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If you are making a distinction between two types of sugar, for example, lump sugar and granulated sugar, then it is correct. If you're ordering it for your coffee or tea, you'd ask for "lumps of sugar."
"a lump of sugar", "a cube of sugar". Sugar used to be chopped off sugar cones - lump - for house hold use. Now most sugar is granulated or pressed into cubes.
If you are making a distinction between two types of sugar, for example, lump sugar and granulated sugar, then it is correct. If you're ordering it for your coffee or tea, you'd ask for "lumps of sugar."
lump sugar 
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