Plattwalzwerk = plate rolling mill/flat rolling mill
Blechwalzwerk = sheet rolling mill

Both can be described as rolling mills, it just depends on how thick the resulting product was/is - which becomes even more confusing when you get to Grobblechwalzwerke and Feinblechwalzwerke :-)

The earlier, and simpler, form of the plate rolling mill was a flat rolling mill , in which iron bars were squeezed between a set of rollers to make strips of plate that were further processed into rods or wire in a slitting mill.

I'll try to pop in some references when I have time, I must have something in my archives on ironstone mining and the beginnings of iron and steel making in the Cleveland region of North Yorkshire in the 18th and 19th centuries.

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Note added at 5 hrs (2014-07-20 22:38:46 GMT)
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BTW: one difference between a plate rolling mill and a sheet rolling mill was the diameter of the rolls (rollers). Plate mill rolls were generally between 28 and 44 inches and sheet rolls between 20 and 32 inches. The rolls for producing rods and wires were correspondingly smaller (only 8 to 12 inches)

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Note added at 5 hrs (2014-07-20 22:41:50 GMT)
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Early mills were water-powered, but these were later followed by steam driven mills and, eventually were constructed with electrically driven rollers - a typical conversion/modernisation was the replacement of the waterwheel with a Pelton turbine and generator as a source of power.

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Note added at 5 hrs (2014-07-20 22:54:58 GMT)
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Here's a mention of one of the biggest operators in Cleveland/Teesside in Yorkshire:

"Bolckow, Vaughan, & Co. (Limited.)- This is one of the largest firms of its kind in the kingdom. Its Middlesborough Iron-works are situated in the borough, and are the oldest and most extensive works in the Cleveland district; they cover something like twenty-one acres of ground between the river Tees and Vulcan Street. Within this site, when boring for water about ten years ago, a valuable deposit of salt was discovered; to get which shafts are now being put down by the company. Besides the Middlesborough Ironworks, Messrs. Bolckow, Vaughan, & Co. have large works at Witton Park, about thirty-two miles west of Middlesborough, the Cleveland Ironworks at Eston, and the Gorton Steelworks at Gorton, near Manchester. Their Cleveland iron-stone royalties are being worked at their Eston mines, Chaloner mines, near Guisborough, and the North Skelton mines, near Saltburn-by-the-Sea.

Bowesfield Ironworks consist of mills for rolling iron plates and sheets, and occupy a site of twenty-five acres, situate on the Tees at Stockton, and adjoining the Stockton and Darlington branch of the North- eastern Railway. "

Source: from my archives (found somewhere on a website about the development of Middlesbrough)

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Note added at 5 hrs (2014-07-20 23:07:05 GMT)
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Plate rolling mill at the Low Moor Ironworks in Bradford:

"In 1842 the company installed a new mill to roll iron plate for engine boilers."

From my archives: History of the Low Moor Ironworks, Bradford