate may be used to react with the zinc

sulfate solution. Suitable aqueous metal carbonate solutions may be prepared comprising

an alkali metal carbonate, including but not limited to any one or more of the following

alkali metals: sodium, potassium. In a preferred embodiment, the aqueous metal

carbonate solution is a sodium-containing carbonate, more preferably, sodium carbonate.

The chemical reaction between zinc sulfate solution and aqueous sodium carbonate

produces solid zinc carbonate and aqueous sodium sulfate. The precipitation of zinc

carbonate from solution may be accompanied by the undesirable co-precipitation of one

or more contaminants, including but not limited to contaminants containing one or more of

the following: sulfates, sodium, magnesium, calcium, lead, nickel. Any sulfate

contaminants present in solution originate mainly from the aqueous sodium sulfate

byproduct formed during the reaction between zinc sulfate solution and aqueous sodium

carbonate, as well as from other zinc sulfate compounds that may have been introduced,

for example, as sulfuric acid, during the leaching stage or earlier. The amount of sulfate

that co-precipitates with the zinc carbonate product may depend on the pH and

temperature of the reaction conditions. In a preferred embodiment, co-precipitation of

sulfate contaminants may be reduced by maintaining a solution pH of > 7.0 at low

temperature, for example, around 25O.

Any sodium contaminants present in solution may originate from any one or more sodium-

containing compounds that have been introduced as, for example, sodium carbonate,

during the precipitation stage or earlier. The amount of sodium that co-precipitates with

the zinc carbonate product may depend on the pH and temperature of the reaction

conditions. In a preferred embodiment, co-precipitation of sodium contaminants may be

reduced by maintaining a solution pH of < 7.0 at a raised temperature, for example,

>60O.

Any calcium contaminants present in solution may originate from the formation of two

compounds: (i) calcite (calcium carbonate); and (i) disordered dolomite (calcium

magnesium carbonate), both of which may have been introduced during the feed stage or

earlier. The amount of calcium and magnesium contaminants that co-precipitates with the

zinc carbonate product may depend on the pH and temperature of the reaction conditions.

In a preferred embodiment, calcium and magnesium co-precipitation may be reduced by

maintaining a solution pH of < 7.0 at low temperature, for example, around 25O.

Any lead contaminants present in solution may originate from any one or more lead-

containing compounds that have been introduced during the feed stage or earlier. The

amount of lead that co-precipitates with the zinc carbonate product may depend on the pH

and temperature of the reaction conditions. In a preferred embodiment, co-precipitation of

lead contaminants may be reduced by maintaining a solution pH of < 7.0 at low

temperature, for example, around 25O.

Any nickel contaminants present in solution may originate from any one or more nickel-

containing compounds that have been introduced during the feed stage or earlier. The

amount of nickel that co-precipitates with the zinc carbonate product may depend on the

pH of the reaction conditions. The amount of nickel contaminant in solution may be

reduced by manipulating the feed pre-treatment and leaching conditions, and by

introducing DMG as a chelating or sequestering agent during the leaching stage. In a

preferred embodiment, co-precipitation of nickel contaminants may be reduced by

maintaining a solution pH of < 7.0 at low temperature, for example, around 25O.

In a preferred embodiment of the zinc oxide purification process, sodium carbonate

solution is added to the zinc sulfate solution in the reactor feed tank 1 1 at a constant

temperature of 55O. The rate at which the sodium carb onate is added into the reactor

feed tank 1 1 is ideally controlled to maintain a solution pH of 7.15. This allows the solid

form of zinc carbonate to precipitate, from solution to afford a zinc carbonate slurry.

Maintaining a constant pH of 7.15 and a constant temperature of 55O may minimise the

inclusion of small amounts of impurities contained within the resulting zinc carbonate

slurry and is essential to producing a high purity zinc oxide product. Tests have shown

that this process should produce crystals of sufficiently large size to obtain good recovery

using a suitable separating process, for example, centrifugation. Optionally, it may be

desirable to incorporate a seed recycle step (not shown) at a later stage if a larger particle

size is preferred. Since the co-precipitation of sulfate contaminants from the zinc

carbonate slurry may have been reduced by maintaining the solution pH of > 7.0 at low

temperature, for example, around 25O, it will be a ppreciated that a large amount of

sulfate contaminants may be present in the zinc carbonate slurry.

Zinc Carbonate Separation and Washing

Following the zinc carbonate precipitation reaction, the next stage in the zinc oxide

purification process is to separate the zinc carbonate from the zinc carbonate slurry. In

particular, it is desirable to separate the zinc carbonate product from the sodium sulfate

solution.

The zinc carbonate product may be separated from the sodium sulfate solution using any

suitable method. For example, separation may be achieved using a vacuum belt filter (not

shown) or a Peeler centrifuge 13. As described in the zinc carbonate precipitation stage,

maintaining the temperature around 55O may minimise the inclusion of small amounts of

impurities contained within the resulting zinc carbonate slurry. Similarly, maintaining the

temperature in this range during the separation stage may reduce the amount of

impurities in the separated zinc carbonate product and also aid settling.

In a preferred embodiment, the zinc carbonate slurry from the zinc carbonate precipitation

stage is collected in a slurry tank 12, where it may be stirred at a constant temperature of

55O prior to separation.The zinc carbonate slurry is th en fed in batches to a Peeler

centrifuge 13 where the zinc carbonate crystals may be separated from the zinc carbonate

slurry.

Desirable separation conditions are shown in Table 5 .

Table 5

The isolated zinc carbonate crystals are ideally washed with, for example, with water and

then fed to the zinc carbonate drying stage.

Following centrifugation of the zinc carbonate slurry, the remaining centrifugate, or mother

liquor, is preferably collected in a precipitation tank 14 for calcium and magnesium

removal. In a preferred embodiment, the solution is heated to around 100 C and a

suitable alkali metal carbonate solution, for example, sodium carbonate solution, is added

to achieve a pH of 9.1 . The solution may then be filtered on a plate and frame filter (15) to

recover the calcium magnesium carbonate contaminants for disposal.

Zinc Carbonate Drying and Calcining

The zinc carbonate crystals may be dried and bagged for storage, or they may be

converted to the zinc oxide product directly.

As shown in the preferred embodiment in Figure 2 , the zinc carbonate crystals isolated

using the Peeler centrifuge 13 may be collected in any suitable receptacle, for example, a

feed hopper 17 and fed to a suitable drying device, for example a flash drier 18 to dry the

crystals. The dried zinc carbonate crystals may be then collected in a bag filter 19 for

storage.

The dried zinc carbonate crystals may be converted to zinc oxide by heating the crystals

to a suitable temperature over an appropriate length of time, where the main byproduct is

carbon dioxide. Heating the zinc carbonate crystals at too high a temperature may

produce an adversely coloured zinc oxide product if certain contaminants are present in

the crystals. Preferably, the zinc carbonate crystals are heated to a temperature in the

range between 400 to 440O to avoid undesirable discol ouration of the zinc oxide product.

Good results have been obtained by heating the crystals at 420O for a period of 60

minutes.

Heating the zinc carbonate crystals may be performed using any suitable heating method,

including but not limited to any one of the following heating methods: fluidized bed reactor,

directly heated calciner, flash dryer, batch electric furnace, rotary furnace. Preferably the

zinc carbonate crystals are heated in a rotary calciner 20. The rotary action of the calciner

20 ensures that the zinc carbonate crystals may be heated uniformly.

Desirable calcination conditions are shown in Table 6 .

Table 6

As shown in the preferred embodiment in Figure 2 , the zinc carbonate crystals stored in

the bag filter 19 are continuously fed to an indirectly heated rotary calciner 20 where the

zinc carbonate crystals are calcined at 420O for a perio d of 60 minutes to form the zinc

oxide product.

The zinc oxide product isolated from the rotary calciner 20 may be collected through a

rotary valve (not shown) and bagged in any suitable container, for example a bulk product

storage bag 21. The ventilation gas comprising the carbon dioxide byproduct, is removed

from the rotary calciner passes through a bag filter (not shown) prior to discharge to the

atmosphere.

Sodium Sulfate Crystallization and Recovery

As an aside to the zinc oxide purification process, it may be desirable to purify the sodium

sulfate solution isolated during the zinc carbonate precipitation stage.

In a preferred embodiment, the sodium sulfate solution may be transferred from the

Peeler centrifuge 13 to a precipitation tank 14, where sulfuric acid is added to achieve a

slightly acidic solution pH of, for example pH 6.5. A slightly acidic solution pH may help to

prevent scaling of any impurities remaining in solution. Preferably, the precipitation tank

14 is maintained at a temperature of around 100O to m aximize the removal of unwanted

contaminants. The acidified sodium sulfate solution is then evaporated in a Mechanical

Vapour Recompression (MVR) unit 16, where around 80% of the water is desirably

removed. Evaporated water from the solution is desirably collected and recycled and the

more concentrated solution exits this stage close to the saturation point of the sodium

sulfate. The concentrated solution may then be fed to an atmospheric crystallizer (not

shown) where sodium sulfate is crystallized. . It will be appreciated by persons skilled in

the art that the large magnitude of solution evaporation may raise the concentration of any

contaminants present to significant levels. Accordingly, removal of calcium and

magnesium contaminants prior to the sodium sulfate crystallization and recovery stage

may prevent fouling of the evaporation equipment. There may also be a quantity of

chloride contaminants within the liquor, which may have originated from the feed and/or

make-up water. Removal of any chloride contaminants may be achieved by operating the

crystallizer at the saturation point of sodium chloride (265 g/L Cl). To maintain and control

the reaction, a purge stream is desirably removed from the system at a rate equal to the

mass of chlorides being entered into the reactor from the MVR unit 16. The chloride

concentration within the reactor therefore remains constant. A stream from the crystallizer

(not shown) may then be fed into a suitable separation device, for example, a batch

operated pusher centrifuge (not shown) to dewater the sodium sulfate crystals present in

the slurry, or may be first recovered using a thickener (not shown) before being fed to the

pusher centrifuge (not shown). The remaining filtrate may then be returned to the MVR

unit 16 and the recovered sodium sulfate crystals transferred into a storage device, for

example a feed hopper (not shown). It may be desirable to scrub the off-gas from the drier

using a wet scrubber (not shown) prior to venting it to the atmosphere.

Now that a preferred embodiment for a method of purifying a zinc sulfate solution in the

production of zinc oxide has been described it will be apparent to those skilled in the art

that it has the following advantages:

1. The zinc oxide purified by the process of the present invention is of an

exceptionally high grade compared to zinc oxide produced by known

hydrometallurgical processes;

2 . All major contaminants within the feed are removed during the purification stage

and hence are not included within the final zinc oxide product.

3 . The purification stage occurs over two simple steps and uses cost effective

reagents adding to the commercial viability of the invention.

4 . The use of a zinc residue to reprocess into a high grade zinc oxide.

It will be appreciated by persons skilled in the art that numerous variations and/or

modifications may be made to the invention as shown in the specific embodiments without

departing from the spirit or scope of the invention as broadly described. The present

embodiments are, therefore, to be considered in all respects as illustrative and not

restrictive.

CLAIMS

1. A method of purifying a zinc sulfate solution comprising mixing the zinc sulfate

solution together with an aqueous metal carbonate to produce a zinc carbonate

slurry.

2 . A method of purifying a zinc sulfate solution in the production of zinc oxide, said

method comprising the steps of:

mixing the zinc sulfate solution together with an aqueous metal carbonate

to produce a zinc carbonate slurry;

separating zinc carbonate from the zinc carbonate slurry; and

heat treating the zinc carbonate to produce the zinc oxide.

3 . A method of purifying a zinc sulfate solution as defined in either of claims 1 or 2

wherein the step of mixing the zinc sulfate solution together with an aqueous metal

carbonate comprises controlling the rate of addition of the aqueous metal

carbonate.

4 . A method of purifying a zinc sulfate solution as defined in claim 3 wherein the step

of mixing comprises controlling the rate of addition of the aqueous metal carbonate

so that the mixing occurs at a pH of between 5 to 9 .



so that the mixing occurs at a pH of between 6 to 8 .



so that the mixing occurs at a pH of between 7 to 7.2.


of mixing the zinc sulfate solution together with an aqueous metal carbonate

comprises controlling the rate of addition of the aqueous metal carbonate so that

the mixing occurs at a temperature of between 5OO to 6OO and at a pH of

between 7 to 7.2.

8 . A method of purifying a zinc sulfate solution as defined in any one of the preceding

claims wherein the aqueous metal carbonate comprises one or more types of

alkali metal carbonates.

9 . A method of purifying a zinc sulfate solution as defined in claim 8 wherein the one

or more types of alkali metal carbonates include those of sodium, potassium.

10. A method of purifying a zinc sulfate solution as defined in any one of the preceding

claims further comprising a preliminary step of removing one or more

contaminants from the zinc sulfate solution.

11. A method of purifying a zinc sulfate solution as defined in claim 10 wherein the

step of removing one or more contaminants from the zinc sulfate solution includes

the addition of an alkaline solution and an oxidant in a temperature range of

between 5OO to 100O.


alkaline solution comprises one or more types of alkali metal cations, and the

oxidant is selected from the group consisting of: hydrogen peroxide, sodium

hypochlorite, potassium permanganate, oxygen.

13. A method of purifying a zinc sulfate solution as defined in claim 12 wherein the one

or more types of alkali metal cations include those of sodium or potassium and the

oxidant is potassium permanganate.

14. A method of purifying a zinc sulfate solution as defined in any one of claims 10 to

13 wherein the one or more contaminants includes nickel, aluminium, iron, calcium

or manganese.


step of removing nickel from the zinc sulfate solution comprises adding a chelating

agent material.


chelating agent material is dimethyl glyoxime.

17. A method of purifying a zinc sulfate solution as defined in claim 2 wherein the step

of separating zinc carbonate from the zinc carbonate slurry involves centrifuging

the zinc carbonate slurry.

18. A method of purifying a zinc sulfate solution as defined in claim 2 wherein the step

of heat treating the zinc carbonate to produce the zinc oxide involves calcining the

zinc carbonate at a temperature of between 400O to 4 4OO.


step of calcining the zinc carbonate occurs at a temperature of 420O.

INTERNATIONALSEARCH REPORT International application No.

PCT/AU2010/000206

A. CLASSIFICATION OF SUBJECT MATTER

Int. Cl.

COlG 9/02 (2006.01) COlG 9/00 (2006.01) .

According to International Patent Classification (IPC) or to both national classification and IPC

B, FIELDS SEARCHED

Minimum documentation searched (classification system followed by classification symbols)

Documentation searched other than minimum documentation to the extent that such documents are included in the fields searched

Electronic data base consulted during the international search (name of data base and, where practicable, search terms used)EPODOC WPI, JAPIO, CAPLUS: & keywords: zinc oxide, zinc sulfate, zinc carbonate, & similar terms

C. DOCUMENTS CONSIDERED TO BE RELEVANT

Category* Citation of document, with indication, where appropriate, of the relevant passages Relevant toclaim No.

DE 4223300 Al (BAYER AG) 20 January 1994X Column 1 lines 1-13, column 2 lines 39-49, examples 1-9.Y Column 1 lines 1-13, column 2-lines 39-49, examples 10-17

US 2144299 A (SESSIONS et al.) 17 January 1939X - Page 2 1-3, 8-10, 14Y Page 2 11-13, 15-17

CN 1927722 A (UNIV LANZHOU TECHNOLOGY) 14 March 2007. English abstractretrieved from EPODOC database

X Abstract 1-5, 8, 9Y Abstract 10-17

X Further documents are listed in the continuation of Box C X See patent family annex

* Special categories of cited documents:"A" document defining the general state of the art which is "T" later document published after the international filing date or priority date and not in

not considered to be of particular relevance conflict with the application but cited to understand the principle of theoryunderlying the invention

"E" earlier application or patent but published on or after the "X" document of particular relevance; the claimed invention cannot be considered novelinternational filing date or cannot be considered to involve an inventive step when the document is taken

alone"L" document which may throw doubts on priority claim(s) "Y" document of particular relevance; the claimed invention cannot be considered to

or which is cited to establish the publication date of involve an inventive step when the document is combined with one or more otheranother citation or other special reason (as specified) such documents, such combination being obvious to a person skilled in the art

"O" document referring to an oral disclosure, use, exhibitionor other means "&" document member of the same patent family

"P" document published prior to the international filing datebut later than the priority date claimed

Date of the actual completion of the international search Date of mailing of the international search report

18 March 20 10 2 MAR 2010

Name and mailing address of the ISA/AU Authorized officer

AUSTRALIAN PATENT OFFICEROBYN KNOCK

PO BOX 200, WODEN ACT 2606, AUSTRALIA AUSTRALIAN PATENT OFFICE

E-mail address: [email protected] (ISO 9001 Quality Certified Service)Facsimile No. +61 2 6283 7999 Telephone No : +61 2 6283 3149

Form PCT/ISA/210 (second sheet) (July 2009)

INTERNATIONAL SEARCH REPORT International application No.

PCT/AU2010/000206

C (Continuation). DOCUMENTS CONSIDERED TO BE RELEVANT

Category* Citation of document, with indication, where appropriate, of the relevant passages Relevant to claimNo.

CN 101049956 A (UNIV SHANGHAI) 10 October 2007. English abstract retrievedfrom EPODOC database

X Abstract 1, 2, 8, 9, 18, 19Y Abstract 10-17

JP 04-280814 A (HAYASHI KINZOKU KOGYOSHO KK) 6 October 1992. Englishabstract retrieved from EPODOC database

X Abstract 1-6, 8, 9Y Abstract 10-17

US 2001/0031272 A l (NOGUCHI et al.) 18 October 2001X Paragraphs [0022]-[0032] 1-4, 8, 9 18, 19Y Paragraphs [0022]-[0032] 10-17

US 6171580 B l (KATSUYAMA et al.) 9 January 2001X Column 2 1-9, 18, 19Y Column 2 10-17

GB 190317785 A (ARMBRUSTER et al.) 1 October 1903X Page 1 1, 3, 8, 9Y Page l 10-17

CN 141 8972 A (WANG) 2 1 May 2003. English abstract retrieved from EPODOCdatabase

Y Abstract 10, 14

US 3493334 A (LTJBENGOOD) 3 February 1970Y Column 1 lines 27-40 10-14

US 6248241 Bl (CHRISTENSEN et al.) 19 June 2001Y Column 3 lines 18-21 10-14

US 5635073 A (AKTOR et al.) 3 June 1997Y Column 3 lines 38-42 10-14

US 3148944 A (VAN DIJK et al.) 15 September 1964Y Column 2 lines 39-45 10-14

Form PCT/ISA/210 (continuation of second sheet) (July 2009)

INTERNATIONAL SEARCH REPORT International application No.

PCT/AU2010/000206

C (Continuation) DOCUMENTS CONSIDERED TO BE RELEVANT

Category* Citation of document, with indication, where appropriate, of the relevant passages Relevant toclaim No.

US 5278743 A (BENGEL et al.) 11 January 1994Y Whole document 10-14

US 4828712 A (REYNOLDS et al.) 9 May 1989Y Abstract, column 2 lines 13-36 10, 14-16

US 1279888 (LOSTUMO et al.) 24 September 1918Y Page 1 lines 38-41 17

US 1362078 A (BONNER) 14 December 1920Y Page 1 lines 64-65 17

Form PCT/ISA/210 (continuation of second sheet (2)) (July 2009)

erna ona ap ca on o.

Information on patent family members PCT/AU2010/000206

This Annex lists the known "A" publication level patent family members relating to the patent documents cited in theabove-mentioned international search report. The Australian Patent Office is in no way liable for these particularswhich are merely given for the purpose of information.

Patent Document Cited in Patent Family MemberSearch Report

D E 4223300 NONE

U S 2144299 NONE

CN 1927722 NONE

CN 101049956 NONE

JP 4280814 NONE

U S 2001031272 CN 1235016 EP 0958809 JP 11302625

US 6447759

us 6171580 AU 10544/99 CN 1248228 EP 0992455

JP 2007182382 WO 9925654

G B 190317785 NONE

CN 1418972 NONE

U S 3493334 BE 741436 DE 1958304 FR 2082021

GB 1285596 NL 6916375

us 6248241 AU 45504/97 DK 112396 EP 0938453

NO 991684 WO 9816476

us 5635073 AU 49461/93 EP 0660804 WO 9406717

us 3148944 BE 651345 DE 1494573

U S 5278743 NONE

us 4828712 NONE

us 1279888 NONE

us 1362978 NONE

Due t c data integration issues this family listing may not include 10 digit Australian applications filed since May 2001.

END OF ANNEX

Form PCT/ISA/210 (patent family annex) (July 2009)

Documents

WO 2010/096862 Al