Plants using waste materials are usually geographically disconnected from their raw material’s ultimate source. The most significant category of waste usage is the large number of small plants that grew up using chalk ballast. Coal from the northeast of England has been the preferred fuel in London virtually since its foundation two thousand years ago. Since London produced no low-value cargoes for the coal ships on their return journeys, it became normal practice to ballast them using the cheapest available bulk material – chalk quarried from the banks of the Thames estuary. The result was the uninterrupted series of quarries along the Thames in the Grays and Northfleet areas, and vast stockpiles of chalk surrounding the dock areas in northeast ports such as Hull, Tees-side, Hartlepool and Tyneside. The latter were massive: the old Ordnance Survey maps label them “ballast hill”. Several were high enough and permanent enough to have trigonometric points on their summits. The small cement plants that grew up on the river banks literally quarried them. They were not an entirely reliable material source, since various other ballast materials such as gravel or china clay tailings were indiscriminately mixed in with the chalk. The North-east became the only area outside the Thames/Medway district making an appreciable amount of Portland cement. A contemporary description of the district is given here. As the chalk stockpiles depleted, and when iron coal-ships with pumpable water ballast were developed, these plants ended up shipping chalk from the Thames as commercial cargoes, until they gave up the ghost due to the resulting high production cost. This occurred even when quite good limestones were available locally – a result of the mystical cement-making qualities originally attributed to Thames chalk.
Some other industries produced calcium carbonate as a waste, notably the alkali industry on the Mersey, Tyne and Tees. Although the chemical industries needed calcium carbonate as a raw material, precipitated calcium carbonate, obtained as a cake, mud or sludge, could not be re-cycled in old-fashioned lime kilns before the advent of the rotary lime kiln, and vast quantities were discarded, usually by literally flushing them “down the drain”. On the other hand, a wet process cement plant could handle them without much trouble.
In the Leblanc Process for making sodium carbonate, calcium sulfide is produced as waste:
2NaCl + H2SO4 → Na2SO4 + 2HCl
Na2SO4 + CaCO3 + 2C → Na2CO3 + CaS + 2CO2
Under pressure from the Alkali Inspectorate, the Chance Process was developed to consume the huge noxious mountains of calcium sulfide that had built up in the nineteenth century:
CaCO3 → CaO + CO2
CaS + CO2 + H2O → CaCO3 + H2S
2H2S + O2 → 2S + 2H2O (using a Fe2O3 catalyst)
The calcium carbonate produced, being too fine to re-burn as lime, was used as part of a cement rawmix. The only plant dedicated to using this process appears to have been that attached to Chance’s own alkali plant at Oldbury, although Ditton and Jarrow used it among a cocktail of other wastes.
The process of conversion of sodium carbonate to sodium hydroxide also produced calcium carbonate as a fine filter cake that was otherwise dumped:
CaCO3 → CaO + CO2
CaO + H2O → Ca(OH)2
Na2CO3 + Ca(OH)2 → 2NaOH + CaCO3
Many plants used this at least for part of their lives, those after 1895 being Crosfield's, Ditton, Gateshead and Jarrow. This “causticisation” process was also used in soda recovery in the paper industry:
Lignin + NaOH → Sodium Lignate
Sodium Lignate → Na2CO3 (by pyrolysis)
Na2CO3 + Ca(OH)2 → 2NaOH + CaCO3
At least one plant (South Hylton) used the carbonate sludge as a raw material. This source dried up with the conversion of paper making to the sulfite process.
At Billingham, the adjacent synthetic ammonia plant used the Haber process to make ammonia. Ammonia was reacted with combustion gas CO2 to form ammonium carbonate, and this was reacted with finely-ground anhydrite to form ammonium sulfate and calcium carbonate as “sulfate plant mud” that was previously run into the North Sea:
2NH3 + H2O + CO2 → (NH4)2CO3
(NH4)2CO3 + CaSO4 → (NH4)2SO4 + CaCO3
The conventional cement plant started using this in the 1920s.
At least one sugar refinery had an attached cement plant: lime was used to carry down impurities in sugar liquor:
CaCO3 → CaO + CO2
CaO + H2O → Ca(OH)2
Sugar + Ca(OH)2 → Calcium saccharate
Calcium saccharate + CO2 → Sugar + CaCO3
Until rotary lime kilns were developed, the resulting calcium carbonate was too fine to re-burn.
Plants using carbonate waste began to proliferate after 1890, the imperatives driving this being typified by Crosfield's, who had been prosecuted for dumping their waste in the Mersey. Nonetheless, this source was of limited duration, because the chief sources in the alkali industry were at the same time moving towards more efficient waste-free processes – notably the electrolytic processes for making sodium hydroxide. Carbonate waste was further eliminated by the development of rotary lime kilns which could easily re-burn the otherwise intractable carbonate sludges. The one long-standing plant at Billingham remained – by cement industry standards – extremely inefficient. As a minor excrescence of ICI’s business, there was little pressure to drive down costs as long as favourable pricing and ignored incidental expenses kept the balance sheet more or less in the black.
Around the beginning of the twentieth century, the lack of locally produced cement in Scotland prompted a number of Scottish iron works to consider re-burning their blast-furnace slag with extra limestone to make a clinker, subsequently inter-ground with more slag to make a pbfc. “Activated slag” cement had been made for some time by inter-grinding water-granulated slag with calcium and/or sodium hydroxides, but this was too slow a cement even by the standards of the day. In the USA, rejection of this product led to the making of portland clinker from slag and limestone in 1900 , and accounted for 13% of US output by 1912, peaking at over two million tonnes per year. The largest plant using the process, at Buffington IN, had a capacity at that time of 4800 t/day from 40 kilns, making it the largest plant in the US, and therefore the world. The use of slag in Portland rawmixes had been examined in the UK, but rejected because of the weakness and friability of the briquetted rawmix, which prevented its use in static kilns. The process had to wait for the introduction of the rotary kiln. In the UK, the process began to be used when it was already in decline in the US, and it never developed outside Scotland, where conventional cement raw materials were hard to find. Only low-magnesia slag could be used, and the 20th century trend towards higher magnesia in slags made the process progressively less viable. In Scotland, used at Coltness, Gartsherrie and Wishaw, it provided the only indigenous cement between the closure of Cousland in 1923 and the opening of Dunbar in 1963.
The Scottish slag-burning plants all started out by installing the Colloseus patent process for making ground activated slag. Substantial capital was invested before, as happened in the USA, the complete rejection of the product by the market led them to investigate making a Portland product. Others, notably in the North-east, had examined the possibility of making the Colloseus product – Consett iron works was to supply the slag – but the idea was never carried through (although a great deal of cement with slag in it was undoubtedly sold surreptitiously). This left only the Scottish plants. All had lost their on-site blast furnaces by 1930, and continued by quarrying the massive slag-"bings" that had been built in the previous century, and by buying in slag from the diminishing number of plants remaining in Scotland. The availability of low-Mg slag in particular diminished after WWII, and the slight raison d’être of the plants was eliminated with the prospect of a large conventional plant in the form of Dunbar. The rotary kilns exclusively used were all dry process, the early difficulties with this being less in evidence due to the forgiving nature of the slag rawmix. In principle, high efficiency should have been possible, but the operations were far too small to allow the necessary investment to bring this about, and fuel consumptions remained mediocre.
Included for convenience here are the plants making sulfuric acid by the “Anhydrite Process” , in which cement clinker itself was a by-product. In this process, anhydrite (calcium sulfate) replaces limestone in a cement rawmix, and under reducing conditions, sulfur dioxide is evolved instead of carbon dioxide. The sulfur dioxide is converted to sulfuric acid by the Contact Process using a vanadium pentoxide catalyst.
CaSO4 + 2C → CaS + 2CO2
3CaSO4 + CaS + 2SiO2 → 2Ca2SiO4 (belite) + 4SO2
3CaSO4 + CaS → 4CaO + 4SO2
Ca2SiO4 + CaO → Ca3OSiO4 (alite)
2SO2 + O2 → 2SO3
SO3 + H2O → H2SO4
The reaction is driven forward by the irreversible combination of calcium with shale minerals to produce Portland clinker. Typically plants produced around 104 tonnes of sulfuric acid for every 100 tonnes of clinker (although not all the clinker may have been of cement making quality and may have been written-off).
The process was invented in Germany (by I G Farben, Leverkusen) during WWI, when the usual sources of sulphur (native sulphur and pyrite) were cut off due to blockade. The clinker was at that time discarded, and only in the mid-1920s started to be used for cement. ICI began their Billingham plant in 1929, purely because a huge anhydrite resource happened to be there, and essentially developed their own technology (although the initial concept, as with the Haber process, was acquired as “spoils of war”). There followed twenty years of slow, faltering development, during which time the sulphuric acid industry in general became increasingly dependent upon elemental sulphur as a raw material, until in 1951 an unexpected US government-imposed restriction on sulphur exports caused a crisis in the industry. This made the anhydrite process easily the cheapest method of acid manufacture in terms of operating cost (although by no means in terms of capital cost). Furthermore, in the 1950s, the UK government maintained pressure, to the point of coercion, for industry to use indigenous raw materials.
There followed a major scale-up in anhydrite process. ICI installed a big new kiln, and launched United Sulphuric Acid Corporation to construct a plant at Widnes. This was a “non-profit-making” consortium consisting of ICI and ten of its major customers , and the plant was constructed and operated using ICI’s technology. At the same time, Albright and Wilson (née Marchon) installed a new plant (which ultimately became the biggest) at Whitehaven, using I G Farben technology, and having an agreement with APCM for cement marketing. These all came on stream in 1954-1955. The optimisation of the clinker-manufacturing part of the process was of low-to-zero priority. The kilns used were necessarily dry process, but by the standards of the cement industry, their fuel consumptions were high and their outputs very low. This is partly due to the much larger inherent endothermic heat of reaction: 3.7 MJ per kg of clinker compared with 1.7 MJ for a conventional mix. However, this was exacerbated by the use of coarse rawmix, made with the mistaken idea that this would reduce dust pickup. By raising burning zone temperatures, this increased gas velocities and viscosities, thus increasing the potential for dust pickup. All these kilns were limited by dust generation. Conceivably, a single stage preheater might have dramatically improved performance, but all the kilns were simple tubes with no internals. In addition to poor performance, the kilns were also subject to the inevitable reduced availability associated with a very complex plant with multiple sources of potential mechanical failure. The majority of short stops were associated with downstream problems in the acid plant, while the long-term effect of these led to long stops for major refractory failure. Subsequent expansion brought production to a peak in 1968, before a major fall in the price of elemental sulphur and the rise in energy prices wiped it out, the last production being in 1976.
The requirement for a reducing environment in the kiln meant that clinker mineralogy was variable and often unfavourable, and this problem was exacerbated by the fact that producers regarded clinker as little more than a waste by-product. The longest-lived plant (Billingham) always blended the product with conventional clinker. The last of the plants in this group (Whitehaven) shut down in 1976. There is little immediate prospect of a revival, although it is conceivable that an anhydrite process with greatly enhanced efficiency and perhaps a sulfoaluminate cement product might be considered in the future. The process will also be increasingly attractive by virtue of the fact that its CO2 emissions are much lower than those of the conventional Portland cement manufacturing process.