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By Chao Li, Shuai Wang, Hongxu Li, Lifeng Zhang, Xie Yang

The method of chloride-hypochlorite leaching has the advantages of short process, low pollution and higher in reaction rate compared with the traditional leaching method. In this research, NaCl - NaClO solution is used to leaching gold from a refractory gold concentrate, and the influence of NaCl and NaClO concentration, reaction time to the leaching rate were investigated in this experiment. The raw materials and the leaching residue were characterized by a series of analysis methods such as XRD, XRF, SEM and EDS. Results indicate that the leaching rate and the desulfurization rate can be achieved at 96.26% and 98% with most optimum conditions as follows: NaCl concentration 50g/L, NaClO concentration 200g/L, Liquid-solid ratio 10:1, the best reaction time was 150 mins. And these conditions should be guaranteed pH<8.0, ORP>900mV.

Jan 1, 2015

By Elias Rigopoulos

In the present study the reductive behavior of dust from the rotary kilns (RK), in ferronickel production from nickeliferous Greek laterites, is investigated. The study concerns dust of various origins (washing towers, electrostatic filters, polycyclones) and pellets made from the dust. Reducibility was studied in the temperature range of 700 °C to 850 °C in a laboratory tube furnace. Experimental results showed that reduction degree increases with temperature up to about 800 °C. The rate of reduction is great during the first minutes, giving a reduction degree R% equal to about 25%, in only a few minutes. After this, the rate is decreasing until it diminishes to zero after about 10-20 minutes of reaction. Maximum reduction degree achieved is 26 to 28%. Rates and reduction degrees at various temperatures are higher for the dust samples than those of the respective pellet samples. Keywords: Ferronickel, metallurgy, laterites, pellets, reduction

Jan 1, 2004

By M. E. Huckman

In this paper, we review the equations needed to construct an ion exchange column model. These models contain three major components: 1) a differential material balance around the colunm, 2) a differential material balance around an ion exchange particle, and 3) a constitutive equation for diffusive flux We then examine the ability of a fairly simple mathematical model to predict ion exchange column experimental data. This model contains the traditional absorption theory equations and is used to predict the behavior of a column filled with a hydrous crystalline silico-titanate and which is removing trace amounts of Cs+ from a high pH, complex electrolyte solution. Finally, we propose replacing Fick&apos;s law with the Nernst-Planck equation as the constitutive equation for diffusive flux We use a Nernst-Planck type model to predict batch experimental data, then evaluate it by comparing it&apos;s ability to predict experimental data to that of a more traditional Fick&apos;s law type model? The data from this comparison suggests that the Nemst-Planck type model yields better results.

Jan 1, 1996

By Stephen Northey, Nawshad Haque, Terry Norgate

"Life cycle assessment (LCA) is a recognized tool to evaluate various processing routes for metal production. Declining ore grades and higher specific energy requirements for primary metal production put greater emphasis on recycling. Greenhouse gas (GHG) emissions of steel and aluminium metal production were quantified with recycling scenarios using material recovery facility (MRF) data from the database of SimaPro LCA software. The GHG footprint of the MRF is relatively minor compared with that of associated transport during collection (i.e. 10 times more than MRF) of curbside recyclable material. Additionally, if the bulk recyclable material is sent overseas (i.e. Australia to China) from the MRF for further processing, the GHG footprint of shipping can significantly be larger compared with the sum of the collection and MRF (assuming electricity is from same source). Thus opportunities exist for reducing GHG emissions from secondary metal production if it is processed close to the MRF.IntroductionIt is a challenge to recycle metals significantly, although theoretically infinite recycling is possible due to their elemental nature. Leaks from the metal stocks in society occur through corrosion, wear and dispersive uses, or via land filling or similar activities that return metals to the earth. In order to provide a technically sound and transparent assessment of metal recycling, a methodology such as life cycle assessment (LCA) should be used. By taking a life cycle perspective, the beneficial recycling properties of metals can be evaluated in a manner that enables appropriate comparisons with other materials or product systems that do not have recycling loops. In practice, mixtures of primary and secondary metals are often used in new products, and also at the end-of-life various processing methods are used for recycling.The difficulty of introducing recycling into LCA is to set the right boundaries for the different flows ending in different product systems. It is a question of which observed material flow belongs to the first product system and which one to the second or subsequent systems. Recycling can be part of any product LCA. However, it is often a complex issue which requires specific considerations. As pointed out by Yellishetty et al. and Birat et al. [l, 2], LCA practitioners are left with much freedom in allocation of environmental burdens to account for recycling, thus making subjective judgments on recycling and allocation of credits to recycling. This often makes it difficult to compare the results of LCA studies conducted by two different practitioners even on the same processes. As LCA is often used to define policy in government, business and society circles, it should be based on a sound, objective and unbiased description of recycling."

Jan 1, 2014

By A. G. Jackson, S. J. P. Laube

"Control of thin films during the growth process is dependent on several parameters that are difficult to monitor, including flux of film material, pressure, substrate temperature, composition, and thickness. One means for easing this difficulty is to take advantage of emission spectra associated with the process and the film. For pulsed laser deposition (PLD), the ion cloud generated by the laser has characteristic emission spectra that can be used to control the flux and, hence, the film. For chemical vapor deposition, the species present can be determined using mass spectroscopy and specific peaks can be used to control the deposition. Raman spectroscopy affords the possibility of real-time control by sensing characteristic peaks associated with film composition and thickness. These spectroscopic methods offer a real-time alternative to process control that is very attractive, because of the ability to accurately control films to achieve the engineered structures and properties sought. Examples are presented to illustrate the capabilities of these methods for thin film preparation.IntroductionThe drive to reduce costs and improve efficiencies of operation sets the tone for much of research, basic or applied. Basic research is limited by the ability of sensors and associated instrumentation to measure parameters accurately and reproducibly. Applied research has the same limitations and also the added constraints of time and costs. Hence, there are strong incentives to explore means for refining existing sensors with respect to performance and cost, as well as finding new sensors and techniques for more accurately and rapidly measuring quantities of interest in experiments and in various processes."

Jan 1, 1997

By F. A. Myzenkov

The technology developed by the authors uses new methods for raw material smelting in electric furnaces, a new autogenous method and apparatus for subliming smelting for antimony trioxide production and separation of antimony and gold. Concentrate produced from the ore deposits in Yakutia has the following average contents of main constituents, % by weight: 29.4 Sb, 1.2 Fe, 13.1 S, 40.0 SiO2, 0.2 As, 2.6 CaO and 45 g/t of Au. Unlike the technology used abroad a new autogenous method makes it possible to increase considerably specific capacity and reduce gold losses with antimony trioxide by several times. At the same time mineralogical composition of trioxide has improved considerably ? it has mainly a cubic base with prevalence of senarmontite. The content of a main component (Sb2O3) in the final product has increased from 99.0% up to 99.6% which means better quality and makes it suitable for use in special technologies. Despite a lower antimony content of the concentrate (29.4% as compared with over 33% as common in the world practice), an antimony recovery of 85.3% into a commercial-grade product has been achieved, as well as a gold recovery of 96.65% into a gold-antimony alloy. The technology for processing gold-antimony materials developed and introduced at the Ryaztsvetmet plant is unique in the world with respect to gold recoveries.

Jan 1, 2006

By A. S. Usmani

The flow associated with the pouring of molten metal into moulds has been modelled on the basis of certain simplifying assumptions. A finite element based Navier-Stokes equation solver has been used to analyse the flow. The velocities obtained from the Navier-Stokes equation solver are used to advect a pseudo-concentration function for modelling the fluid front. A simple problem with a known analytical solution is solved first to test the model. Subsequently, several more examples with varied geometrical configurations are modelled, including one with curved flow boundaries.

Jan 1, 1992

By Vladimir Panjkovic, Ostrovski. Oieg, Campbell Cripps Clark, John Truelove

"Effects of properties of raw materials and blast on the performance of an iron bath reactor have been examined by a mathematical model. The model consists of three modules: 1) chemical reactions in the gas phase and heat transfer from gas to droplets ejected from the bath; 2) heat losses from the vessel and radiative heat transfer from the freeboard to the bath; and 3) heat and mass balances for the bath. The following parameters of the process are calculated: coal, ore, flux and oxygen consumption, post-combustion ratio and heat-transfer efficiency, temperature and composition of the off-gas, and slag and metal chemistry.IntroductionOne of the anticipated advantages of smelting reduction over conventional ironmaking is the use of lower-grade raw materials. In this work, the effect of-properties of raw materials on the performance of an iron-bath reactor is examined with a numerical model. The model is also used to assess the effect of properties of the blast (flowrate, temperature and oxygen enrichment) and ejected droplets (generation rate, size and ratio of total weights of slag and iron droplets) on the key parameters of the process. In the process coal, oxygen, ore (which may be partly prereduced) and fluxes for specified slag composition are charged to the bath. Slag and iron droplets are ejected from the bath into the gas phase, where they are heated before returning to the bath. Iron droplets undergo decarburisation and reoxidation with CO2 and H2O. Dust is also generated from the bath, and oxygen or preheated air for post combustion is injected from the top of the vessel.The model consists of three modules: 1) reactions in the gas phase and heat transfer from gas to droplets ejected from the bath; 2) heat losses from the vessel and radiative heat transfer from the freeboard to the bath; and 3) heat and mass balances for the bath. The structure of the model and the data exchange between the modules are shown in Fig. 1. The following parameters of the process are calculated: coal, ore, flux and oxygen consumption, postcombustion ratio (PCR), heat transfer efficiency (HTE), off-gas temperature and composition, slag and metal chemistry, and rates of decarburisation and reoxidation of the iron droplets."

Jan 1, 1996

By Guangliang Wu, Dongchang Huang, Xinbin Liu

Oxidation Kinetics of SPHC was analyzed by TGA, and the curve of oxidation weight gain with different temperature was acquired. It is found that, when the oxidation temperature is between 500 and 600 , all the curves are linear; when the oxidation temperature is above 700 , the curves are close to parabolic. At the temperature of 900 , the oxidation weight gain increases dramatically. The TGA results indicate that the activation energy of SPHC steel is 146.19 kJ/mol at the temperature of 500 900 , and an oxidation kinetic model was set up for variable temperature. Based on the temperature change during rolling process, numerical simulation of oxide scales evolution was carried out for SPHC steel, and the simulation result agreed well with measured ones. It can provide a basis for estimating the scale growth and controlling the temperature on site.

Jan 1, 2016

By David G. Dixon, Maziar Eghbalnia

"Gold and silver nanoparticles (NPs) were deposited on pyrite at ambient temperature from colloidal solutions prepared by reduction of hydrogen tetrachloroaurate or silver nitrate by sodium sulfide or sodium tetrahydridoborate. The nanoparticles were characterized using Ultraviolet-Visible (UV-Vis) spectroscopy and the experimental spectra were fitted using Mie - Gans modeling. The modified pyrite was investigated using Electrochemical Impedance Spectroscopy (EIS). Electrodes were prepared from both natural and modified pyrite samples and were modeled using an equivalent circuit (EC) based on the EIS data. The comparison of the results revealed a decrease in charge transfer resistance for the ferric ferrous redox couple associated with the modified pyrite. From this behavior it is concluded that the electronic transport between the nanoparticles and the electrode surface over distances in nanometer scale is enhanced. Nanoparticles below a certain size act as quantum dots due to electronic confmement. Decreased charged transfer resistance at the nanometer scale may be caused by electron transport through tunneling mechanism via quantum dots.IntroductionIn recent years, considerable attention has been focused on the synthesis and study of Metal nanoparticles due to their potential applications in the fields of physics, chemistry, biology, medicine, material science and their interdisciplinary fields[!]. When material is nano-sized, the physical properties deviate markedly from those of bulk material, however, this effect is highly dependent on the particle size, interparticle distance, and morphology of the nanoparticles [2].An extremely small region of space defmed in a semiconductor material in order of nanometers is called a quantum dot, a term coined by Reed [3]. The three dimensional confinement of quantum dots can change properties of the excitons in comparison to free excitons in a bulk material [4]. Nanoparticles behave electronically as zero-dimensional (restricted in all three dimensions) quantum dots when the de Broglie wavelength of the valence electrons and the particle are in the same size range.As Reed [3] explains electrons confined in a plane have no freedom to move the third dimension. Those confined in a quantum wire are only free to move in one dimension, and those confined in a quantum dot are not free in to move in any dimension. An electron inside a cube of material IO nm on a side is confined to a point."

Jan 1, 2012

By Z. Sun, H. A. Davies

"Current understanding of the mechanisms by which glassy and crystalline thin ribbons are formed in high speed continuous casting processes is considered. A computer model of the combined heat and momentum transfer during melt spinning has been developed. The influence of roll velocity and of heat transfer coefficient (between the melt and the roll) on the temperature and velocity distributions within the melt puddle is analysed and the thickness of the extracted ribbon computed. The relative importance of heat and momentum transfer under different conditions is assessed and differences between the casting of crystalline and amorphous strip highlighted.IntroductionHigh speed continuous casting of thin metallic ribbon and strip is currently of considerable interest both to metal processers and to those studying process mechanisms. The practical interest arises both from the potential economic benefits to be gained from a shorter and more energy-efficient production route and from the ability to obtain extended and uniform thin strip having a metastable microcrystalline or amorphous structure with enhanced or unique properties. The latter arise from the rapid solidification associated with the cooling of very thin sections of melt brought into intimate thermal contact with one or two rapidly moving heat sinks.The principal processes for casting thin strip include free jet, chill-block melt-spinning (GBMS) (1), melt drag (MD) (2), planar flow casting (PFG) (3) and twin-roll casting (TRG) (4). For the first three of these, all essentially variants of melt-spinning, melt flows through a nozzle onto the circumferential surface of a rapidly rotating roll, whereas for TRG the melt passes into the nip between two contra-rotating rolls. We shall be concerned in this paper with the single-roll melt spinning processes only. For these, a puddle is established where the melt contacts the roll surface and the length of this puddle l is a function of the process conditions, notably the circumferential velocity of the roll Us and the volumetric flow rate of the melt Q. The contact time between any point on the roll surface and this puddle, or residence time eR(= l /Us) determines the thickness of ribbon t extracted from the puddle. This thickness, in turn, partly governs the cooling rate T and the amount of undercooling in the melt and thus the microstructure of the solidified ribbon."

Jan 1, 1986

By Joël P. T. Kapusta, Romeo U. Pagador, Noparut Wachgama, Chumnoom Khuankla

"Thai Copper Industries (TCI) is a custom copper smelter and refinery in Rayong, Thailand, with an annual capacity of 165,000 tons of cathodes. It employs the El Teniente Technology as the primary smelting reactor to produce white metal from copper concentrates. The white metal is further treated in Hoboken Siphon Converters to produce blister copper then refined in anode furnaces before being cast into anodes for electro-refining in the on-site tankhouse.Thai Copper Industries is the first copper smelter to install and successfully operate on a commercial scale with the Air Liquide Shrouded Injection (ALSI™) Technology in the Hokoben converters with sonic injection of oxygen enriched air and nitrogen for cooling of the injector tip areas. Following commissioning and the establishment of operating practices, the performance of the Hoboken converters with ALSI™ Technology produced outstanding results. The unique combination of an El Teniente and Hoboken siphon converters, both of which are bath smelting operations, offers very high blowing and oxygen efficiencies. As a consequence, reverts recycling was accelerated with minimal build up on the mouth, end-plates or even below the siphon that is typical in Hoboken converters.These outstanding results in the Hoboken Siphon Converters with ALSI™ Technology helped the TCI Plant reach a production of about 75% of its capacity within four months despite the fact that the TCI staff is quite young and its converter know-how and experience are not yet fully developed."

Jan 1, 2009

By Masaru Imaizumi, Hiroyuki Nomura, Mitsuharu Takita

"Cast iron possesses many superior characteristics caused from precipitated graphite structure, for example, castability, wear resistance, solid lubrication, damping capacity. But the more graphite hyper-eutectic cast iron contains, the lower its mechanical strength becomes, because graphite gives a negative effect to strength of cast iron. In addition, primary graphite tends to segregate during solidification, so uniform materials are difficult to be produced.In this study, authors introduced semi-solid processing in developing new and high quality cast iron. A series of semi-solid processing experiments were carried out to clarify the effect of stirring condition; solidification temperature and etc. on microstructures and characteristics of cast iron. In particular much attention is paid to the appearance of ternary eutectic zone at the final stage of solidification. As the results advanced hyper-eutectic cast iron was obtained with high quality of both the mechanical strength and wear resistance. As for the mechanical strength, 300MPa and 0.60% are obtained as the tensile strength and the elongation values, respectively, under the controlled stirring condition. This indicates that semi-solid processed hyper-eutectic cast iron is used prospectively without spheroidization.IntroductionCast iron has various good properties such as wear resistance, solid lubrication, damping capacity and machinability. The characteristics of cast iron are mainly decided by graphite structure, namely graphite shape and its distribution in the matrix. Mechanical strength of cast iron is large in the case of small quantity of graphite particles which are precipitated uniformly, however, it becomes small in the case of large quantity of graphite precipitated flakily."

Jan 1, 2000

By Shu-cai Zhou

"To understand the solidification behavior of austenitic heat-resistant stainless steel (TP34 7) in rotary electromagnetic-field, the influence of electromagnetic stirring in mold (M-EMS) on solidification structure of TP34 7 heat resistant steel in a round billet continuous casting was investigated based on industrial experiments. The results indicate that the excellent billet solidification structure of TP34 7 heat resistant steel can be obtained with appropriate electromagnetic stirring parameters. The solidification structure of TP34 7 heat resistant steel can be remarkably refmed, central porosity and shrinkage cavity can be remarkably decreased and equiaxed grains zone widens was enlarged. The industrial trials verify that the stirring intensity of TP34 7 heat resistant austenitic stainless steel should be higher comparing with plain carbon steel. Due to a higher viscosity, rotating speed of molten stainless steel was 20-30% lower than that of molten carbon steel in the same magnetic flux density.IntroductionIn recent years, the development of ultra super critical (USC) plants is accelerated by the demands of environment protection and energy saving, and USC plants with steam temperature and pressure conditions in excess of 593°C and 24.lMPa respectively were constructed. To achieve the goal, it is important to develop new materials with excellent high temperature strength, superior resistance to oxidation and corrosion at high temperature. Besides, their cost must be low enough to meet the requirement of plants [ 1]. TP34 7H steel is promising austenitic heat-resistant steel, which is used as superheater/reheater tube material and is extensively used for high temperature components and fossil-fired power plants, and suitable to be used in USC boilers plant. This kind steel contains with about 0. 7%Nb and it keeps higher allowable stress and creep rupture strength than TP304 and TP321H [1-2]. TP347H has conventionally been produced by ingot casting followed by forging and rolling. Recently, in order to decrease the production cost, challenging trials to produce these types of alloys by a continuous casting (CC) process have been attempted. However, the solidification structure of austenitic stainless steel is one of the key issues that attract more attention. Due to the low thermal conductivity of TP347H heat resistant stainless steel (much smaller than that of plain carbon steel), pronounced defects during continuous casting of austenitic heat-resistant steel such as coarse columnar, center porosity and coarse bridging, greatly influence mechanical properties of fmished products.A widely used technique in metallurgy is the inductive drive of molten metals using rotational or traveling magnetic fields. Recently, rotary electromagnetic stirring (EMS) has been applied to reducing various defects and improving increase the solidification structure of the metals in the continuous casting of steel, Al-based alloys and Mg-based alloys [3-8]. Electromagnetic stirring of plain carbon and low alloy steel in the continuous casting bloom and slab is well established for improving the quality of cast products [9-11] and is widely used in industry. However, very few literatures on electromagnetic casting of heat resistant stainless steel have reported. In this work, an industrial plant trial was performed to clarify the effect ofM-EMS on the macroscopic quality ofTP347heat resistant steel billet, in which the regularity and mechanism of the influence of EMS on solidification casting billet were discussed."

Jan 1, 2014

By K. Alibhai

Greek laterites, which generally had nickel concentrations of < 1 %, are amenable to chemical leaching,. although there is considerable variability in the ease of leaching with different ore types. With low grade ores(< 1 % Ni content); >70% of the nickel was leached in columns with a strong selectivity for nickel over iron. Heterotrophic microbially assisted leaching could be a viable alternative to classical hydrometallurgical and/or pyrometallurgical techniques, with a very high recovery (greater than 80% ) being found with some of these mineral microbial interactions. Critic acid was probably the effective chelating agent.

Jan 1, 1991

By Robert Matusewicz

Ausmelt Technology has been applied to the recycling of spent pot liner (SPL) generated from primary alnminum production. The process is centered on the Ausmelt furnace, in which the SPL is converted under high temperature, oxidizing conditions to an HF containing gas stream and inert slag. fluorine is recovered from the SPL as HF gas and is concentrated and used to manufacture AIF) for recycle to the aluminum smelters. Approximately 87% of the fluorine can be recovered and converted to AIF), with the remainder being effectively tied up in the by product slag. Cost projections show that the recycling process will be competitive with other processes for SPL. The production and recycling of AIF) provides available credit that significantly offsets the operating cost of the facility. Ausmelt Technology for recycling of SPL is currently being marketed in North America by Ausmelt Technology Corporation in Denver, Colorado, and in the rest of the world by Ausmelt Limited of Dandenong, Australia.

Jan 1, 1996

By J. Daniel Bryant

Deformation processing of rapidly solidified powder alloys presents a technical challenge different in several ways from the processing of ingot based alloys. The metastable phases formed during planar flow casting of aluminum alloy 8009 (FVS0812) must be safe-guarded from high temperature excursions during processing to avoid decomposition; this restriction prohibits liquid phase sintering of particles during consolidation. The development of strong particle-particle adhesion, therefore, relies on the imposition of very large strains during forming operations to insure sufficient mechanical meshing of the aluminum particles and break-up of prior particle oxide boundaries. These strains are generally much greater than those required for metallurgical transformations in ingot based alloys. In an attempt to quantify the deformation required to improve interparticle bonding, the finite element method (FEM) was employed. Preliminary results which predict the metal flow and strain distribution in several workpiece configurations are discussed. These predictions are then correlated to measurements made on experimental subscale forgings. Lastly, experimentally measured quantities which reflect the degree of interparticle bondingg are related back to FEM model predictions.

Jan 1, 1992

By Paula Ana Paiva

The high economic value and unique technological properties of the platinum-group metals (PGMs), plus their growing scarcity in the Earth’s crust, justify the crucial importance of developing recycling practices for PGMs end-of-life materials. Examples of top devices relying on the use of PGMs are automotive and industrial catalysts, and electrical and electronics equipment. This article critically describes the most recent research on the use of solvent extraction to recover one PGM, palladium, from spent catalysts. Some groups focus on the development of schemes involving commercial extractants, while others prefer to design specific molecules to efficiently and selectively recover palladium from these particular complex leaching solutions. Examples of commercial extractants proposed for the former schemes are Alamine® 308, TBP and LIX® 84I; while on the other hand, sulfur-containing diamides, thioamides, thiocarbamates, and dithioethers have recently been developed. Ionic liquids have to be mentioned too.

By A. C. Mollison

In November of 1983 a fire inside the autoclave caused the curtailment of operation of the Kidd Creek pressure leaching facility. Subsequent repairs and recommissioning of the autoclave in 1984 allowed Kidd Creek to increase its production of zinc as originally planned. This paper describes the operation of this process from 1984 to 1988 and details major mechanical work carried out on the vessel in 1.988:

Jan 1, 1990

By H. Soto, J. Jara, F. Nava

A method has been developed to recover cyanide and copper from solutions containing copper complexes and thiocyanate. Copper is precipitated as copper thiocyanate or as copper cyanide depending on pH and the concentrations of thiocyanate and cyanide in the effluent. The copper precipitate assays over 50% copper and presents good dewatering characteristics. After settling it can be separated from the effluent by filtration. The solution containing the bulk of the cyanide is then oxidized with ozone to transform the remaining thiocyanate into cyanide. This results in a regenerated solution rich in free cyanide that can be recycled to the cyanidation process. Careful control of the pH of the solution is required. First, pH of the effluent is lowered to optimize the precipitation and oxidation stages, then the pH is restored to an alkaline value to regenerate the cyanide solution. Recoveries of over 96% copper and cyanide were obtained in laboratory tests using industrial solutions. With this procedure, cyanocidic gold/copper ores could be treated economically by heap leaching, since the cyanide consumed by the copper impurities could be regenerated at very low cost (most of the cost of cyanide regeneration would be compensated by the copper credits). In order to achieve this, a bleed of the barren solution could be treated by the proposed method, and then recycled to the leaching step. The volume of solution to be treated should be large enough to keep a cyanide/copper molar ratio higher than 3:1. This would assure that copper does not cause undue interferences in the leaching or the carbon adsorption stages. If the thiocyanate content is low, the oxidation stage can be avoided, and the solution can be recycled to the leaching stage directly after the solid-liquid separation stage.

Jan 1, 1997