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Hydrothermal processing — An emerging technology
Hydrothermal Processing- An Emerging Technology Edgel P Stambaugh, Battelle Columbus Division, 505 King Avenue, Columbus, Ohio 43201-2693 USA
Abstract Hydrothermal processing involves the chemistry of hot water under pressure to carry out leaching and precipitation reactions. These reactions are usually conducted at temperatures ranging from I O0 to 350°C andpressures in the range from a few to 3000 psig. The reaction medium may be water alone or in combination with inorganic and~or organic acids and bases. The chemistry of this technology permits selective dissolution (leaching) or selective precipitation of metal species to be achieved leading to potentially novel treatment methods using a wide variety o fores and raw materials to produce high quality inorganic products. Thus, hydrothermal technology offers alternate or new approaches which can be technically superior to conventional processing for producing a wide variety of high technology inorganic oxides for specific applications, recovery of valuable components from low grade ores, and converting waste material to marketable products. Furthermore, because of short reaction times, simplicity of the reactor and recycle of reagents, the application of hydrothermal technology frequently leads to processes which can be less energy intensive, less pollutant and/or less capital intensive. Hydrothermal technology has a/ready been used in many successful commercial processes. Now, with the combination of feedstock shortages, rising energy costs, more restrictive pollution abatement regulations, inflationary pressures, need for higher quality products and a better understanding of the chemistry of hot water, this technology is emerging as a viable alternative to more conventional high temperature processing. The technology is expected to impact on several industrial areas, including primary and secondary ferrous and non.ferrous metals, ceramics, electronics, pigments, petroleum, utilities, refractories and inorganic chemicals.
Introduction More restrictive pollution abatement regulations, feedstock shortages, rising energy costs, inflationary pressures, and the need for higher quality products at lower costs are forcing industry to search out and develop alternative processes. Hydrothermal processing, one altemative technology, has been used in minerals processing for many decades. The most notable and well-known commercial application of hydrothermal processing is the Bayer aluminia process. This process was first commercialised in 1939 for the recovery of aluminia from bauxite or~'. Other commercial applications since then include the production of molecular sieves, synthetic rutile 2~, and nickel and cobalt metal powders. Because of its versatility, hydrothermal processing is reemerging in minerals processing and being evaluated for the production of advanced inorganic oxides and the conversion of industrial effluents to recyclable a n d / o r added-value products.
Process description Hydrothermal processing
involves
the chemistry of hot water under pressure to carry out leaching and precipitation reactions. These reactions are conducted at temperatures ranging from 100 to 350°C and pressures in the range of a few to about 3000 psig. The reaction medium may be water alone or in combination with inorganic and/or organic acids and bases. Such process chemistry has been and is being used to accelerate leaching reactions, aid physical processes during crystal growth, improve quality of inorganic products, and recover valuable materials from a wide variety of feedstocks. Examples of these are ores, concentrates, and waste materials. A potential application of this technology for the industry is broad. A summary of s o m e of these applications for specified industries is shown in Table 1.
Definition of hydrothermal processing Hydrotherma] processing, as the name implies, is based on processes using the chemistry of hot water under pressure. As shown in Figure 1, two conventional processing appli-
MATERIALS & DESIGN Vol. 10 No. 4 JULY/AUGUST 1989
cations are leaching and precipitation. Each of these is discussed in the following. Hydrothermal leaching Hydrothermal leaching may be used to extract selectively impurities from a raw material, leaving the product as the leach residue. An example of this application is the production of synthetic rutile via the benelite process~2L In this example, the iron value is selectively extracted from an ilmenite ore, leaving a crude TiO2 product ( > 9 0 percent TiO2) as the leach residue. The crude titanium dioxide product, referred to as synthetic rutile, is used as a substitute for natural rutile for the production of titanium dioxide pigment by the chloride vapour phase route. Another example in which hydrothermal leaching may be employed is for the treatment of basic oxygen furnace (BOF) dust~3~.Impurities such as zinc, calcium, manganese, lead, magnesium, and copper are selectively extracted from the BOF dust to produce a high purity ferric oxide which may be used as a pigment or raw material for the production of ferrites and metallic iron.
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Hydrothermal leaching can also be used to extract selectively the major component from feedstock to produce a pregnant liquor from which the major component may be recovered by hydrothermal precipitation or electro-winning. An example of this application is the hyclrothermalleaching of zinc sulphide concentrate to produce elemental sulphur and a zinc sulphate solutiont4L The zinc value is subsequently recovered from the sulphate liquor by electro-winning. Another example is the recovery of aluminium value from bauxite ore by the Bayer processc'L In this example, the aluminium value is selectively extracted, leaving impurit!es such as silica and iron in the residue. The aluminium value is subsequently recovered from the pregnant liquor by hydrolysis of the sodium aluminate.
Table I Potential industrial applications of hydrothermal technology Industry
Application
Iron/steel
Upgrading ol iron ores Wastes to markefable products Treatment of sludges/aqueous streams Conversion of non-coking coals Io coking coals Desulphurisation/deashing of coking coals Metal powders Treatment of electroplalers wastes Critical metals recovery Minerals processing Ore beneficiation Advanced inorganic oxides for specific applications Treatment of spent refractory pottery wastes
Non-ferrous metals
Ceramic magnetic electronic Pelroleum
Production of catalysts Oil from tar sands Regeneration/treatment ol catalysts Desulphurisation of petroleum coke Treatment of sludges, solid wastes, and aqueous waste streams Direction precipitation of pigments Solid/sludge/waste water treatment Silicate production Chromate production Deashing of coal Coal-oil/water mixtures Coal chemicals Treatment of fly ash and coal ash Detoxification ot coal Treatment of incinerator ash Destruction of organic wastes Conversion of biomass to organic feedstocks Treatmenl of cellulostic malerials
Pigment Inorganic Chemicals Coal/utilities
Hydrothermal precipitation Hydrothermal precipitation is an alternate approach to conventional processing for the production of high quality, advanced inorganic oxides from pure and impure solutions and from hydroxide and hydrated oxides. Basically, this process consists of heating an aqueous solution containing soluble metal species or an aqueous slurry in an autoclave at temperatures less than the critical of water, and the corresponding pressure (steam alone or steam pressure plus an added over-pressure of an inert, oxidising, or reducing gas) for a time sufficient to promote the formation of the desired product. These products may be doped and undoped, simple and complex inorganic oxides. "Fhe process may be operated batchwise, continuously, or semicontinuously. It may also be operated as a single stage process whereby a single oxide is precipitated from the aqueous solution. An example of a single-stage hydrothermal process is the precipitation of zirconia from an aqueous zirconium oxychloride solution(5.6L In this example, anhydrous zirconia is precipitated from a zirconium oxychloride solution by heating the solution to a temperature of about 200°C and the corresponding steam pressure. Doped simple oxides such as yttrium-doped zirconia are produced directly by heating an aqueous alkaline slurry of yttrium hydroxide and zirconium hydroxide in an autoclave at temperatures greater than about
176
Food Agriculture
/
FEEDSTOCK
Precipitation
Leaching
Extract Impurit=es
l Product
I
Extract Primary Component
I
i
1
Precipitate I Impurities
Product
I Precipitate Primary Component
1
Product
Product
1 Final Product
Intermedlate Product
( Iron Oxide)
(Synthetic Ruttle)
Fig 1
Electrolytic
Winning
Hydrotherma 1 Precipitation
Versatility of hydrothermal processing to extract products from feedstock
MATERIALS & DESIGN Vol. 10 No. 4 JULY/AUGUST 1989
190°C and under the corresponding steam pressure(7.8.9). Complex oxides, ie oxides containing two or more metal ions, are normally produced hydrothermally by heating the corresponding hydroxides. For example, BaFe~20~9 is prepared by heating an aqueous slurry of barium hydroxide and ferric hydroxid~'°.~L Lead zirconate titanate is formed by heating an aqueous alkaline slurry of the reactants, having a pH ranging from about 7 to 14, at temperatures greater than about 250°C and the corresponding steam pressures('2L The multi-stage mode of operation offers the opportunity for production of high purity oxides from impure feedstocks. An example is the recovery of high purity zirconia from zircon as shown in Figure 2. In this example, a zircon frit is dissolved in hydrochloric acid. High purity ZrO2 is recovered by first selectively precipitating SiO 2 and Ti02 by heating the solution to about 150°C, leaving the Zr02, etc in solution. After separating the precipitated SiO2 and TiO 2 from the solution, the ZrO2 is selectively precipitated by heating the liquor to a higher temperature and pressure (about 180°C and under the corresponding steam pressure) leaving the impurities - AI203 and MnO2 - in solution. AI203 can be precipitated by heating the liquor to a still higher temperature and pressure~13L
ZIRCON FRIT
------I DISSOLVER J
J
FILTER _ INSOLUBLE -- MATERIAL
STAGE 1
SIO2 PRECIPITATOR
L
FILTER PRESS
L
j
FILTER PRESS PRIMARY -~- FILTRATE FOR RECYCLE
STAGE2
WASHSOLUTIOR
m[
MIXER
I
%)
Table II Typical hydrothermal processing conditions Solvent Temperature Pressure pH Time Atmosphere
Hydrothermal processing may be operated in a batch or continuous mode of operation and as a single stage or multi-stage process for both leaching and precipitation. In the batch mode of operation, the reactants are simply mixed and heated in a tank type autoclave to achieve the
FILTRATE ~-- FOR RECYCLE
Flow diagram of the selective recovery of silicon and zirconium dioxides (SiO2 and ZrO~) from zircon frit by hydrothermal precipitation~s2~
Modes of operation
Batch operation
]
[
ZrO2 (99.9 +
F g2
SiO2
J
Lp Zr02 RECIPITATOR
Processing conditions The range of processing conditions under which hydrothermal processing can be conducted are shown in Table IL As shown, hydrothermal processing is a low to moderate temperature and pressure operation, not a high temperature, high pressure operation. Reaction time is very short (5 to 60 minutes), making hydrothermal precipitation as well as hydrothermal leaching amenable to continuous operation. Furthermore, hydrothermal precipitations can be conducted over the entire PH range and under a steam, oxidising, or reducing atmosphere.
IICI
Mode of Operation
Not water Hot water & inorganic or organic acids Hot water & alkali and/or alkaline earths Less than 350C Less than 3000 psig Less than 1 to 14 5 to 60 min. Steam (inert) Steam & oxidising agent (02) Steam & reducing agenl (H2) Batch or continuous Single-stage Multi-stage
desired reaction, whether it be leaching or precipitation. An example of a laboratory batch-type autoclave is shown in Figure 3. The autoclave on
MATERIALS & DESIGN Vol. 10 No. 4 JULY/AUGUST 1989
the left is equipped with a sampling set-up to withdraw samples as a function of time and temperature for studying the kinetics of the leaching
177
rig3
Fig4
Batch- type autoclaves for conducting hydrothermal research
PREPARATION
TREATMENT
H
1
SEPARATION
H
Commercial synthetic rutile plant (courtesy of United Mcgill Corp.)
RECOVERY
F
L HYOROTHERMAL TREATMENT
Fig 5
Schematic of an integrated continuous hydrothermal process
or precipitation i-eaction. An example of a commercial batch operation is depicted in Figure 4. These autoclaves are spherically shaped vessels. In this example, synthetic rutile is produced as noted above by heating ilmenite ore in an aqueous hydrochloric acid solution to selectively extract iron and a number of other impurities, leaving the synthetic rutile containing greater than about 95 percent titanium dioxide as the residueI"h Other examples are given in F Habashi's paper on "Recent Advances in Hydrometallurgy 'c~5~.
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Continuous operation A continuous hydrothe'rmal processing system can be very simple (Figure 5). The system consists of: feed preparation heat exchanger tank product storage feed storage tank tank pump filter and wash autoclave tanks In a typical tank operation, the feed solution or slurry is prepared and pumped to the feed storage tank. From here, the solution or slurry is pumped through the heated reactor to carry out the reaction. The reaction
may involve hydrothermal leaching or hydrothermal precipitation. The product slurry or solution is then cooled in the heat exchanger and discharged through a pressure letdown valve into the product storage tank. The product is then processed normally by conventional processing. Hydrothermal processes were originally developed for batch operation. However, because of short reaction time, better understanding of the chemistry of hydrothermal processing, and the more favourable economies of continuous processing, the trend is now toward the continuous mode of operation. An example of a continuous bench scale hydrothermal facility is shown in Figure 6. In this example, geothite (Fe00H) is being hydrothermally converted to ferric oxide by continuously pumping an aqueous slury of the geothite through the heated tubular reactor. The schematic of a continuous pilot plant facility is shown in Figure 7. Current s t a t u s of h y d r o t h e r m a l processing Hydrothermal processing is receiving increased attention throughout the world. Because of its versatility, this technology is expected to replace many high temperature, hydrometallurgy type operations.
MATERIALS & DESIGN Vol. 10 No. 4 J U L Y / A U G U S T 1 9 8 9
O~sPF~pla~en(
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Continuous hydrothermal facility for producing inorganic oxides
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F~g8 Applications for hydrothermally derived inorganic oxides Hydrothermal leaching - mineraLs processing Hydrothermal leaching has made significant progress in the last few years and will continue to progress(~6L Primary emphasis has been on the recovery of aluminium, nickel, cobalt, copper, zinc, uranium, and titanium for a wide variety of concentrates and waste materials. Some of these processes employ leaching and a combination of leaching and precipitation. The Bayer process for the recovery of aluminia from bauxite is still the largest and the oldest leaching operation in the worldm. This process originally developed as a batch operation has been converted into a continuous operation. Hydrothermal processing is also being applied in the treatment of a variety of nickel and cobalt feedstocks for nickel and cobalt recovery
Fig 7
Battelle's hydrothermal pilot facility
employing leaching to extract the metals and a multi-stage process for selective recovery of the metal values from the leachate by hydrogen reduction. One of these plants is operated by Sherritt Gordon Mines, Ltd. This plant produces nickel and cobalt powders and nickel - or cobaltcoated powdersWL Hydrothermal leaching is being applied in the zinc industry by Cominco, Ltd, which opened the world's first zinc hydrothermal leaching plant at Trail, B C, Canada in 1981(~8LThis is a continuous process whereby a zinc concentrate is leached at a moderate temperature and pressure to produce zinc sulphate solution, an iron-lead residue and a molten elemental sulphur. The use of hydrothermal leaching has been practiced for many years for uranium recovery and interest still
remains high on uranium recovery~% A recent development is the uranium extraction plant operated by Vereinigte Aluminium Werke-I ippenwerk, Lunen, West Germany. Hydrothermal leaching has also received worldwide attention in the production of synthetic rutile as a substitute for natural rutile for the production of titanium dioxide pigment. In this case, iron, along with other impurities are selectively separated from the titanium dioxide by hydrothermal leaching leaving a residue containing 90 to 97 percent titanium dioxide. Examples of plants producting synthetic rutile by hydrothermal leaching are Kerr McGee in US, Ishihara Sangyo Daisha, Ltd, in Japan, Murphyores, Inc, PTY, Ltd, and Western Titanium N I_,in Australia. Hydrothermal leaching is now receiving attention in the precious
/
MATERIALS & DESIGN Vol. 10 No. 4 JULY/AUGUST 1989
179
Table III HydrotherTnically produced inorganic oxides Simple Oxides Zr02(5,6,ll) Cr02(24,25)
Simple
Doped Oxides
Y203-Zr02(7,8,9,22)
CdO(21)
Cr203(26)
Y203 , CaO-,MgO-Zr02(8)
CoO(21)
Mn02(27)
Eu-Hf02(31)
Fe203(21,]I)
U02(28)
ZnO-Bi203-Mn203 (36)
Complex
Oxides
BaFe12019 (lO,I]) Y3Fes012(32)
Homogeneous Single crysta}s Controlled doping Non-agglomerated High purity
PbZrO3-Pbri03(12, 34,37)
Th02(28)
BaTi03(35)
MgO(21)
Si02(29)
BaZr03(22)
NiO(21)
Nb205(30)
(Ba,Sr) Ti03(38)
ZnO(21,22)
A1203(22~
Ti02(19)
Mn304(21)
Hydrothermal precipitation production of advanced inorganic oxides Hydrothermal precipitation technology has not advanced as much as hydrothermal leaching technology. However, because of the need for improved inorganic oxide powders for high tech applications and the ability for hydrothermal processing to produce these powders, hydrothermal processing is becoming a worldwideemerging technology for the production of advanced inorganic oxides for several applications. Examples of these applications are shown in Figure 8. Table III presents a partial listing of oxides which have been produced by hydrothermal processing.
Anhydrous Controlled crystal size and shape Narrow particle size distribution High purity
CaSi03(33)
Fe304(21)
metals industry and increased attention in the zinc industry. Homestake Mining has successfully employed a combination of leaching and precipitation for liberating gold and silver from a complex orc~2°L Kidd Creek Mining has recently constructed a plant for the recovery of zinc from a zinc sulphide concentrate by hydrothermal leaching~4L Thus, hydrothermal processing leaching alone or a combination of leaching and precipitation - is receiving increased attention in the minerals industry for the recovery of a wide variety of metal values from complex ores and concentrates (both highgrade and low-grade feedstocks).
180
Table IV Characteristics of hydrothermically produced oxides
acicular, platy, and equiaxed particles. Specific examples are shown in Figures 9 through 12. In addition, while not shown, the composition as well as the crystal (particle) morphology can be controlled to within very narrow specifications by hydrothermal precipitationCSL
An alternate manufacturing approach
Nature of hydrothermally precipi. tated oxides Inorganic oxides having the characteristics shown in Table IV are precipitated from aqueous solutions or slurries by hydrothermal processing as high purity, non-agglomerated, anhydrous, single crystal particulates of a controlled size and shape. These preciptated oxides exhibit a high degree of homogeneity and a uniform particle size distribution. Since the process involves chemical reactions that are carried out at low to moderate temperatures and pressures, the oxides contain fewer structural defects than those produced by high temperature calcination/roasting processes. Since the oxides contain little or no chemically bound water, only a simple drying is required to removed physically bound water. Normally, high temperature calcination or roasting is required to produce anhydrous oxides. Since the oxides are non-agglomerated, no milling or grinding is required to reduce the material to the ultimate particle size; thus, contamination by the grinding medium and the introduction of structural defects by grinding is avoided. In fact, the physical, chemical, and electronic properties can be tailored for specific applications by hydrothermal processing. Through the manipulation of the processing parameters, the shape of the crystals as well as the size of the crystals can be controlled to produce
A number of universities and research institutions are gearing up to assist industry. Examples of these are Battelle Columbus Division, Tokyo Institute of Technology and the University of Utah. While hydrothermal precipitation is in an early stage of development, industry is beginning to view hydrothermal precipitation as an alternative to conventional processes such as calcination, roasting, and sol-gel. Consequently, commercial plants have been constructed, are being constructed, or plans to build a commercial plant have been announced. In support of this commercialisation activity, the major activity in commercialisation of hydrothermal precipitation for the production of advanced inorganic oxides is in the general areas of toughened ceramics, electronic oxides, and pigments. For example, Cabot has developed a hydrothermal process through the pilot plant stage for the production of barium titanate-based dielectric powders(35L Sakai Chemical Industry Co has announced completion of a mass production plant for producing ultrafine ceramic powder using a hydrothermal synthesis process originally developed for the manufacture of magnetic iron oxide(39~.According to the report, this powder, used for functional ceramic products, has a grain size about one-tenth that of conventional products and can be sintered at low temperatures. Chichibu Cement Co, together with the Tokyo Institute of Technology, is planning to construct facilities at its Kumogaya
MATERIALS & DESIGN Vol. 10 No. 4 JULY/AUGUST 1989
a. 2.0 iz diameter Fig 9
b. 1.0 # diameter
Crystal morphology of zirconia produced by manipulation of hydrothermal processing conditions ~'JT~
Fig I0 Particle shape of hydrothermally produced partially stabilized zirconia ,.m
Fig 11 Hydrothermally produced acicular ferric oxide i, factory to produce fine zirconia powder by hydrothermal processingC4% According to Chichibu Cement, the powder is very uniform in size and can be sintered at temperatures about 100°C lower than required in the case of zirconia powders produced by conventional processing. H y d r o t h e r m a l p r o c e s s i n g to rec o v e r materials from industrial effluents
Fig 12 Barium ferrite (BaFe12019) produced by hydrothermal precipitatior~l ,
MATERIALS & DESIGN VoL 10 No. 4 JULY/AUGUST 1989
Hydrothermal processing provides a very attractive solution to a mounting waste m a n a g e m e n t problem. With industry generating millions of tons of solid and liquid effluents each year, waste m a n a g e m e n t and material recycling are a big-and-growing business worldwide. One source, for example, estimates that in the US alone, the waste m a n a g e m e n t industry will grow from $15.3 billion in 1987 to $23.9 billion in 1992 and that material recycling will grow from $7.8
181
billion to $10.3 billion in the same time period.{4'L A major reason for the increasing emphasis on waste management is that many traditional effluent disposal methods, such as ocean dumping, deep welling, and ]andfilling, are being strongly regulated, or even banned. Thus, industries are seeking alternative means of effluent disposal that are not only environmentally sound but also economical. Industries that generate effluents containing hazardous or potentially hazardous pollutants are especially hard pressed to find economical alternatives. For these wastes, disposal costs can be as high as $600 per ton and are expected to increase. As a result, some companies appear to be shutting down or changing their mode of operation to a less polluting process. But there is another alternative: converting effluents to recycleable, often marketable products. One company's waste material may actually be another company's raw material. In many cases, the effluent requires little or no treatment for use as a raw material. For example, waste iron oxide generating from treating pickle liquor can be used in the manufacture of barium ferrite. In other cases, the effluents or their components do require treatment before recycling or use as raw materials. An example is spent catalysts from the petroleum industry, which contain molybdenum, vanadium, nickel, and cobalt that can be recovered for recycling for the production of catalysts or can be used as raw material for the production of pigments. Many processes have been developed for treating waste materials to reduce pollution problems and to take advantage of profit opportunities. Many of these processes recover effluent components as high-purity oxides that can be used directly as raw materials. One common approach for obtaining these oxides is to use high temperature calcination/roasting processes, but an alternative - hydrothermal processing - shows promise as a technically and economically superior way to convert solid and liquid effluents into marketable, recyclable, and/or environmentally safe products.
Applications to recover useful products Hydrothermal processing has advantages which exceed those of the high
182
-Fable V High purity ferric oxide from electric arc furnace dust Concentration %
Metal
Zn
Before
15.0
After Treatment(1 )
< 0.01
Cu
0.05
< 0.01
Pb
0.1
< 0.01
Mg
1.7
< 0.01
Mn
1.1
< 0.01
(1) Nol detected: B. Sb. As, Co. Cd, Ge, Be. A g
High purity ferric oxide of controlled particle size from electric arc furnace dust by hydrotherrnal processin~ ~' temperature calcination/roasting processes in addition to a variety of applications for the treatment of effluents. Because of the unique chemistry of aqueous solutions, hydrothermal leaching and precipitation technology can be used to treat effluents to recover useful products. One example is the hydrothermal treatment of electric-arc furnace dust, which the US Environmental Protection Agency (EPA) has classified as a hazardous waste material. In this process, all hazardous or potentially hazardous metals as shown in Table V are extracted, resulting in a pigmentgrade (high purity) ferric oxide of
controlled particle size (Figure 13) for use as a pigment or in the production of magnetic oxides or metallic iron. Subsequent treatment of the extract produces a crude zinc product for use as a raw material in zinc smelters. Upon further treatment by hydrothermal processing, high purity heavy metal oxides such as zinc oxide can be produced '2~''. Another example is the recovery of high purity ferric oxide from basic oxygen furnace dust as shown in Table VI~22'. The treatment of fly ash by a combination of hydrothermal leaching and hydrothermal precipitation is another application. In this case, an
MATERIALS & DESIGN Vol. 10 No. 4 J U L Y / A U G U S T 1989
Table VI Pigment grade ferric oxide from basic oxygen furnace dust Concentration %
Metal
Before
Zinc Manganese Magnesium Lead Copper Calcium
2-4 10 05 10 0 02 5-10
After Treat ment(~
< <: < < < <
03 0 02 00t 001 001 001
(1) Others not detected
a. Spherically shaped particles of fly ash (before treatment)
4 ~ ,:.
b. Fly ash converted to acicular shaped particles (after treatment) Conversion of fly ash from spherically shaped particles to acicular shaped particles by hydrothermal processing ~'"
MATERIALS & DESIGN Vol. 10 No. 4 JULY/AUGUST 1989
alumina product of more than 99.9 per cent purity (see Table VII) was recovered for use in the aluminium and pigment industries. In addition, the shape of the fly ash particles was modified from spherical to acicular Ishown in Figure 14{~2~].This modified ash may be used for strengthening concrete, plastics, and other building materials. Hydrothermal treatment can also produce useful materials from the sludges that many industries, eg electroplating, iron/steel, and brassgenerate, recovering the heavy metals in a form suitable for use as raw materials in other industries. In the case of hydrothermal treatment of pickle liquor sludge, a highly reactive ferric oxide was precipitated. This oxide was found to be an excellent raw material for producing wire cord for the tyre industry2~L Another application is the cleanup of aqueous streams containing heavy metals. As shown in Table VIII, hydrothermal treatment of an aqueous stream containing barium, chromium, copper, lead, and ziiac resulted in essentially complete precipitation of these heavy metals. Leachability tests of hydrothermally precipitated oxides from a waste stream indicate that the product is environmentally safe ~22~. Thus, it could be disposed of in a landfill or further treated to recover heavy metals such as high purity oxides. Other applications include treatment of electroplating wastes, regeneration/ treatment of spent catalysts, treatment of incinerator ashes and dest_ruction of organic wastes as shown in Table I.
The future of hydrothermal processing Hydrothermal processing is receiving increased attention worldwide. Primary emphasis has been on the recovery of aluminium, nickel, cobalt, zinc, copper, molybdenum, tungsten, uranium, and titanium from a wide variety of ores, concentrates, and waste materials. Consequently, commercial plants are found in a number of countries throughout the world. In recent years, increased understanding of a q u e o u s chemistry ,~424~444546, has been moving hydrothermal technology from an "art" to a technology based on sound scientific and engineering principles. With the
183
Table VII High purity alumina from fly ash by hydrothermal processing Concentration of Impurities in Alumina Product, ppm(2)
Concentration of Metals
Element
in Fly Ash, ppm(1)
Be B Si P S Ti Fe Cu Ni
5 15 30% 1000 600 3% 10% 200 200
0.15 1 400 300 200 15 10 6 10
(1) ppm except where noted. (2) Others, Mn, Co, V, As and rare earths, less than 5 ppm.
Table VIII Hydrothermal precipitation of metals from a waste stream Concentration mg/I Metal
Ba Cr Cu Pb Zn Fe
Untreated
4000 9000 3000 5000 16000 0
realisation of the potential of hydrothermal processing, more types of raw materials (including industrial effluents) are being treated hydrothermally to produce a wide variety of products. Examples are the recovery of precious metals, ~ chromium, and manganese and the production of ceramics, pigments, and catalysts. Some of these processes, notably precious metals recovery and advanced ceramics production, have already reached the commercial stage. Others are in the pilot plant stage of development or near-commercial stage; still others are in the laboratory stage. The increased need for higher quality products at lower costs, more restrictive pollution abatement regulations, feedstock shortages, rising energy costs, and inflationa~ pressures have enhanced the superiority of hydrothermal processing over that of conventional high temperature processing. Thus, hydrothermal processing is emerging as an alternate technology to more conventional processing for the conversion of a wide variety of raw materials to
184
Treated
% Removal
3.8 14.9 4.3 540.0 780.0 0.4
99.9 99.8 99.9 89.2 95.1
added-value products. References
1
F A Peters et al, "A Cost Estimate of the Bayer Process for Producing/Muminia," Report of Investigation 6730, Bureau of Mines (1966). 2 H C Hsu, "Processes for Rutile Substitute," Report of N/V~kB, Nat Res Council, pp. 81-88 (June, 1972). 3 "HydrothermalTreatment of BOF Dust," Battelle Columbus Division (Unpublished). 4 J Mouland, Kidd Creek Mines, P O Box 2002, Timonins, Ontario, Canada P4N7K1 (Private communication). 5 E P Stambaugh and R A Roos, US Patent No 3,090,670 (May 21, 1963). 6 W B Blumenthal, ChemicalBehaviourof Zirconium, p 157 (1958). 7 A R Burkin, H Saricimen, and B C H Steele, "preparation of Yttria Stabilised Zirconia (YSZ) Powders by High Temperature Hydrolysis (HTH)," Trans J Bdt Ceram Society, Vol 79, pp 10.5-108 (1980). 8 E PStambaughand J H Adair,US Patent No 4,619,817 (October28, 1986). 9 S Somiya and M Yoshimura, "Hydrothermal Processing of Ultrafine Single Crystalline Zirconia and Hafnia Powders With Homogeneous Dopants," 1986 Conference on CeramicPowderScience and Technology,Boston,~ (August36, 1986). 10 D Barbet al, RumanianPatentNo RO79845B (August 30, 1982).
11
E P Stambaugh and W J Dawson, "Technology For New/Improved Hydrothermal Processes," Hydrothermal Group Program, Battelle Columbus Division (]982-1987). 12 K Beal, "Precipitation of Lead Zirconate Titanate Solid Solutions Under Hydrothermal Conditions," presented at 1986 Conference on Ceramic Powder Science and Technology, B o s t o n , / ~ (August 36, 1986). ] 3 R A Foos and E P Stambaugh, US Patent No 3,098,708 (July 23, 1963). 14 E P Stambaugh and D W Neuendorf, US Patent No 4,321,236 (March 23, 1982). 1.5 F Habashi, "Recent Advances in Hydrometallurgy," Proceedings of XIII International Mineral Processing Congress, Warszawa (1979). 16 F Habashi, "Recent Advances in Pressure Hydrometallurgy," Proceedings of International Conference on Advances in Chemical Metallurgy, Bhabu Atomic Research Centre, Bombay (1979). 17 D J I Evans, "Production of Metals by Gaseous Reduction from Solution Process and Chemistry," Inst Min and Met, London, pp 183-907 (1967). 18 E (3 Parker et al, "Zinc Pressure Leaching at Cominco's Trail Operation," Pro Hydrometallurgy Research, Development and Plant Practice, ed K Aseo-Asare and J D Miller. 19 F Habashi, "Hydrometallurgy", Chem & Engr Hews, pp 46-58 (February 8, ] 982). 20 ( 3 0 Argall, Jr, "Perseverance and Winning Ways at McLaughlin Gold", Engineering and Mining Journal (October 1986). 21 F Habashi, Principles of Extractiue Metal. lurgy, Vol 2, 0 255 (1970). 22 Battelle Columbus Division (Unpublished). 23 J L Keats, US Patent No 2,345,980 (April 4, 1944). 24 Paul Hagenmuller, "New Hydrothermal Methods With Oxidizing Solutions: Application to Chromium Dioxide Synthesis," Proceedings of the First International Symposium on Hydrothermal Reactions, Tokyo Institute of Technology (March 22-26, 1986). 25 N I Ingraham et al, US Patent No 2,923,683 (February 6, 1960). 26 R A Foos and E P Stambaugh, US Patent No 3,06.5,095 (November 20, 1962). 27 J Koslov, US Patent No 2,779,6.59 (January 29, 1957). 28 R A Foos and E P Stam baugh, US Patent No 3,098,708 (July 23, 1963). 29 E P Stambaugh and R A Foos, US Patent No 3,119,661 (January 28, 1964). 30 E P Stambaugh, US Patent No 3,607,006 (September 21, 1971 ). 31 S Somiya et al, "Hydrothermal Processing of Ultra-Fine Single Crystal Zirconia and Hafnia Powders with Homogeneous Dopants," 1986 Conference on Ceramic Powder Science and Technology, Boston, (August 5-6, 1986). 32 D L Wood, E D Kolb, and J P Remeika, J Applied Physics, Vol 39, p 1139 (1968). 33 F J Anderson et al, US Patent No 2,42],918 (June 10, ]947). 34. M Zonezawa, US Patent No 3,963,630 (June 15, 1976). 35 J Menashi et al, "Characterisation of Hydrothermically Synthesized BaTiO 3 and BaTiO 3 Based Dielectric Powders,"
MATERIALS & DESIGN Vol. 10 No. 4 JULY/AUGUST 1989
36
37
38
presentedat 1986 Conferenceon Ceramic Powder and Science Technology, Boston, MA (August 5-6, 1986). W J Dawson, E P Stambaugh and S L Swartz, "Hydrothermal Processing - An Aqueous Route to Improved Ceramic Powders," presented at the AICHE Conference on Emerging Technologies, Minneapolis, MN (August 1987). W J Dawson, S L Swartz and E P Stambaugh, "Hydrothermal Synthesis of PZT," presented at the American Ceramic Society Conference, Seattle (October 1986). W J Dawson, "Hydrothermal Synthesis of
39 40 41 42 43
Advanced Ceramic Powders," American Ceramic Society Bulletin. Vo167, No 10, 1673-1678 (October 1988). "Sakai Chemical Bares Fine Ceramic Powder Technology," Japan Chemical Week, p 4 (June 12, 1986). "Chichibu Develops Fine, Uniform Zirconia Powder," Japan Chemical Week, P 3 (June 12, 1986). "Business Opportunity Report," Business Communications Co, Inc, P O Box 2070C, Stamford, CT 06906. S Somiya, Toyko Institute of Technology, Japan (Private communication). M E Wadsworth, University of Utah
MATERIALS & DESIGN Vol. 10 No. 4 JULY/AUGUST 1989
44 45 46 47
(Private communication). F Habashi, Laval University, Quebec City, Canada (Private communication). E Peters, University of British Columbia, Canada (Private communication). S Hirano, Nagoya University, Japan (Private communication). W J Dawson, E P Stambaugh and S L Swart.z, "Tetragonal Zirconia Polycrystalline Ceramics by Hydrothermal Processing," presented at the 90th Annual American Ceramic Society Meeting (May 1988).
185
Abstract Hydrothermal processing involves the chemistry of hot water under pressure to carry out leaching and precipitation reactions. These reactions are usually conducted at temperatures ranging from I O0 to 350°C andpressures in the range from a few to 3000 psig. The reaction medium may be water alone or in combination with inorganic and~or organic acids and bases. The chemistry of this technology permits selective dissolution (leaching) or selective precipitation of metal species to be achieved leading to potentially novel treatment methods using a wide variety o fores and raw materials to produce high quality inorganic products. Thus, hydrothermal technology offers alternate or new approaches which can be technically superior to conventional processing for producing a wide variety of high technology inorganic oxides for specific applications, recovery of valuable components from low grade ores, and converting waste material to marketable products. Furthermore, because of short reaction times, simplicity of the reactor and recycle of reagents, the application of hydrothermal technology frequently leads to processes which can be less energy intensive, less pollutant and/or less capital intensive. Hydrothermal technology has a/ready been used in many successful commercial processes. Now, with the combination of feedstock shortages, rising energy costs, more restrictive pollution abatement regulations, inflationary pressures, need for higher quality products and a better understanding of the chemistry of hot water, this technology is emerging as a viable alternative to more conventional high temperature processing. The technology is expected to impact on several industrial areas, including primary and secondary ferrous and non.ferrous metals, ceramics, electronics, pigments, petroleum, utilities, refractories and inorganic chemicals.
Introduction More restrictive pollution abatement regulations, feedstock shortages, rising energy costs, inflationary pressures, and the need for higher quality products at lower costs are forcing industry to search out and develop alternative processes. Hydrothermal processing, one altemative technology, has been used in minerals processing for many decades. The most notable and well-known commercial application of hydrothermal processing is the Bayer aluminia process. This process was first commercialised in 1939 for the recovery of aluminia from bauxite or~'. Other commercial applications since then include the production of molecular sieves, synthetic rutile 2~, and nickel and cobalt metal powders. Because of its versatility, hydrothermal processing is reemerging in minerals processing and being evaluated for the production of advanced inorganic oxides and the conversion of industrial effluents to recyclable a n d / o r added-value products.
Process description Hydrothermal processing
involves
the chemistry of hot water under pressure to carry out leaching and precipitation reactions. These reactions are conducted at temperatures ranging from 100 to 350°C and pressures in the range of a few to about 3000 psig. The reaction medium may be water alone or in combination with inorganic and/or organic acids and bases. Such process chemistry has been and is being used to accelerate leaching reactions, aid physical processes during crystal growth, improve quality of inorganic products, and recover valuable materials from a wide variety of feedstocks. Examples of these are ores, concentrates, and waste materials. A potential application of this technology for the industry is broad. A summary of s o m e of these applications for specified industries is shown in Table 1.
Definition of hydrothermal processing Hydrotherma] processing, as the name implies, is based on processes using the chemistry of hot water under pressure. As shown in Figure 1, two conventional processing appli-
MATERIALS & DESIGN Vol. 10 No. 4 JULY/AUGUST 1989
cations are leaching and precipitation. Each of these is discussed in the following. Hydrothermal leaching Hydrothermal leaching may be used to extract selectively impurities from a raw material, leaving the product as the leach residue. An example of this application is the production of synthetic rutile via the benelite process~2L In this example, the iron value is selectively extracted from an ilmenite ore, leaving a crude TiO2 product ( > 9 0 percent TiO2) as the leach residue. The crude titanium dioxide product, referred to as synthetic rutile, is used as a substitute for natural rutile for the production of titanium dioxide pigment by the chloride vapour phase route. Another example in which hydrothermal leaching may be employed is for the treatment of basic oxygen furnace (BOF) dust~3~.Impurities such as zinc, calcium, manganese, lead, magnesium, and copper are selectively extracted from the BOF dust to produce a high purity ferric oxide which may be used as a pigment or raw material for the production of ferrites and metallic iron.
175
Hydrothermal leaching can also be used to extract selectively the major component from feedstock to produce a pregnant liquor from which the major component may be recovered by hydrothermal precipitation or electro-winning. An example of this application is the hyclrothermalleaching of zinc sulphide concentrate to produce elemental sulphur and a zinc sulphate solutiont4L The zinc value is subsequently recovered from the sulphate liquor by electro-winning. Another example is the recovery of aluminium value from bauxite ore by the Bayer processc'L In this example, the aluminium value is selectively extracted, leaving impurit!es such as silica and iron in the residue. The aluminium value is subsequently recovered from the pregnant liquor by hydrolysis of the sodium aluminate.
Table I Potential industrial applications of hydrothermal technology Industry
Application
Iron/steel
Upgrading ol iron ores Wastes to markefable products Treatment of sludges/aqueous streams Conversion of non-coking coals Io coking coals Desulphurisation/deashing of coking coals Metal powders Treatment of electroplalers wastes Critical metals recovery Minerals processing Ore beneficiation Advanced inorganic oxides for specific applications Treatment of spent refractory pottery wastes
Non-ferrous metals
Ceramic magnetic electronic Pelroleum
Production of catalysts Oil from tar sands Regeneration/treatment ol catalysts Desulphurisation of petroleum coke Treatment of sludges, solid wastes, and aqueous waste streams Direction precipitation of pigments Solid/sludge/waste water treatment Silicate production Chromate production Deashing of coal Coal-oil/water mixtures Coal chemicals Treatment of fly ash and coal ash Detoxification ot coal Treatment of incinerator ash Destruction of organic wastes Conversion of biomass to organic feedstocks Treatmenl of cellulostic malerials
Pigment Inorganic Chemicals Coal/utilities
Hydrothermal precipitation Hydrothermal precipitation is an alternate approach to conventional processing for the production of high quality, advanced inorganic oxides from pure and impure solutions and from hydroxide and hydrated oxides. Basically, this process consists of heating an aqueous solution containing soluble metal species or an aqueous slurry in an autoclave at temperatures less than the critical of water, and the corresponding pressure (steam alone or steam pressure plus an added over-pressure of an inert, oxidising, or reducing gas) for a time sufficient to promote the formation of the desired product. These products may be doped and undoped, simple and complex inorganic oxides. "Fhe process may be operated batchwise, continuously, or semicontinuously. It may also be operated as a single stage process whereby a single oxide is precipitated from the aqueous solution. An example of a single-stage hydrothermal process is the precipitation of zirconia from an aqueous zirconium oxychloride solution(5.6L In this example, anhydrous zirconia is precipitated from a zirconium oxychloride solution by heating the solution to a temperature of about 200°C and the corresponding steam pressure. Doped simple oxides such as yttrium-doped zirconia are produced directly by heating an aqueous alkaline slurry of yttrium hydroxide and zirconium hydroxide in an autoclave at temperatures greater than about
176
Food Agriculture
/
FEEDSTOCK
Precipitation
Leaching
Extract Impurit=es
l Product
I
Extract Primary Component
I
i
1
Precipitate I Impurities
Product
I Precipitate Primary Component
1
Product
Product
1 Final Product
Intermedlate Product
( Iron Oxide)
(Synthetic Ruttle)
Fig 1
Electrolytic
Winning
Hydrotherma 1 Precipitation
Versatility of hydrothermal processing to extract products from feedstock
MATERIALS & DESIGN Vol. 10 No. 4 JULY/AUGUST 1989
190°C and under the corresponding steam pressure(7.8.9). Complex oxides, ie oxides containing two or more metal ions, are normally produced hydrothermally by heating the corresponding hydroxides. For example, BaFe~20~9 is prepared by heating an aqueous slurry of barium hydroxide and ferric hydroxid~'°.~L Lead zirconate titanate is formed by heating an aqueous alkaline slurry of the reactants, having a pH ranging from about 7 to 14, at temperatures greater than about 250°C and the corresponding steam pressures('2L The multi-stage mode of operation offers the opportunity for production of high purity oxides from impure feedstocks. An example is the recovery of high purity zirconia from zircon as shown in Figure 2. In this example, a zircon frit is dissolved in hydrochloric acid. High purity ZrO2 is recovered by first selectively precipitating SiO 2 and Ti02 by heating the solution to about 150°C, leaving the Zr02, etc in solution. After separating the precipitated SiO2 and TiO 2 from the solution, the ZrO2 is selectively precipitated by heating the liquor to a higher temperature and pressure (about 180°C and under the corresponding steam pressure) leaving the impurities - AI203 and MnO2 - in solution. AI203 can be precipitated by heating the liquor to a still higher temperature and pressure~13L
ZIRCON FRIT
------I DISSOLVER J
J
FILTER _ INSOLUBLE -- MATERIAL
STAGE 1
SIO2 PRECIPITATOR
L
FILTER PRESS
L
j
FILTER PRESS PRIMARY -~- FILTRATE FOR RECYCLE
STAGE2
WASHSOLUTIOR
m[
MIXER
I
%)
Table II Typical hydrothermal processing conditions Solvent Temperature Pressure pH Time Atmosphere
Hydrothermal processing may be operated in a batch or continuous mode of operation and as a single stage or multi-stage process for both leaching and precipitation. In the batch mode of operation, the reactants are simply mixed and heated in a tank type autoclave to achieve the
FILTRATE ~-- FOR RECYCLE
Flow diagram of the selective recovery of silicon and zirconium dioxides (SiO2 and ZrO~) from zircon frit by hydrothermal precipitation~s2~
Modes of operation
Batch operation
]
[
ZrO2 (99.9 +
F g2
SiO2
J
Lp Zr02 RECIPITATOR
Processing conditions The range of processing conditions under which hydrothermal processing can be conducted are shown in Table IL As shown, hydrothermal processing is a low to moderate temperature and pressure operation, not a high temperature, high pressure operation. Reaction time is very short (5 to 60 minutes), making hydrothermal precipitation as well as hydrothermal leaching amenable to continuous operation. Furthermore, hydrothermal precipitations can be conducted over the entire PH range and under a steam, oxidising, or reducing atmosphere.
IICI
Mode of Operation
Not water Hot water & inorganic or organic acids Hot water & alkali and/or alkaline earths Less than 350C Less than 3000 psig Less than 1 to 14 5 to 60 min. Steam (inert) Steam & oxidising agent (02) Steam & reducing agenl (H2) Batch or continuous Single-stage Multi-stage
desired reaction, whether it be leaching or precipitation. An example of a laboratory batch-type autoclave is shown in Figure 3. The autoclave on
MATERIALS & DESIGN Vol. 10 No. 4 JULY/AUGUST 1989
the left is equipped with a sampling set-up to withdraw samples as a function of time and temperature for studying the kinetics of the leaching
177
rig3
Fig4
Batch- type autoclaves for conducting hydrothermal research
PREPARATION
TREATMENT
H
1
SEPARATION
H
Commercial synthetic rutile plant (courtesy of United Mcgill Corp.)
RECOVERY
F
L HYOROTHERMAL TREATMENT
Fig 5
Schematic of an integrated continuous hydrothermal process
or precipitation i-eaction. An example of a commercial batch operation is depicted in Figure 4. These autoclaves are spherically shaped vessels. In this example, synthetic rutile is produced as noted above by heating ilmenite ore in an aqueous hydrochloric acid solution to selectively extract iron and a number of other impurities, leaving the synthetic rutile containing greater than about 95 percent titanium dioxide as the residueI"h Other examples are given in F Habashi's paper on "Recent Advances in Hydrometallurgy 'c~5~.
178
Continuous operation A continuous hydrothe'rmal processing system can be very simple (Figure 5). The system consists of: feed preparation heat exchanger tank product storage feed storage tank tank pump filter and wash autoclave tanks In a typical tank operation, the feed solution or slurry is prepared and pumped to the feed storage tank. From here, the solution or slurry is pumped through the heated reactor to carry out the reaction. The reaction
may involve hydrothermal leaching or hydrothermal precipitation. The product slurry or solution is then cooled in the heat exchanger and discharged through a pressure letdown valve into the product storage tank. The product is then processed normally by conventional processing. Hydrothermal processes were originally developed for batch operation. However, because of short reaction time, better understanding of the chemistry of hydrothermal processing, and the more favourable economies of continuous processing, the trend is now toward the continuous mode of operation. An example of a continuous bench scale hydrothermal facility is shown in Figure 6. In this example, geothite (Fe00H) is being hydrothermally converted to ferric oxide by continuously pumping an aqueous slury of the geothite through the heated tubular reactor. The schematic of a continuous pilot plant facility is shown in Figure 7. Current s t a t u s of h y d r o t h e r m a l processing Hydrothermal processing is receiving increased attention throughout the world. Because of its versatility, this technology is expected to replace many high temperature, hydrometallurgy type operations.
MATERIALS & DESIGN Vol. 10 No. 4 J U L Y / A U G U S T 1 9 8 9
O~sPF~pla~en(
F~g6
Continuous hydrothermal facility for producing inorganic oxides
P,o.o~ I
I Collecl,on ~ l e z o e l e c t t cs
Capacllor$
Me(Ill
Refaxors
~
]
'
so,d.
L,oo,o
I
~
Separal,on I
~w~'--~] "1
\ Tao.
I
(B,nder I
Componenl~
Mlgnell¢$
I
F~g8 Applications for hydrothermally derived inorganic oxides Hydrothermal leaching - mineraLs processing Hydrothermal leaching has made significant progress in the last few years and will continue to progress(~6L Primary emphasis has been on the recovery of aluminium, nickel, cobalt, copper, zinc, uranium, and titanium for a wide variety of concentrates and waste materials. Some of these processes employ leaching and a combination of leaching and precipitation. The Bayer process for the recovery of aluminia from bauxite is still the largest and the oldest leaching operation in the worldm. This process originally developed as a batch operation has been converted into a continuous operation. Hydrothermal processing is also being applied in the treatment of a variety of nickel and cobalt feedstocks for nickel and cobalt recovery
Fig 7
Battelle's hydrothermal pilot facility
employing leaching to extract the metals and a multi-stage process for selective recovery of the metal values from the leachate by hydrogen reduction. One of these plants is operated by Sherritt Gordon Mines, Ltd. This plant produces nickel and cobalt powders and nickel - or cobaltcoated powdersWL Hydrothermal leaching is being applied in the zinc industry by Cominco, Ltd, which opened the world's first zinc hydrothermal leaching plant at Trail, B C, Canada in 1981(~8LThis is a continuous process whereby a zinc concentrate is leached at a moderate temperature and pressure to produce zinc sulphate solution, an iron-lead residue and a molten elemental sulphur. The use of hydrothermal leaching has been practiced for many years for uranium recovery and interest still
remains high on uranium recovery~% A recent development is the uranium extraction plant operated by Vereinigte Aluminium Werke-I ippenwerk, Lunen, West Germany. Hydrothermal leaching has also received worldwide attention in the production of synthetic rutile as a substitute for natural rutile for the production of titanium dioxide pigment. In this case, iron, along with other impurities are selectively separated from the titanium dioxide by hydrothermal leaching leaving a residue containing 90 to 97 percent titanium dioxide. Examples of plants producting synthetic rutile by hydrothermal leaching are Kerr McGee in US, Ishihara Sangyo Daisha, Ltd, in Japan, Murphyores, Inc, PTY, Ltd, and Western Titanium N I_,in Australia. Hydrothermal leaching is now receiving attention in the precious
/
MATERIALS & DESIGN Vol. 10 No. 4 JULY/AUGUST 1989
179
Table III HydrotherTnically produced inorganic oxides Simple Oxides Zr02(5,6,ll) Cr02(24,25)
Simple
Doped Oxides
Y203-Zr02(7,8,9,22)
CdO(21)
Cr203(26)
Y203 , CaO-,MgO-Zr02(8)
CoO(21)
Mn02(27)
Eu-Hf02(31)
Fe203(21,]I)
U02(28)
ZnO-Bi203-Mn203 (36)
Complex
Oxides
BaFe12019 (lO,I]) Y3Fes012(32)
Homogeneous Single crysta}s Controlled doping Non-agglomerated High purity
PbZrO3-Pbri03(12, 34,37)
Th02(28)
BaTi03(35)
MgO(21)
Si02(29)
BaZr03(22)
NiO(21)
Nb205(30)
(Ba,Sr) Ti03(38)
ZnO(21,22)
A1203(22~
Ti02(19)
Mn304(21)
Hydrothermal precipitation production of advanced inorganic oxides Hydrothermal precipitation technology has not advanced as much as hydrothermal leaching technology. However, because of the need for improved inorganic oxide powders for high tech applications and the ability for hydrothermal processing to produce these powders, hydrothermal processing is becoming a worldwideemerging technology for the production of advanced inorganic oxides for several applications. Examples of these applications are shown in Figure 8. Table III presents a partial listing of oxides which have been produced by hydrothermal processing.
Anhydrous Controlled crystal size and shape Narrow particle size distribution High purity
CaSi03(33)
Fe304(21)
metals industry and increased attention in the zinc industry. Homestake Mining has successfully employed a combination of leaching and precipitation for liberating gold and silver from a complex orc~2°L Kidd Creek Mining has recently constructed a plant for the recovery of zinc from a zinc sulphide concentrate by hydrothermal leaching~4L Thus, hydrothermal processing leaching alone or a combination of leaching and precipitation - is receiving increased attention in the minerals industry for the recovery of a wide variety of metal values from complex ores and concentrates (both highgrade and low-grade feedstocks).
180
Table IV Characteristics of hydrothermically produced oxides
acicular, platy, and equiaxed particles. Specific examples are shown in Figures 9 through 12. In addition, while not shown, the composition as well as the crystal (particle) morphology can be controlled to within very narrow specifications by hydrothermal precipitationCSL
An alternate manufacturing approach
Nature of hydrothermally precipi. tated oxides Inorganic oxides having the characteristics shown in Table IV are precipitated from aqueous solutions or slurries by hydrothermal processing as high purity, non-agglomerated, anhydrous, single crystal particulates of a controlled size and shape. These preciptated oxides exhibit a high degree of homogeneity and a uniform particle size distribution. Since the process involves chemical reactions that are carried out at low to moderate temperatures and pressures, the oxides contain fewer structural defects than those produced by high temperature calcination/roasting processes. Since the oxides contain little or no chemically bound water, only a simple drying is required to removed physically bound water. Normally, high temperature calcination or roasting is required to produce anhydrous oxides. Since the oxides are non-agglomerated, no milling or grinding is required to reduce the material to the ultimate particle size; thus, contamination by the grinding medium and the introduction of structural defects by grinding is avoided. In fact, the physical, chemical, and electronic properties can be tailored for specific applications by hydrothermal processing. Through the manipulation of the processing parameters, the shape of the crystals as well as the size of the crystals can be controlled to produce
A number of universities and research institutions are gearing up to assist industry. Examples of these are Battelle Columbus Division, Tokyo Institute of Technology and the University of Utah. While hydrothermal precipitation is in an early stage of development, industry is beginning to view hydrothermal precipitation as an alternative to conventional processes such as calcination, roasting, and sol-gel. Consequently, commercial plants have been constructed, are being constructed, or plans to build a commercial plant have been announced. In support of this commercialisation activity, the major activity in commercialisation of hydrothermal precipitation for the production of advanced inorganic oxides is in the general areas of toughened ceramics, electronic oxides, and pigments. For example, Cabot has developed a hydrothermal process through the pilot plant stage for the production of barium titanate-based dielectric powders(35L Sakai Chemical Industry Co has announced completion of a mass production plant for producing ultrafine ceramic powder using a hydrothermal synthesis process originally developed for the manufacture of magnetic iron oxide(39~.According to the report, this powder, used for functional ceramic products, has a grain size about one-tenth that of conventional products and can be sintered at low temperatures. Chichibu Cement Co, together with the Tokyo Institute of Technology, is planning to construct facilities at its Kumogaya
MATERIALS & DESIGN Vol. 10 No. 4 JULY/AUGUST 1989
a. 2.0 iz diameter Fig 9
b. 1.0 # diameter
Crystal morphology of zirconia produced by manipulation of hydrothermal processing conditions ~'JT~
Fig I0 Particle shape of hydrothermally produced partially stabilized zirconia ,.m
Fig 11 Hydrothermally produced acicular ferric oxide i, factory to produce fine zirconia powder by hydrothermal processingC4% According to Chichibu Cement, the powder is very uniform in size and can be sintered at temperatures about 100°C lower than required in the case of zirconia powders produced by conventional processing. H y d r o t h e r m a l p r o c e s s i n g to rec o v e r materials from industrial effluents
Fig 12 Barium ferrite (BaFe12019) produced by hydrothermal precipitatior~l ,
MATERIALS & DESIGN VoL 10 No. 4 JULY/AUGUST 1989
Hydrothermal processing provides a very attractive solution to a mounting waste m a n a g e m e n t problem. With industry generating millions of tons of solid and liquid effluents each year, waste m a n a g e m e n t and material recycling are a big-and-growing business worldwide. One source, for example, estimates that in the US alone, the waste m a n a g e m e n t industry will grow from $15.3 billion in 1987 to $23.9 billion in 1992 and that material recycling will grow from $7.8
181
billion to $10.3 billion in the same time period.{4'L A major reason for the increasing emphasis on waste management is that many traditional effluent disposal methods, such as ocean dumping, deep welling, and ]andfilling, are being strongly regulated, or even banned. Thus, industries are seeking alternative means of effluent disposal that are not only environmentally sound but also economical. Industries that generate effluents containing hazardous or potentially hazardous pollutants are especially hard pressed to find economical alternatives. For these wastes, disposal costs can be as high as $600 per ton and are expected to increase. As a result, some companies appear to be shutting down or changing their mode of operation to a less polluting process. But there is another alternative: converting effluents to recycleable, often marketable products. One company's waste material may actually be another company's raw material. In many cases, the effluent requires little or no treatment for use as a raw material. For example, waste iron oxide generating from treating pickle liquor can be used in the manufacture of barium ferrite. In other cases, the effluents or their components do require treatment before recycling or use as raw materials. An example is spent catalysts from the petroleum industry, which contain molybdenum, vanadium, nickel, and cobalt that can be recovered for recycling for the production of catalysts or can be used as raw material for the production of pigments. Many processes have been developed for treating waste materials to reduce pollution problems and to take advantage of profit opportunities. Many of these processes recover effluent components as high-purity oxides that can be used directly as raw materials. One common approach for obtaining these oxides is to use high temperature calcination/roasting processes, but an alternative - hydrothermal processing - shows promise as a technically and economically superior way to convert solid and liquid effluents into marketable, recyclable, and/or environmentally safe products.
Applications to recover useful products Hydrothermal processing has advantages which exceed those of the high
182
-Fable V High purity ferric oxide from electric arc furnace dust Concentration %
Metal
Zn
Before
15.0
After Treatment(1 )
< 0.01
Cu
0.05
< 0.01
Pb
0.1
< 0.01
Mg
1.7
< 0.01
Mn
1.1
< 0.01
(1) Nol detected: B. Sb. As, Co. Cd, Ge, Be. A g
High purity ferric oxide of controlled particle size from electric arc furnace dust by hydrotherrnal processin~ ~' temperature calcination/roasting processes in addition to a variety of applications for the treatment of effluents. Because of the unique chemistry of aqueous solutions, hydrothermal leaching and precipitation technology can be used to treat effluents to recover useful products. One example is the hydrothermal treatment of electric-arc furnace dust, which the US Environmental Protection Agency (EPA) has classified as a hazardous waste material. In this process, all hazardous or potentially hazardous metals as shown in Table V are extracted, resulting in a pigmentgrade (high purity) ferric oxide of
controlled particle size (Figure 13) for use as a pigment or in the production of magnetic oxides or metallic iron. Subsequent treatment of the extract produces a crude zinc product for use as a raw material in zinc smelters. Upon further treatment by hydrothermal processing, high purity heavy metal oxides such as zinc oxide can be produced '2~''. Another example is the recovery of high purity ferric oxide from basic oxygen furnace dust as shown in Table VI~22'. The treatment of fly ash by a combination of hydrothermal leaching and hydrothermal precipitation is another application. In this case, an
MATERIALS & DESIGN Vol. 10 No. 4 J U L Y / A U G U S T 1989
Table VI Pigment grade ferric oxide from basic oxygen furnace dust Concentration %
Metal
Before
Zinc Manganese Magnesium Lead Copper Calcium
2-4 10 05 10 0 02 5-10
After Treat ment(~
< <: < < < <
03 0 02 00t 001 001 001
(1) Others not detected
a. Spherically shaped particles of fly ash (before treatment)
4 ~ ,:.
b. Fly ash converted to acicular shaped particles (after treatment) Conversion of fly ash from spherically shaped particles to acicular shaped particles by hydrothermal processing ~'"
MATERIALS & DESIGN Vol. 10 No. 4 JULY/AUGUST 1989
alumina product of more than 99.9 per cent purity (see Table VII) was recovered for use in the aluminium and pigment industries. In addition, the shape of the fly ash particles was modified from spherical to acicular Ishown in Figure 14{~2~].This modified ash may be used for strengthening concrete, plastics, and other building materials. Hydrothermal treatment can also produce useful materials from the sludges that many industries, eg electroplating, iron/steel, and brassgenerate, recovering the heavy metals in a form suitable for use as raw materials in other industries. In the case of hydrothermal treatment of pickle liquor sludge, a highly reactive ferric oxide was precipitated. This oxide was found to be an excellent raw material for producing wire cord for the tyre industry2~L Another application is the cleanup of aqueous streams containing heavy metals. As shown in Table VIII, hydrothermal treatment of an aqueous stream containing barium, chromium, copper, lead, and ziiac resulted in essentially complete precipitation of these heavy metals. Leachability tests of hydrothermally precipitated oxides from a waste stream indicate that the product is environmentally safe ~22~. Thus, it could be disposed of in a landfill or further treated to recover heavy metals such as high purity oxides. Other applications include treatment of electroplating wastes, regeneration/ treatment of spent catalysts, treatment of incinerator ashes and dest_ruction of organic wastes as shown in Table I.
The future of hydrothermal processing Hydrothermal processing is receiving increased attention worldwide. Primary emphasis has been on the recovery of aluminium, nickel, cobalt, zinc, copper, molybdenum, tungsten, uranium, and titanium from a wide variety of ores, concentrates, and waste materials. Consequently, commercial plants are found in a number of countries throughout the world. In recent years, increased understanding of a q u e o u s chemistry ,~424~444546, has been moving hydrothermal technology from an "art" to a technology based on sound scientific and engineering principles. With the
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Table VII High purity alumina from fly ash by hydrothermal processing Concentration of Impurities in Alumina Product, ppm(2)
Concentration of Metals
Element
in Fly Ash, ppm(1)
Be B Si P S Ti Fe Cu Ni
5 15 30% 1000 600 3% 10% 200 200
0.15 1 400 300 200 15 10 6 10
(1) ppm except where noted. (2) Others, Mn, Co, V, As and rare earths, less than 5 ppm.
Table VIII Hydrothermal precipitation of metals from a waste stream Concentration mg/I Metal
Ba Cr Cu Pb Zn Fe
Untreated
4000 9000 3000 5000 16000 0
realisation of the potential of hydrothermal processing, more types of raw materials (including industrial effluents) are being treated hydrothermally to produce a wide variety of products. Examples are the recovery of precious metals, ~ chromium, and manganese and the production of ceramics, pigments, and catalysts. Some of these processes, notably precious metals recovery and advanced ceramics production, have already reached the commercial stage. Others are in the pilot plant stage of development or near-commercial stage; still others are in the laboratory stage. The increased need for higher quality products at lower costs, more restrictive pollution abatement regulations, feedstock shortages, rising energy costs, and inflationa~ pressures have enhanced the superiority of hydrothermal processing over that of conventional high temperature processing. Thus, hydrothermal processing is emerging as an alternate technology to more conventional processing for the conversion of a wide variety of raw materials to
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Treated
% Removal
3.8 14.9 4.3 540.0 780.0 0.4
99.9 99.8 99.9 89.2 95.1
added-value products. References
1
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MATERIALS & DESIGN Vol. 10 No. 4 JULY/AUGUST 1989
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presentedat 1986 Conferenceon Ceramic Powder and Science Technology, Boston, MA (August 5-6, 1986). W J Dawson, E P Stambaugh and S L Swartz, "Hydrothermal Processing - An Aqueous Route to Improved Ceramic Powders," presented at the AICHE Conference on Emerging Technologies, Minneapolis, MN (August 1987). W J Dawson, S L Swartz and E P Stambaugh, "Hydrothermal Synthesis of PZT," presented at the American Ceramic Society Conference, Seattle (October 1986). W J Dawson, "Hydrothermal Synthesis of
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44 45 46 47
(Private communication). F Habashi, Laval University, Quebec City, Canada (Private communication). E Peters, University of British Columbia, Canada (Private communication). S Hirano, Nagoya University, Japan (Private communication). W J Dawson, E P Stambaugh and S L Swart.z, "Tetragonal Zirconia Polycrystalline Ceramics by Hydrothermal Processing," presented at the 90th Annual American Ceramic Society Meeting (May 1988).
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