Kings at castle

Posted on 12 Mar 2009 by The Manufacturer

Castle Cement is investing in the future, through equipment, processes and people. Ruari McCallion got the lowdown from David Holgate

It’s in the nature of cement production that companies don’t stay in one place, so the fact that Castle Cement has been located at its Ribblesdale plant in Clitheroe, Lancashire, is worthy of note.

“The original company was established here in the mid-1980s,” says David Holgate, engineering manager at the site. A number of different companies either combined into or influenced the development of Castle to the company it is today. RTZ was pretty keen on the cement business in the 1980s and its takeover of the original business and acquisitions of Tunnel Holdings and Thomas Ward went a long way to creating today’s company. It built it up to four plants and then sold the group to a Scandinavian company, which went on to create Scancem. This was quite an important development, for reasons that will be come apparent.

In 1999, the company was purchased by Heidelberg Cement and, in 2007, Heidelberg acquired Hanson. Castle Cement is now an operating unit of Heidelberg, within the cementation division of Hanson. Castle Cement itself now has three facilities: at Ribblesdale in Clitheroe Lancashire; Padeswood near Mold in North Wales; and at Ketton, near Stanford in Lincolnshire. The Ribblesdale facility employs around 112 people – not including the delivery drivers – and has turnover in the region of £50m.

Why locate in Ribblesdale – or indeed any of those locations? The answer is, essentially, logistical. The cement plants are built where they can have effective access to their main raw material, which is limestone. It helps if other raw materials such as silica stone, gypsum and clay are nearby, too.

The layperson may be unaware but the finished cement product is made by a chemical reaction. It is made up of calcium silicates and smaller amounts of calcium aluminates, which react with water and make it set. The misconception may be that simply removing the water causes it to harden but that is not the case – the reaction is what gives cement its strength. Clearly, that strength varies, depending on the application – and the job of the cement producer is to mix and blend the raw materials into the right quantities, in order to produce the right qualities for the end user. You don’t have the same strength and resistance in cement that will be used for fixing bricks together on a house as you will need for a bridge on a motorway, which may carry 40,000 to 50,000 vehicles a day.

Cement is made by blending various raw materials and firing them at a high temperature in order to achieve precise chemical proportions of lime, silica, alumina and iron in the finished product – which is known as cement clinker. Castle uses high calcium limestone from its own quarry, which is right there on the site.

“We’re sat on a limestone seam; so we can simply dig down and quarry the limestone we need. Once extracted, the limestone is crushed to lumps of 70mm and less,” says Holgate. “From previous analysis, we do a rough adjustment to the chemistry by adding pulverised fuel ash (PFA – a by-product from the power generation industry) for alumina, and sandstone for silica. The crushed product is then held in a 20-tonne store until required. Then we add various sandstones and gritstones, which are purchased in from another quarry about four miles away.” That’s the great thing about the north of England – you have millstone grit sitting right on top of limestone, with sandstones in close attendance. No wonder the northwest was the heart of the UK’s industrial revolution – everything needed was right there.

“Until a couple of years ago, we would also add iron oxide, but now we can get the iron content required from one of our alternative fuels – processed tyres. The alumina required is present in the limestone, but in its natural form it contains high alkalis, so we again add more PFA to get the chemistry just right. It is now ground down to something with a consistency of talcum powder and stored in a 10,000 tonne storage silo.”

This material produced is called raw meal which is used to feed the kiln feed, and it’s about 80% to 90% limestone. Clay and other raw materials accounts for 10% to 15%; precise proportions vary. The reason why the chemistry of the rock is checked is because there are some impurities, found in limestone, which will compromise the cement’s quality. Magnesium carbonate is the main impurity and companies like Castle Cement will strive to keep it below three per cent as an absolute maximum.

Other materials to avoid are alkalis such as sodium oxide and potassium oxide. Some siliceous aggregates (silica materials) react with alkalis and form a gel, which swells and compromises durability. Portland cement is the most widely known and produced in the UK but it is not suitable for every application. Blended cements are made by blending finelyground cement clinker with other materials, like blast furnace slag, natural or artificial pozzolanas (volcanic ash is the natural version; metakaolins are artificial), siliceous fly ash, limestone fines, shale and gypsum. Blended types are standardised under BS:EN:97-1, which covers five main types of cement and indicates relative portions of Portland cement clinker (the second main ingredient), 28-day strength of class and rate of early strength-gain. Other varieties include rapid hardening, low-heat, sulphate-resistant, and low-alkali. Cement clinker is manufactured by heating the blended and ground raw material to partial fusion.

“Our kiln is a four-stage, pre-calciner,” explains Holgate. “The ground material is fed from the storage silo into the top of an 80-metre high tower, where it is pre-heated before entering the kiln, which turns at about three rpm. The tower heats the raw material by mixing it with the hot gases exiting the kiln and then separating the raw meal from the gases, and dropping it into the next stage, where it gets even hotter. To do this, the tower contains four cyclones, not unlike those in a Dyson. This is repeated in four stages, taking the temperature from 40C at the top to 900C at the exit. This process takes less than a minute. The final stage is called calcination, where additional fuel is added and burned to drive off CO2.”

The material then enters the kiln, where it is heated again from 900C to between 1400C and 1600C. “When it’s cooled, it turns into 10mm to 50mm balls of ash called cement clinker. The clinker is cooled as quickly as possible and the heat is recovered back into the process. The cement clinker is stored before being ground to make the final cement. Around five per cent gypsum is added to delay the setting, and depending on how fine the cement is ground, can determine product and quality.”

Given the level of heat that the process needs, and the CO2 present in the raw material, it’s not much of a surprise to learn that the cement industry is putting a lot of effort into reducing its CO2 emissions. Each tonne of cement produces about 0.5 tonnes of CO2. It is estimated that, globally, the cement industry is responsible for around seven per cent of man-made CO2 (to put that in context, the airline industry is responsible for about five per cent). The British cement industry has agreed to reduce its primary energy consumption by 25.6% by 2010 from a 1990 baseline, so finding alternative sources of fuel is obviously a major priority.

“Our turnover at Ribblesdale is around £50m a year and one of our biggest challenges is energy,” says Holgate. “Our electricity bill is in the region of £6m per year – and we saw a price hike of more than 30% last year. We’re looking at ways to reduce it – running off-peak, for example. We know every half-hour of every day throughout the year what the kilowatt-hour charge will be. So wherever possible we can optimise our operation to use the cheapest power, normally overnight and at weekends. Some of the processes, such as the kiln, have to keep going constantly.” Effective off-peak operation is easier to achieve and manage at the moment, as the market is weaker. When demand was high, the plant was running above 90% capacity and was able to sell everything it produced, but with less opportunity to save on off-peak running. The crushing and moving of millions of tonnes of powders uses a tremendous amount of electricity. The drives on the mills weigh in at an impressive 2.2 MW each. They simply could not crush rock without that level of power. And the continuous operation of the kiln means that raw heat is high up the agenda. It’s here that Castle has been making headway – and for some time.

“Castle Cement has been an industry leader in alternative fuel technology,” Holgate says. “Going back to 1992, we were the first plant to introduce secondary liquid fuels as an alternative fuel for cement production.” This goes back to the period under Scandinavian ownership. The company at that time created something that became branded as Cemfuel. “It was developed with the Solvent Recovery Manufacturing Company, which Castle then acquired, and is made of reprocessed solvents from the print, auto, paint and spraying industries.” The intentions were not just innovative, they also had an honourable dimension – without Castle Cement using these compounds, they would have simply been disposed of as a waste in landfill. However, the initiative was not welcomed by the green lobby.

“It actually created more problems than it solved. The press release stirred up a hornet’s nest, with local protesters claiming we were burning hazardous waste!” says Holgate. “We have installed a wet gas scrubber and state-of-theart monitoring technology that allows us to use this material without releasing dangerous levels of emissions.” Solvents do contain unpleasant compounds but the kiln temperature was – and remains – easily high enough to neutralise them.

The drive to control costs and reduce CO2 has led the company to further innovations. “Now, we use chipped tyres. They are as they say: old tyres that have been shredded into chips, around 50mm squares,” he explains. “Also, three or four years ago, we installed another fuel handling system that allows us to burn meat and bone meal – waste from the rendering industry. We could now burn pretty much anything with a high calorific value. It has made a difference and has helped us reduce our carbon footprint replacing our traditional fuel for cement production – coal – which is a fossil fuel.”

Castle is well on the way to eliminating coal from the process. “We are now above 60% to 65% replacement fuels, rather than coal,” he says. “Cemfuel is permitted to burn in a coal flame but we could, conceivably, get to the point where we use coal only for start-up; we would run the kiln completely on replacement and alternative fuels.” The drive for efficiency improvements is being manifested outside the pure power area as well. Castle Cement has invested more than £1.5m in a new site control system – and there’s more to come.

“Our plan is to invest between £2.5m and £3m in total on a complete new system for the kilns and cement mills,” says Holgate. “We’ve spent £1.5m over the past 18 months on automation. The company previously used control systems, which was based on old technology. It was pretty good when it first went in – state-of-the-art, in fact – but that was back in 1982. Things have moved on a bit since then, it has to be acknowledged.

“We were getting to the stage where the technology was obsolete and wasn’t being supported. It used electronic cards and hard wired systems which were high maintenance. It cost an arm and a leg to maintain; it needed specialist technicians and the annual service costs were going through the roof,” he explains. “In another two years, we’d been told, support for the hardware and software would have stopped completely.” One could see that happening, as some of the software was loaded and stored on 5.25 inch floppy discs. The new control system is supplied by a number of vendors – F.L.Smidth, Siemens and Dell – and will become standard technology at all three Castle sites.

“It gives much more control and much better information using a standard format throughout the system,” says Holgate. “We’re able to introduce and adapt automation and apply diagnostics much easier with the new software. We can easily monitor and trend any parameters we measure such as temperatures, pressures, vibration and the like. If we identify a potential problem with a piece of machinery, we can click on any or all of the parameters, where we can view, display and/or trend the information, which is a brilliant diagnostic tool. The software allows us to easily set up alarm limits and trip points to protect the machinery and other assets if any parameters go outside the set points. The start-up sequence for complex systems has now been simplified; it’s got lots of add-ons, for future management reporting, optimising energy usage, and so on.” You can tell when a man is pleased with the way things are going.

And there’s more.

“We’ve just gone through a 12-week MIP (maintenance improvement programme) with Heidelberg and consultancy company Proudfoot,” Holgate reveals. “It took maintenance right back to basics: looking at how jobs are identified, how they go into the system, how they’re executed, what resources are needed and planned, and so on. We’ve just completed the process and the early indications are that we’ve identified some significant savings. We had previously already identified savings; this will help us deliver them.” The focus is on efficiency in planning, execution and developing the necessary maintenance performance indicators (MPIs) to monitor how we are performing against plan.

“It should help us to reduce breakdowns,” says Holgate. “We now plan and schedule every PM job with a high priority and use corrective jobs as buffer work. It’s about doing the things that are critical to the plant operation and also what process people want you to do.”

At the end of the day, Castle is in the business of manufacturing cement – and it wants to do it better. Its geographical market is generally within 50 miles of its bases; the cost of transport makes it sensible to locate production as close to market as possible. Its production is split between readymix (just over half total output) pre-cast (20%, which includes block paving, concrete blocks, and beams and structures) and the remainder, 25%, as bagged cement. There are many different types of blend and all the major players are experimenting with them – Castle Cement included.

“It’s all about producing the right quality and quantity at the right time and cost. This means getting the best out of your people and equipment. The investment we have received in technology, equipment and the re-invention of the maintenance organisation has given us the tools to exceed both our maintenance and production targets,” confirms Holgate.

The Ribblesdale site is located on the edge of some of England’s most beautiful scenery – the Lake District and the northern Pennines. The rugged beauty of the Pennines in particular is a product of the same geological processes that created Castle’s raw material. Carboniferous limestone has low porosity and, therefore, low moisture content, so Castle Cement uses dry process manufacturing. The deposits to the east and north of Clitheroe are thick, consistent and of pretty high purity. They took a long time to lay them down and they may well have been waiting eons for exploitation. They’ve been there a while, and Castle Cement intends on being there a while, as well.