[2111USMC]

Yep, it’s a standard run of the mill bulk storage tank.

The rectangular flanges you mentioned are for attaching the legs to it. The bottom is conical shaped in order to ensure the ability to drain/empty it completely.

[bert]

..... and fitted with what appears to be a ball valve

[Cvengr]

Concur. It appears to be designed to be mounted vertically, with the conical section facing down. Retaining ring (painted white, with signs of carbon steel corrosion) with flanges might be for legs or stabilization anchor bolted to a frame or foundation. Insulation appears to be on the sides. Bottom flange and bolts appear too small in diameter for any deep subsurface design and no signs of burning to indicate aerospace design. Stainless fittings would seem to support perhaps foodstuffs or process bin. Probably cheaper to rebuild a new one than fetch it and repair it.

[AnAmericanMother]

Yup, looks like a milk storage tank to me too.