And the result in the end is unimpressive by modern standards. At every stage of the process, there is no guidance and no guarantee of success. Does this dirt contain iron ore? Not sure… it looks kind of red, so maybe.

Regular skin examinations to detect erythema, epitheliomata or dermatitis are also prudent, and extra protection can be provided by alginate-base barrier creams. Workers doing hot work should be instructed prior to the onset of hot weather to increase fluid intake and heavily salt their food. They and their supervisors should also be trained to recognise incipient heat-induced disorders in themselves and their co-workers.

All those working here should be trained to take the proper measure necessary to prevent the occurrence or progression of the heat disorders. Workers exposed to high noise levels should be supplied with hearing protection equipment such as earplugs which allow the passage of low-frequency noise to allow perception of orders but reduce the transmission of intense, high-frequency noise. Moreover, workers should undergo regular audiometric examination to detect hearing loss.

Finally, personnel should also be trained to give cardiopulmonary resuscitation to victims of electric shock accidents.

The potential for molten metal splashes and severe burns are widespread at many sites in reduction plants and associated operations. In addition to protective clothing e. Individuals using cardiac pacemakers should be excluded from reduction operations because of the risk of magnetic field induced dysrhythmias.

The hazards to workers, the general population and the environment resulting from the emission of fluoride-containing gases, smokes and dusts due to the use of cryolite flux have been widely reported see table 1. In children living in the vicinity of poorly controlled aluminium smelters, variable degrees of mottling of permanent teeth have been reported if exposure occurred during the developmental phase of permanent teeth growth.

Among smelter workers prior to , or where inadequate control of fluoride effluents continued, variable degrees of bony fluorosis have been seen. The first stage of this condition consists of a simple increase in bone density, particularly marked in the vertebral bodies and pelvis. As fluoride is further absorbed into bone, calcification of the ligaments of the pelvis is next seen. Finally, in the event of extreme and protracted exposure to fluoride, calcification of the paraspinal and other ligamentous structures as well as joints are noted.

While this last stage has been seen in its severe form in cryolite processing plants, such advanced stages have rarely if ever been seen in aluminium smelter workers. Apparently the less severe x-ray changes in bony and ligamentous structures are not associated with alterations of the architectural or metabolic function of bone. By proper work practices and adequate ventilatory control, workers in such reduction operations can be readily prevented from developing any of the foregoing x-ray changes, despite 25 to 40 years of such work.

Finally, mechanization of potroom operations should minimize if not totally eliminate any fluoride associated hazards. Fluoride—both gaseous and particulates, carbon dioxide, sulphur dioxide, carbon monoxide, C 2 F 6 ,CF 4 and perfluorinated carbons PFC.

Since the early s an asthma-like condition has been definitively demonstrated among workers in aluminium reduction potrooms. This aberration, referred to as occupational asthma associated with aluminium smelting OAAAS , is characterized by variable airflow resistance, bronchial hyperresponsiveness, or both, and is not precipitated by stimuli outside the workplace.

Its clinical symptoms consist of wheezing, chest tightness and breathlessness and non-productive cough which are usually delayed some several hours following work exposures. The latent period between commencement of work exposure and the onset of OAAAS is highly variable, ranging from 1 week to 10 years, depending upon the intensity and character of the exposure.

The condition usually is ameliorated with removal from the workplace following vacations and so on, but will become more frequent and severe with continued work exposures. While the occurrence of this condition has been correlated with potroom concentrations of fluoride, it is not clear that the aetiology of the disorder arises specifically from exposure to this chemical agent.

Given the complex mixture of dusts and fumes e. It presently appears that this condition is one of an increasingly important group of occupational diseases: occupational asthma.

The causal process which results in this disorder is determined with difficulty in an individual case. Signs and symptoms of OAAAS may result from: pre-existing allergy-based asthma, non-specific bronchial hyperresponsiveness, the reactive airway dysfunction syndrome RADS , or true occupational asthma.

Diagnosis of this condition is presently problematic, requiring a compatible history, the presence of variable airflow limitation, or in its absence, production of pharmacologically induced bronchial hyperresponsivity.

But if the latter is not demonstrable, this diagnosis is unlikely. However, this phenomenon can eventually disappear after the disorder subsides with removal from work exposures.

Since this disorder tends to become progressively more severe with continued exposure, affected individuals most usually need be removed from continued work exposures.

While individuals with pre-existent atopic asthma should initially be restricted from aluminium reduction cell rooms, the absence of atopy cannot predict whether this condition will occur subsequent to work exposures. There are presently reports suggesting that aluminium may be associated with neurotoxicity among workers engaged in smelting and welding this metal.

It has been clearly shown that aluminium is absorbed via the lungs and excreted in the urine at levels greater than normal, particularly in reduction cell room workers. However, much of the literature regarding neurological effects in such workers derives from the presumption that aluminium absorption results in human neurotoxicity. Accordingly, until such associations are more reproducibly demonstrable, the connection between aluminium and occupational neurotoxicity must be considered speculative at this time.

Such episodes are most likely to occur when the weather initially changes from the moderate to hot, humid conditions of summer. In addition, work practices which result in accelerated anode changing or employment over two successive work shifts during hot weather will also predispose workers to such heat disorders. Heat stroke has occurred but rarely among aluminium smelter workers except among those with known predisposing health alterations e.

Exposure to the polycyclic aromatics associated with breathing of pitch fume and particulates have been demonstrated to place Soderberg-type reduction cell personnel in particular at an excessive risk of developing urinary bladder cancer; the excess cancer risk is less well-established.

Workers in carbon electrode plants where mixtures of heated coke and tar are heated are assumed to also be at such risk. Hence the reduction cells utilizing prebaked electrodes have not been as clearly shown to present an undue risk of development of these malignant disorders. Other neoplasia e. In the vicinity of the electrolytic cells, the use of pneumatic crust breakers in the potrooms produce noise levels of the order of dBA. The electrolytic reduction cells are run in series from a low-voltage high-amperage current supply and, consequently, cases of electric shock are not usually severe.

However, in the power house at the point where the high-voltage supply joins the series-connection network of the potroom, severe electrical shock accidents may occur particularly as the electrical supply is an alternating, high voltage current. Because health concerns have been raised regarding exposures associated with electromagnetic power fields, the exposure of workers in this industry has been brought into question.

It must be recognized that the power supplied to electrolytic reduction cells is direct current; accordingly, the electromagnetic fields generated in the potrooms are mainly of the static or standing field type. Such fields, in contrast to low frequency electromagnetic fields, are even less readily shown to exert consistent or reproducible biological effects, either experimentally or clinically.

In addition, the flux levels of the magnetic fields measured in present day cell rooms are commonly found to be within presently proposed, tentative threshold limit values for static magnetic fields, sub-radio frequency and static electric fields.

Exposure to ultra-low frequency electromagnetic fields also occur in reduction plants, especially at the far-ends of these rooms adjacent to rectifier rooms. However, the flux levels found in the nearby potrooms are minimal, well below present standards. Finally, coherent or reproducible epidemiological evidence of adverse health effects due to electromagnetic fields in aluminium reduction plants have not been convincingly demonstrated. Workers in contact with pitch fumes may develop erythema; exposure to sunlight induces photosensitization with increased irritation.

Cases of localized skin tumours have occurred among carbon electrode workers where inadequate personal hygiene was practised; after excision and change of job no further spread or recurrence is usually noted. During electrode manufacture, considerable quantities of carbon and pitch dust can be generated. Where such dust exposures have been severe and inadequately controlled, there have been occasional reports that carbon electrode makers may develop simple pneumoconiosis with focal emphysema, complicated by the development of massive fibrotic lesions.

The grinding of coke in ball mills produces noise levels of up to dBA. A variety of exposures have been associated with other diseases e. Gold mining is carried out on a small scale by individual prospectors e. The simplest method of gold mining is panning, which involves filling a circular dish with gold-bearing sand or gravel, holding it under a stream of water and swirling it. The lighter sand and gravel are gradually washed off, leaving the gold particles near the centre of the pan.

More advanced hydraulic gold mining consists of directing a powerful stream of water against the gold-bearing gravel or sand. This crumbles the material and washes it away through special sluices in which the gold settles, while the lighter gravel is floated off. For river mining, elevator dredges are used, consisting of flat-bottomed boats which use a chain of small buckets to scoop up material from the river bottom and empty it into a screening container trommel.

The material is rotated in the trommel as water is directed on it. The gold-bearing sand sinks through perforations in the trommel and drops onto shaking tables for further concentration.

There are two main methods for the extraction of gold from ore. These are the processes of amalgamation and cyanidation. The process of amalgamation is based on the ability of gold to alloy with metallic mercury to form amalgams of varying consistencies, from solid to liquid.

The gold can be fairly easily removed from the amalgam by distilling off the mercury. In internal amalgamation, the gold is separated inside the crushing apparatus at the same time as the ore is crushed.

The amalgam removed from the apparatus is washed free of any admixtures by water in special bowls. Then the remaining mercury is pressed out of the amalgam. In external amalgamation, the gold is separated outside the crushing apparatus, in amalgamators or sluices an inclined table covered with copper sheets.

Before the amalgam is removed, fresh mercury is added. The purified and washed amalgam is then pressed. In both processes the mercury is removed from the amalgam by distillation.

The amalgamation process is rare today, except in small scale mining, because of environmental concerns. Extraction of gold by means of cyanidation is based on the ability of gold to form a stable water-soluble double salt KAu CN 2 when combined with potassium cyanide in association with oxygen. The pulp resulting from the crushing of gold ore consists of larger crystalline particles, known as sands, and smaller amorphous particles, known as silt.

The sand, being heavier, is deposited at the bottom of the apparatus and allows solutions including silt to pass through. The gold extraction process consists of feeding finely ground ore into a leaching tub and filtering a solution of potassium or sodium cyanide through it. The silt is separated from the gold cyanide solutions by adding thickeners and by vacuum filtration.

Heap leaching, in which the cyanide solution is poured over a levelled heap of coarsely crushed ore, is becoming more popular, especially with low grade ores and mine tailings. In both instances, the gold is recovered from the gold cyanide solution by adding aluminium or zinc dust. In a separate operation, concentrated acid is added in a digest reactor to dissolve the zinc or aluminium, leaving behind the solid gold.

Under the influence of carbonic acid, water and air, as well as the acids present in the ore, the cyanide solutions decompose and give off hydrogen cyanide gas. In order to prevent this, alkali is added lime or caustic soda.

Hydrogen cyanide is also produced when the acid is added to dissolve the aluminium or zinc. Another cyanidation technique involves the use of activated charcoal to remove the gold. Thickeners are added to the gold cyanide solution before slurrying with activated charcoal in order to keep the charcoal in suspension.

The gold-containing charcoal is removed by screening, and the gold extracted using concentrated alkaline cyanide in alcoholic solution. The gold is then recovered by electrolysis. The charcoal can be reactivated by roasting, and the cyanide can be recovered and reused. Both amalgamation and cyanidation produce metal that contains a considerable quantity of impurities, the pure gold content rarely exceeding per mil fineness, unless it is further electrolytically refined in order to produce a degree of fineness of up to Gold ore occurring in great depths is extracted by underground mining.

This necessitates measures to prevent the formation and spread of dust in mine workings. The separation of gold from arsenical ores gives rise to arsenic exposure of mine workers and to pollution of air and soil with arsenic-containing dust. In the mercury extraction of gold, workers may be exposed to high airborne mercury concentrations when mercury is placed in or removed from the sluices, when the amalgam is purified or pressed and when the mercury is distilled off; mercury poisoning has been reported amongst amalgamation and distilling workers.

The risk of mercury exposure in amalgamation has become a serious problem in several countries in the Far East and South America. In amalgamation processes the mercury must be placed on the sluices and the amalgam removed in such a manner as to ensure that the mercury does not come in contact with the skin of the hands by using shovels with long handles, protective clothing impervious to mercury and so on. The processing of the amalgam and the removal or pressing of mercury must also be as fully mechanized as possible, with no possibility of the hands being touched by mercury; the processing of amalgam and the distilling off of mercury must be carried out in separate isolated premises in which the walls, ceilings, floors, apparatus and work surfaces are covered with material which will not absorb mercury or its vapours; all surfaces must be regularly cleaned so as to remove all mercury deposits.

All premises intended for operations involving the use of mercury must be equipped with general and local exhaust ventilation. These ventilation systems must be particularly efficient in premises where mercury is distilled off. Stocks of mercury must be kept in hermetically sealed metal containers under a special exhaust hood; workers must be provided with the PPE necessary for work with mercury; and the air must be monitored systematically in premises used for amalgamation and distilling.

There should also be medical monitoring. Contamination of the air by hydrogen cyanide in cyanidation plants is dependent on air temperature, ventilation, the volume of material being processed, the concentration of the cyanide solutions in use, the quality of the reagents and the number of open installations.

Medical examination of workers in gold-extracting factories has revealed symptoms of chronic hydrogen cyanide poisoning, in addition to a high frequency of allergic dermatitis, eczema and pyoderma an acute inflammatory skin disease with pus formation. Proper organization of the preparation of cyanide solutions is particularly important.

If the opening of drums containing cyanide salts and the feeding of these salts into dissolving tubs is not mechanized, there can be substantial contamination by cyanide dust and hydrogen cyanide gas. Cyanide solutions should be fed in through closed systems by automatic proportioning pumps.

In gold cyanidation plants, the correct degree of alkalinity must be maintained in all cyanidation apparatus; in addition, cyanidation apparatus must be hermetically sealed and equipped with LEV backed up by adequate general ventilation and leak monitoring. All cyanidation apparatus and the walls, floors, open areas and stairs of the premises must be covered with non-porous materials and regularly cleaned with weak alkaline solutions.

The use of acids to break down zinc in the processing of gold slime may give off hydrogen cyanide and arsine.

These operations must therefore be performed in specially equipped and separated premises, with the use of local exhaust hoods. Smoking should be prohibited and workers should be provided with separate facilities for eating and drinking.

Workers must be supplied with personal protective clothing impervious to cyanide compounds. There is evidence of exposure to metallic mercury vapour and methylation of mercury in nature, particularly where the gold is processed. In one study of water, settlements and fish from gold mining areas of Brazil, the mercury concentrations in edible parts of locally consumed fish surpassed by almost 6 times the Brazilian advisory level for human consumption Palheta and Taylor In a contaminated area of Venezuela, gold prospectors have been using mercury to separate gold from auriferous sand and rock powders for many years.

Again, I won't tell you details just a few general points. Slag is always a rather complex mixture of several components see above! Since most "rocks" and thus most gangue contain silicates, you will always find silicates in there.

Since "dirty silica" is just another word for glass , slag typically shows glassy behavior. It doesn't melt as it gets hot, it just become less viscous. Make it hot enough, and its fluidicy is almost like that of water. The temperature needed to achieve good fluidicy depends on the ingredients. The essential first point about optimizing slag is that you have enough typically more than metal , and that it will be sufficiently fluid at, say, 1 o C 2 o F.

And that's why you often add some " flux " to your burden, to whatever goes into your smelter. Flux is essential to obtain good fluidicy at "low" temperatures and to get enough of slag. The right flux may also improve slag properties with respect to the other points. Finding the right kind and amount of flux is one of the secrets of superior smelting.

Here is one such secret revealed for cast-iron smelting: The flux is "Braunspat" brown spar and "Eisenkalk" ferric chalk in precise amounts - and both fluxes are in essence siderite or iron carbonate FeCO 3. Siderite is typically a better flux than other iron ores but all iron ores "work" and were used by all and sundry throughout the millennia.

Not only for iron smelting but also for copper smelting! That is the reason why ancient slags from copper smelting or from iron smelting are often quite similar in composition. The fact that iron ores are very good fluxes means that iron smelting can be " self-fluxing ".

Nice, but wasteful because a lot of your iron then will end up in the slag. In fact, you might produce nothing but slag in your iron smelter if you don't know exactly what you are doing. Rather interesting, isn't it?

The chaps who by good luck have some siderite in their iron ores like the Proto-Austrians who were sucking up to the Romans , automatically get better smelting compared to those guys who have to do without it like the Proto-Schleswig-Holsteinians in my area, who fought the Romans. Analyzing the composition of ancient slag, and using the knowledge gained to conclude back to the smelting technique, is a rather complex and frustrating but rewarding enterprise.

It seems to come into its own right now, and it is rather certain that a lot of exciting new insights into the development of metal technologies will result in the near future. Here, however, I will abandon the subject now before it becomes too obvious that I don't know a thing about it.

Slag heap in Cyprus from around BC getting cross-sectioned. Issue December The first large-scale smelting produced copper. It was definitely not done in a smelter as large and complex as the one I used to explain the basics of smelting. A lot of people assume that it wasn't done in a smelter at all but happened accidentally in a camp fire, while making pottery, or during the burning of lime burning. Actually, I haven't read about the last one so I claim the credit. Lime burning, after all, is a technique far older than smelting or pottery, and was already quite advanced before smelting came into being.

Could that be true? Could you smelt something simple like tin, lead or copper accidentially in a campfire or in a kiln? The answer is: It's almost impossible - but not quite. Let's see why. You need to have ore in the fire. Tin ores are rather rare. It is therefore very unlikely - but not impossible - that some ends up in a campfire by accident.

Same thing for lead and copper ore, even so they are not quite as rare. You need carbon monoxide for reducing the ore. Or do you? As we will see further down, you can reduce ore without carbon monoxide but that doesn't help much here. That is very unlikely - but not impossible - to happen in a typical wood-fueled camp fire. If against the odds you do happen to produce a bit of metal, it would melt immediately in the case of lead and tin, getting absorbed in the ashes and coal pieces.

Copper would not melt no camp fire gets that hot but form small flimsy pieces, prone to oxidize immediately again. In other words: if you don't look very closely at the remains of you camp fire, you would not notice that you produced some metal. So how about pottery? You adorn some pots with some colorful copper ores and then fire them. Wouldn't you smelt these ores? Well - maybe. In a proper kiln , run at high temperatures beyond, let's say, 1 o C ; 1 o F and with an intentionally produced reducing atmosphere otherwise it would always be oxidizing , smelting of copper might happen - but pottery this advanced came long after smelting had already been invented!

As long as pots were fired by putting them into a kind of glorified camp fire - see above! After cooling, he finds the metal at the bottom of the hollow. People around the time when smelting copper came into being were obsessed with " green stones ", used for making jewelry. On occasion they run into native copper looking green since oxidized on the outside , and sometimes they recognized that as being something different an worked it into beads and awls. Somehow, and I don't pretend to know how exactly, they must have smelted copper by accident when working with the green stuff one way or other.

Most of the time nobody noticed. Sometimes somebody noticed but couldn't reproduce the "experiment" and lost interest. But eventually somebody somewhere started to make copper intentionally, and the technique caught on.

As present-day evidence strongly suggests, the very early "copper industry" was of the home-work type. You worked the stuff in your home with "imported" copper ore. No signs of copper smelting were ever found at the earliest copper ore exploits; it seems to have been done in settlements somewhere else. First intentional copper smelting was done in the most simple smelter imaginable: a "standard" ceramic bowl, about 20 - 30 cm in diameter and 16 - 30 cm in height.

We call such a bowl a crucible but there is no intrinsic difference to a bowl used for brewing beer, for example. As long as it was made from decent clay and well-fired not necessarily at very high temperatures , it was fine. Below is what it looked like schematically, real bowls are here.

For smelting you pile your charcoal - ore mixture more on one side and dip your blowpipe into the charcoal bed. A few cm behind the opening of your blowpipe - a simple reed, re-enforced a the hot end with some clay - a temperature of o C o F could be reached if everything was just right; enough to produce carbon monoxide CO somewhat further away, reducing the copper ore in the way of the blast.

A bit of slag might form too, becoming liquid, and dripping down. But that only happens during the blast and in a small region.

When you stop blowing because you need to breath in fresh air, everything cools down. At the next blast the temperature soars up again - but in a somewhat different area because the position of you blowpipe has changed a bit, and the way the air blasted in moves through the charcoal bed is also different, because the charcoals have settled somewhat and the geometry is different.

It is messy and inefficient, allright, but there are advantages: You can easily manipulate the contents of your "smelter": pull coals about, add fuel and ore here or there, blow on this or that spot, and so on. You can't do that in a larger blast-furnace type of smelter, at least not easily.

Typically you will not get a pool of liquid copper covered by some liquid slag this way. Your average temperature is too low, and the spatial and temporal uniformity of the process not good enough. What you get is a messy mixture of copper prills embedded in a fused mess of partially liquefied slag, charcoal pieces, and ore remainders.

The early copper "industry", it appears, consisted mainly of processing the crucible contents by banging it with stone hammers to pry off the precious little pieces of copper in there. The next step would be melting and thus also automatically refining the copper. This was done in the same kind of crucible, just considerably smaller; here are examples. Charcoal is essentially pure carbon. The carbon combines with oxygen to create carbon dioxide and carbon monoxide releasing lots of heat in the process.

Carbon and carbon monoxide combine with the oxygen in the iron ore and carry it away, leaving iron metal. In a bloomery, the fire doesn't get hot enough to melt the iron completely. Instead, the iron heats up into a spongy mass containing iron and silicates from the ore. Heating and hammering this mass called the bloom forces impurities out and mixes the glassy silicates into the iron metal to create wrought iron. Wrought iron is hardy and easy to work, making it perfect for creating tools.

Tool and weapon makers learned to smelt copper long before iron became the dominant metal. Archeological evidence suggests that blacksmiths in the Middle East were smelting iron as early as B.