Since calcium oxalate is 3000 times less soluble than calcium sulfate, would it be possible to concentrate sulfuric acid from let's say $\pu{50\%}$ solution using following steps? :
- adding calcium carbonate to cold dilute $\ce{H2SO4}$
- filtering off resulting $\ce{CaSO4}$
- mixing $\ce{CaSO4}$ with oxalic acid in 1:1 molar ratio
- adding just enough water to submerge the solids
- vacuum filtering off the now concentrated (hopefully) $\ce{H2SO4}$
My reasoning behind this is that even though at any given time there's only a small amount of $\ce{CaSO4}$ dissolved, the reaction producing sulfuric acid would eventually go to completion with high yield due to it's the massive kinetic favourability. The only problem I see is that the amount of water needed would limit the final concentration. Would adding merely enough water to wet all the solid still work, improving the final concentration?